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rfc:rfc7868

Independent Submission D. Savage Request for Comments: 7868 J. Ng Category: Informational S. Moore ISSN: 2070-1721 Cisco Systems

                                                              D. Slice
                                                      Cumulus Networks
                                                             P. Paluch
                                                  University of Zilina
                                                              R. White
                                                              LinkedIn
                                                              May 2016
     Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP)

Abstract

 This document describes the protocol design and architecture for
 Enhanced Interior Gateway Routing Protocol (EIGRP).  EIGRP is a
 routing protocol based on Distance Vector technology.  The specific
 algorithm used is called "DUAL", a Diffusing Update Algorithm as
 referenced in "Loop-Free Routing Using Diffusing Computations"
 (Garcia-Luna-Aceves 1993).  The algorithm and procedures were
 researched, developed, and simulated by SRI International.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This is a contribution to the RFC Series, independently of any other
 RFC stream.  The RFC Editor has chosen to publish this document at
 its discretion and makes no statement about its value for
 implementation or deployment.  Documents approved for publication by
 the RFC Editor are not a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7868.

Savage, et al. Informational [Page 1] RFC 7868 Cisco's EIGRP May 2016

Copyright Notice

 Copyright (c) 2016 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.
 This document may not be modified, and derivative works of it may not
 be created, except to format it for publication as an RFC or to
 translate it into languages other than English.

Table of Contents

 1. Introduction ....................................................5
 2. Conventions .....................................................5
    2.1. Requirements Language ......................................5
    2.2. Terminology ................................................5
 3. The Diffusing Update Algorithm (DUAL) ...........................9
    3.1. Algorithm Description ......................................9
    3.2. Route States ..............................................10
    3.3. Feasibility Condition .....................................11
    3.4. DUAL Message Types ........................................13
    3.5. DUAL Finite State Machine (FSM) ...........................13
    3.6. DUAL Operation -- Example Topology ........................18
 4. EIGRP Packets ..................................................20
    4.1. UPDATE Packets ............................................21
    4.2. QUERY Packets .............................................21
    4.3. REPLY Packets .............................................22
    4.4. Exception Handling ........................................22
         4.4.1. Active Duration (SIA) ..............................22
                4.4.1.1. SIA-QUERY .................................23
                4.4.1.2. SIA-REPLY .................................24
 5. EIGRP Operation ................................................25
    5.1. Finite State Machine ......................................25
    5.2. Reliable Transport Protocol ...............................25
         5.2.1. Bandwidth on Low-Speed Links .......................32
    5.3. Neighbor Discovery/Recovery ...............................32
         5.3.1. Neighbor Hold Time .................................32
         5.3.2. HELLO Packets ......................................33
         5.3.3. UPDATE Packets .....................................33
         5.3.4. Initialization Sequence ............................34
         5.3.5. Neighbor Formation .................................35
         5.3.6. QUERY Packets during Neighbor Formation ............35

Savage, et al. Informational [Page 2] RFC 7868 Cisco's EIGRP May 2016

    5.4. Topology Table ............................................36
         5.4.1. Route Management ...................................36
                5.4.1.1. Internal Routes ...........................37
                5.4.1.2. External Routes ...........................37
         5.4.2. Split Horizon and Poison Reverse ...................38
                5.4.2.1. Startup Mode ..............................38
                5.4.2.2. Advertising Topology Table Change .........39
                5.4.2.3. Sending a QUERY/UPDATE ....................39
    5.5. EIGRP Metric Coefficients .................................39
         5.5.1. Coefficients K1 and K2 .............................40
         5.5.2. Coefficient K3 .....................................40
         5.5.3. Coefficients K4 and K5 .............................40
         5.5.4. Coefficient K6 .....................................41
                5.5.4.1. Jitter ....................................41
                5.5.4.2. Energy ....................................41
    5.6. EIGRP Metric Calculations .................................41
         5.6.1. Classic Metrics ....................................41
                5.6.1.1. Classic Composite Formulation .............42
                5.6.1.2. Cisco Interface Delay Compatibility .......43
         5.6.2. Wide Metrics .......................................43
                5.6.2.1. Wide Metric Vectors .......................44
                5.6.2.2. Wide Metric Conversion Constants ..........45
                5.6.2.3. Throughput Calculation ....................45
                5.6.2.4. Latency Calculation .......................46
                5.6.2.5. Composite Calculation .....................46
 6. EIGRP Packet Formats ...........................................46
    6.1. Protocol Number ...........................................46
    6.2. Protocol Assignment Encoding ..............................47
    6.3. Destination Assignment Encoding ...........................47
    6.4. EIGRP Communities Attribute ...............................48
    6.5. EIGRP Packet Header .......................................49
    6.6. EIGRP TLV Encoding Format .................................51
         6.6.1. Type Field Encoding ................................52
         6.6.2. Length Field Encoding ..............................52
         6.6.3. Value Field Encoding ...............................52
    6.7. EIGRP Generic TLV Definitions .............................52
         6.7.1. 0x0001 - PARAMETER_TYPE ............................53
         6.7.2. 0x0002 - AUTHENTICATION_TYPE .......................53
                6.7.2.1. 0x02 - MD5 Authentication Type ............54
                6.7.2.2. 0x03 - SHA2 Authentication Type ...........54
         6.7.3. 0x0003 - SEQUENCE_TYPE .............................54
         6.7.4. 0x0004 - SOFTWARE_VERSION_TYPE .....................55
         6.7.5. 0x0005 - MULTICAST_SEQUENCE_TYPE ...................55
         6.7.6. 0x0006 - PEER_INFORMATION_TYPE .....................55
         6.7.7. 0x0007 - PEER_ TERMINATION_TYPE ....................56
         6.7.8. 0x0008 - TID_LIST_TYPE .............................56
    6.8. Classic Route Information TLV Types .......................57
         6.8.1. Classic Flag Field Encoding ........................57

Savage, et al. Informational [Page 3] RFC 7868 Cisco's EIGRP May 2016

         6.8.2. Classic Metric Encoding ............................57
         6.8.3. Classic Exterior Encoding ..........................58
         6.8.4. Classic Destination Encoding .......................59
         6.8.5. IPv4-Specific TLVs .................................59
                6.8.5.1. IPv4 INTERNAL_TYPE ........................60
                6.8.5.2. IPv4 EXTERNAL_TYPE ........................60
                6.8.5.3. IPv4 COMMUNITY_TYPE .......................62
         6.8.6. IPv6-Specific TLVs .................................62
                6.8.6.1. IPv6 INTERNAL_TYPE ........................63
                6.8.6.2. IPv6 EXTERNAL_TYPE ........................63
                6.8.6.3. IPv6 COMMUNITY_TYPE .......................65
    6.9. Multiprotocol Route Information TLV Types .................66
         6.9.1. TLV Header Encoding ................................66
         6.9.2. Wide Metric Encoding ...............................67
         6.9.3. Extended Metrics ...................................68
                6.9.3.1. 0x00 - NoOp ...............................69
                6.9.3.2. 0x01 - Scaled Metric ......................70
                6.9.3.3. 0x02 - Administrator Tag ..................70
                6.9.3.4. 0x03 - Community List .....................71
                6.9.3.5. 0x04 - Jitter .............................71
                6.9.3.6. 0x05 - Quiescent Energy ...................71
                6.9.3.7. 0x06 - Energy .............................72
                6.9.3.8. 0x07 - AddPath ............................72
                         6.9.3.8.1. AddPath with IPv4 Next Hop .....73
                         6.9.3.8.2. AddPath with IPv6 Next Hop .....74
         6.9.4. Exterior Encoding ..................................75
         6.9.5. Destination Encoding ...............................76
         6.9.6. Route Information ..................................76
                6.9.6.1. INTERNAL TYPE .............................76
                6.9.6.2. EXTERNAL TYPE .............................76
 7. Security Considerations ........................................77
 8. IANA Considerations ............................................77
 9. References .....................................................77
    9.1. Normative References ......................................77
    9.2. Informative References ....................................78
 Acknowledgments ...................................................79
 Authors' Addresses ................................................80

Savage, et al. Informational [Page 4] RFC 7868 Cisco's EIGRP May 2016

1. Introduction

 This document describes the Enhanced Interior Gateway Routing
 Protocol (EIGRP), a routing protocol designed and developed by Cisco
 Systems, Inc.  DUAL, the algorithm used to converge the control plane
 to a single set of loop-free paths is based on research conducted at
 SRI International [3].  The Diffusing Update Algorithm (DUAL) is the
 algorithm used to obtain loop freedom at every instant throughout a
 route computation [2].  This allows all routers involved in a
 topology change to synchronize at the same time; the routers not
 affected by topology changes are not involved in the recalculation.
 This document describes the protocol that implements these functions.

2. Conventions

2.1. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [1].

2.2. Terminology

 The following is a list of abbreviations and terms used throughout
 this document:
 ACTIVE State:
    The local state of a route on a router triggered by any event that
    causes all neighbors providing the current least-cost path to fail
    the Feasibility Condition check.  A route in Active state is
    considered unusable.  During Active state, the router is actively
    attempting to compute the least-cost loop-free path by explicit
    coordination with its neighbors using Query and Reply messages.
 Address Family Identifier (AFI):
    Identity of the network-layer protocol reachability information
    being advertised [12].
 Autonomous System (AS):
    A collection of routers exchanging routes under the control of one
    or more network administrators on behalf of a single
    administrative entity.

Savage, et al. Informational [Page 5] RFC 7868 Cisco's EIGRP May 2016

 Base Topology:
    A routing domain representing a physical (non-virtual) view of the
    network topology consisting of attached devices and network
    segments EIGRP uses to form neighbor relationships.  Destinations
    exchanged within the Base Topology are identified with a Topology
    Identifier value of zero (0).
 Computed Distance (CD):
    Total distance (metric) along a path from the current router to a
    destination network through a particular neighbor computed using
    that neighbor's Reported Distance (RD) and the cost of the link
    between the two routers.  Exactly one CD is computed and
    maintained per the [Destination, Advertising Neighbor] pair.
 CR-Mode
    Conditionally Received Mode
 Diffusing Computation:
    A distributed computation in which a single starting node
    commences the computation by delegating subtasks of the
    computation to its neighbors that may, in turn, recursively
    delegate sub-subtasks further, including a signaling scheme
    allowing the starting node to detect that the computation has
    finished while avoiding false terminations.  In DUAL, the task of
    coordinated updates of routing tables and resulting best path
    computation is performed as a diffusing computation.
 Diffusing Update Algorithm (DUAL):
    A loop-free routing algorithm used with distance vectors or link
    states that provides a diffused computation of a routing table.
    It works very well in the presence of multiple topology changes
    with low overhead.  The technology was researched and developed at
    SRI International [3].
 Downstream Router:
    A router that is one or more hops away from the router in question
    in the direction of the destination.
 EIGRP:
    Enhanced Interior Gateway Routing Protocol.
 Feasibility Condition:
    The Feasibility Condition is a sufficient condition used by a
    router to verify whether a neighboring router provides a loop-free
    path to a destination.  EIGRP uses the Source Node Condition
    stating that a neighboring router meets the Feasibility Condition
    if the neighbor's RD is less than this router's Feasible Distance.

Savage, et al. Informational [Page 6] RFC 7868 Cisco's EIGRP May 2016

 Feasible Distance (FD):
    Defined as the least-known total metric to a destination from the
    current router since the last transition from ACTIVE to PASSIVE
    state.  Being effectively a record of the smallest known metric
    since the last time the network entered the PASSIVE state, the FD
    is not necessarily a metric of the current best path.  Exactly one
    FD is computed per destination network.
 Feasible Successor:
    A neighboring router that meets the Feasibility Condition for a
    particular destination, hence, providing a guaranteed loop-free
    path.
 Neighbor/Peer:
    For a particular router, another router toward which an EIGRP
    session, also known as an "adjacency", is established.  The
    ability of two routers to become neighbors depends on their mutual
    connectivity and compatibility of selected EIGRP configuration
    parameters.  Two neighbors with interfaces connected to a common
    subnet are known as adjacent neighbors.  Two neighbors that are
    multiple hops apart are known as remote neighbors.
 PASSIVE state:
    The local state of a route in which at least one neighbor
    providing the current least-cost path passes the Feasibility
    Condition check.  A route in PASSIVE state is considered usable
    and not in need of a coordinated re-computation.
 Network Layer Reachability Information (NLRI):
    Information a router uses to calculate the global routing table to
    make routing and forwarding decisions.
 Reported Distance (RD):
    For a particular destination, the value representing the router's
    distance to the destination as advertised in all messages carrying
    routing information.  RD is not equivalent to the current distance
    of the router to the destination and may be different from it
    during the process of path re-computation.  Exactly one RD is
    computed and maintained per destination network.
 Sub-Topology:
    For a given Base Topology, a sub-topology is characterized by an
    independent set of routers and links in a network for which EIGRP
    performs an independent path calculation.  This allows each sub-
    topology to implement class-specific topologies to carry class-
    specific traffic.

Savage, et al. Informational [Page 7] RFC 7868 Cisco's EIGRP May 2016

 Successor:
    For a particular destination, a neighboring router that meets the
    Feasibility Condition and, at the same time, provides the least-
    cost path.
 Stuck In Active (SIA):
    A destination that has remained in the ACTIVE State in excess of a
    predefined time period at the local router (Cisco implements this
    as 3 minutes).
 Successor-Directed Acyclic Graph (SDAG):
    For a particular destination, a graph defined by routing table
    contents of individual routers in the topology, such that nodes of
    this graph are the routers themselves and a directed edge from
    router X to router Y exists if and only if router Y is router X's
    successor.  After the network has converged, in the absence of
    topological changes, SDAG is a tree.
 Topology Change / Topology-Change Event:
    Any event that causes the CD for a destination through a neighbor
    to be added, modified, or removed.  As an example, detecting a
    link-cost change, receiving any EIGRP message from a neighbor
    advertising an updated neighbor's RD.
 Topology Identifier (TID):
    A number that is used to mark prefixes as belonging to a specific
    sub-topology.
 Topology Table:
    A data structure used by EIGRP to store information about every
    known destination including, but not limited to, network prefix /
    prefix length, FD, RD of each neighbor advertising the
    destination, CD over the corresponding neighbor, and route state.
 Type, Length, Value (TLV):
    An encoding format for information elements used in EIGRP messages
    to exchange information.  Each TLV-formatted information element
    consists of three generic fields: Type identifying the nature of
    information carried in this element, Length describing the length
    of the entire TLV triplet, and Value carrying the actual
    information.  The Value field may, itself, be internally
    structured; this depends on the actual type of the information
    element.  This format allows for extensibility and backward
    compatibility.
 Upstream Router:
    A router that is one or more hops away from the router in
    question, in the direction of the source of the information.

Savage, et al. Informational [Page 8] RFC 7868 Cisco's EIGRP May 2016

 VID:
    VLAN Identifier
 Virtual Routing and Forwarding (VRF):
    Independent Virtual Private Network (VPN) routing/forwarding
    tables that coexist within the same router at the same time.

3. The Diffusing Update Algorithm (DUAL)

 The Diffusing Update Algorithm (DUAL) constructs least-cost paths to
 all reachable destinations in a network consisting of nodes and edges
 (routers and links).  DUAL guarantees that each constructed path is
 loop free at every instant including periods of topology changes and
 network reconvergence.  This is accomplished by all routers, which
 are affected by a topology change, computing the new best path in a
 coordinated (diffusing) way and using the Feasibility Condition to
 verify prospective paths for loop freedom.  Routers that are not
 affected by topology changes are not involved in the recalculation.
 The convergence time with DUAL rivals that of any other existing
 routing protocol.

3.1. Algorithm Description

 DUAL is used by EIGRP to achieve fast loop-free convergence with
 little overhead, allowing EIGRP to provide convergence rates
 comparable, and in some cases better than, most common link state
 protocols [10].  Only nodes that are affected by a topology change
 need to propagate and act on information about the topology change,
 allowing EIGRP to have good scaling properties, reduced overhead, and
 lower complexity than many other interior gateway protocols.
 Distributed routing algorithms are required to propagate information
 as well as coordinate information among all nodes in the network.
 Unlike basic Bellman-Ford distance vector protocols that rely on
 uncoordinated updates when a topology change occurs, DUAL uses a
 coordinated procedure to involve the affected part of the network
 into computing a new least-cost path, known as a "diffusing
 computation".  A diffusing computation grows by querying additional
 routers for their current RD to the affected destination, and it
 shrinks by receiving replies from them.  Unaffected routers send
 replies immediately, terminating the growth of the diffusing
 computation over them.  These intrinsic properties cause the
 diffusing computation to self-adjust in scope and terminate as soon
 as possible.
 One attribute of DUAL is its ability to control the point at which
 the diffusion of a route calculation terminates by managing the
 distribution of reachability information through the network.

Savage, et al. Informational [Page 9] RFC 7868 Cisco's EIGRP May 2016

 Controlling the scope of the diffusing process is accomplished by
 hiding reachability information through aggregation (summarization),
 filtering, or other means.  This provides the ability to create
 effective failure domains within a single AS, and allows the network
 administrator to manage the convergence and processing
 characteristics of the network.

3.2. Route States

 A route to a destination can be in one of two states: PASSIVE or
 ACTIVE.  These states describe whether the route is guaranteed to be
 both loop free and the shortest available (the PASSIVE state) or
 whether such a guarantee cannot be given (the ACTIVE state).
 Consequently, in PASSIVE state, the router does not perform any route
 recalculation in coordination with its neighbors because no such
 recalculation is needed.
 In ACTIVE state, the router is actively involved in re-computing the
 least-cost loop-free path in coordination with its neighbors.  The
 state is reevaluated and possibly changed every time a topology
 change is detected.  A topology change is any event that causes the
 CD to the destination over any neighbor to be added, changed, or
 removed from EIGRP's topology table.
 More exactly, the two states are defined as follows:
 o Passive
    A route is considered to be in the Passive state when at least one
    neighbor that provides the current least-total-cost path passes
    the Feasibility Condition check that guarantees loop freedom.  A
    route in the PASSIVE state is usable and its next hop is perceived
    to be a downstream router.
 o Active
    A route is considered to be in the ACTIVE state if neighbors that
    do not pass the Feasibility Condition check provide lowest-cost
    path, and therefore the path cannot be guaranteed loop free.  A
    route in the ACTIVE state is considered unusable and this router
    must coordinate with its neighbors in the search for the new loop-
    free least-total-cost path.
 In other words, for a route to be in PASSIVE state, at least one
 neighbor that provides the least-total-cost path must be a Feasible
 Successor.  Feasible Successors providing the least-total-cost path
 are also called "successors".  For a route to be in PASSIVE state, at
 least one successor must exist.

Savage, et al. Informational [Page 10] RFC 7868 Cisco's EIGRP May 2016

 Conversely, if the path with the least total cost is provided by
 routers that are not Feasible Successors (and thus not successors),
 the route is in the ACTIVE state, requiring re-computation.
 Notably, for the definition of PASSIVE and ACTIVE states, it does not
 matter if there are Feasible Successors providing a worse-than-least-
 total-cost path.  While these neighbors are guaranteed to provide a
 loop-free path, that path is potentially not the shortest available.
 The fact that the least-total-cost path can be provided by a neighbor
 that fails the Feasibility Condition check may not be intuitive.
 However, such a situation can occur during topology changes when the
 current least-total-cost path fails and the next-least-total-cost
 path traverses a neighbor that is not a Feasible Successor.
 While a router has a route in the ACTIVE state, it must not change
 its successor (i.e., modify the current SDAG) nor modify its own
 Feasible Distance or RD until the route enters the PASSIVE state
 again.  Any updated information about this route received during
 ACTIVE state is reflected only in CDs.  Any updates to the successor,
 FD, and RD are postponed until the route returns to PASSIVE state.
 The state transitions from PASSIVE to ACTIVE and from ACTIVE to
 PASSIVE are controlled by the DUAL FSM and are described in detail in
 Section 3.5.

3.3. Feasibility Condition

 The Feasibility Condition is a criterion used to verify loop freedom
 of a particular path.  The Feasibility Condition is a sufficient but
 not a necessary condition, meaning that every path meeting the
 Feasibility Condition is guaranteed to be loop free; however, not all
 loop-free paths meet the Feasibility Condition.
 The Feasibility Condition is used as an integral part of DUAL
 operation: every path selection in DUAL is subject to the Feasibility
 Condition check.  Based on the result of the Feasibility Condition
 check after a topology change is detected, the route may either
 remain PASSIVE (if, after the topology change, the neighbor providing
 the least cost path meets the Feasibility Condition) or it needs to
 enter the ACTIVE state (if the topology change resulted in none of
 the neighbors providing the least cost path to meet the Feasibility
 Condition).
 The Feasibility Condition is a part of DUAL that allows the diffused
 computation to terminate as early as possible.  Nodes that are not
 affected by the topology change are not required to perform a DUAL
 computation and may not be aware a topology change occurred.  This
 can occur in two cases:

Savage, et al. Informational [Page 11] RFC 7868 Cisco's EIGRP May 2016

 First, if informed about a topology change, a router may keep a route
 in PASSIVE state if it is aware of other paths that are downstream
 towards the destination (routes meeting the Feasibility Condition).
 A route that meets the Feasibility Condition is determined to be loop
 free and downstream along the path between the router and the
 destination.
 Second, if informed about a topology change for which it does not
 currently have reachability information, a router is not required to
 enter into the ACTIVE state, nor is it required to participate in the
 DUAL process.
 In order to facilitate describing the Feasibility Condition, a few
 definitions are in order.
 o  A successor for a given route is the next hop used to forward data
    traffic for a destination.  Typically, the successor is chosen
    based on the least-cost path to reach the destination.
 o  A Feasible Successor is a neighbor that meets the Feasibility
    Condition.  A Feasible Successor is regarded as a downstream
    neighbor towards the destination, but it may not be the least-cost
    path but could still be used for forwarding data packets in the
    event equal or unequal cost load sharing was active.  A Feasible
    Successor can become a successor when the current successor
    becomes unreachable.
 o  The Feasibility Condition is met when a neighbor's advertised
    cost, (RD) to a destination is less than the FD for that
    destination, or in other words, the Feasibility Condition is met
    when the neighbor is closer to the destination than the router
    itself has ever been since the destination has entered the PASSIVE
    state for the last time.
 o  The FD is the lowest distance to the destination since the last
    time the route went from ACTIVE to PASSIVE state.  It should be
    noted it is not necessarily the current best distance; rather, it
    is a historical record of the best distance known since the last
    diffusing computation for the destination has finished.  Thus, the
    value of the FD can either be the same as the current best
    distance, or it can be lower.
 A neighbor that advertises a route with a cost that does not meet the
 Feasibility Condition may be upstream and thus cannot be guaranteed
 to be the next hop for a loop-free path.  Routes advertised by
 upstream neighbors are not recorded in the routing table but saved in
 the topology table.

Savage, et al. Informational [Page 12] RFC 7868 Cisco's EIGRP May 2016

3.4. DUAL Message Types

 DUAL operates with three basic message types: QUERY, UPDATE, and
 REPLY.
 o  UPDATE - sent to indicate a change in metric or an addition of a
    destination.
 o  QUERY - sent when the Feasibility Condition fails, which can
    happen for reasons like a destination becoming unreachable or the
    metric increasing to a value greater than its current FD.
 o REPLY - sent in response to a QUERY or SIA-QUERY
 In addition to these three basic types, two additional sub-types have
 been added to EIGRP:
 o  SIA-QUERY - sent when a REPLY has not been received within one-
    half of the SIA interval (90 seconds as implemented by Cisco).
 o  SIA-REPLY - sent in response to an SIA-QUERY indicating the route
    is still in ACTIVE state.  This response does not stratify the
    original QUERY; it is only used to indicate that the sending
    neighbor is still in the ACTIVE state for the given destination.
 When in the PASSIVE state, a received QUERY may be propagated if
 there is no Feasible Successor found.  If a Feasible Successor is
 found, the QUERY is not propagated and a REPLY is sent for the
 destination with a metric equal to the current routing table metric.
 When a QUERY is received from a non-successor in ACTIVE state, a
 REPLY is sent and the QUERY is not propagated.  The REPLY for the
 destination contains a metric equal to the current routing table
 metric.

3.5. DUAL Finite State Machine (FSM)

 The DUAL FSM embodies the decision process for all route
 computations.  It tracks all routes advertised by all neighbors.  The
 distance information, known as a metric, is used by DUAL to select
 efficient loop-free paths.  DUAL selects routes to be inserted into a
 routing table based on Feasible Successors.  A successor is a
 neighboring router used for packet forwarding that has a least-cost
 path to a destination that is guaranteed not to be part of a routing
 loop.
 When there are no Feasible Successors but there are neighbors
 advertising the destination, a recalculation must occur to determine
 a new successor.

Savage, et al. Informational [Page 13] RFC 7868 Cisco's EIGRP May 2016

 The amount of time it takes to calculate the route impacts the
 convergence time.  Even though the recalculation is not processor
 intensive, it is advantageous to avoid recalculation if it is not
 necessary.  When a topology change occurs, DUAL will test for
 Feasible Successors.  If there are Feasible Successors, it will use
 any it finds in order to avoid any unnecessary recalculation.
 The FSM, which applies per destination in the topology table,
 operates independently for each destination.  It is true that if a
 single link goes down, multiple routes may go into ACTIVE state.
 However, a separate SDAG is computed for each destination, so loop-
 free topologies can be maintained for each reachable destination.

Savage, et al. Informational [Page 14] RFC 7868 Cisco's EIGRP May 2016

            +------------+                +-----------+
            |             \              /            |
            |              \            /             |
            |   +=================================+   |
            |   |                                 |   |
            |(1)|             Passive             |(2)|
            +-->|                                 |<--+
                +=================================+
                    ^     |    ^    ^    ^    |
                (14)|     |(15)|    |(13)|    |
                    |  (4)|    |(16)|    | (3)|
                    |     |    |    |    |    +------------+
                    |     |    |    |    |                  \
           +-------+      +    +    |    +-------------+     \
          /              /    /     |                   \     \
         /              /    /      +----+               \     \
        |               |   |            |                |     |
        |               v   |            |                |     v
    +==========+(11) +==========+     +==========+(12) +==========+
    |  Active  |---->|  Active  |(5)  |  Active  |---->|  Active  |
    |          |  (9)|          |---->|          | (10)|          |
    |  oij=0   |<----|  oij=1   |     |  oij=2   |<----|  oij=3   |
 +--|          |  +--|          |  +--|          |  +--|          |
 |  +==========+  |  +==========+  |  +==========+  |  +==========+
 |      ^   |(5)  |      ^         |    ^    ^      |         ^
 |      |   +-----|------|---------|----+    |      |         |
 +------+         +------+         +---------+      +---------+
 (6,7,8)          (6,7,8)            (6,7,8)          (6,7,8)
                    Figure 1: DUAL Finite State Machine
 Legend:
  i   Node that is computing route
  j   Destination node or network
  k   Any neighbor of node i
  oij QUERY origin flag
    0 = metric increase during ACTIVE state
    1 = node i originated
    2 = QUERY from, or link increase to, successor during ACTIVE state
    3 = QUERY originated from successor
  rijk REPLY status flag for each neighbor k for destination j
    1 = awaiting REPLY
    0 = received REPLY
  lik = the link connecting node i to neighbor k

Savage, et al. Informational [Page 15] RFC 7868 Cisco's EIGRP May 2016

 The following describes in detail the state/event/action transitions
 of the DUAL FSM.  For all steps, the topology table is updated with
 the new metric information from either QUERY, REPLY, or UPDATE
 received.
 (1)  A QUERY is received from a neighbor that is not the current
      successor.  The route is currently in PASSIVE state.  As the
      successor is not affected by the QUERY, and a Feasible Successor
      exists, the route remains in PASSIVE state.  Since a Feasible
      Successor exists, a REPLY MUST be sent back to the originator of
      the QUERY.  Any metric received in the QUERY from that neighbor
      is recorded in the topology table and the Feasibility Check (FC)
      is run to check for any change to current successor.
 (2)  A directly connected interface changes state (connects,
      disconnects, or changes metric), or similarly an UPDATE or QUERY
      has been received with a metric change for an existing
      destination, the route will stay in the PASSIVE state if the
      current successor is not affected by the change, or it is no
      longer reachable and there is a Feasible Successor.  In either
      case, an UPDATE is sent with the new metric information if it
      has changed.
 (3)  A QUERY was received from a neighbor who is the current
      successor and no Feasible Successors exist.  The route for the
      destination goes into ACTIVE state.  A QUERY is sent to all
      neighbors on all interfaces that are not split horizon.  Split
      horizon takes effect for a query or update from the successor it
      is using for the destination in the query.  The QUERY origin
      flag is set to indicate the QUERY originated from a neighbor
      marked as successor for route.  The REPLY status flag is set for
      all neighbors to indicate outstanding replies.
 (4)  A directly connected link has gone down or its cost has
      increased, or an UPDATE has been received with a metric
      increase.  The route to the destination goes to ACTIVE state if
      there are no Feasible Successors found.  A QUERY is sent to all
      neighbors on all interfaces.  The QUERY origin flag is to
      indicate that the router originated the QUERY.  The REPLY status
      flag is set to 1 for all neighbors to indicate outstanding
      replies.

Savage, et al. Informational [Page 16] RFC 7868 Cisco's EIGRP May 2016

 (5)  While a route for a destination is in ACTIVE state, and a QUERY
      is received from the current successor, the route remains in
      ACTIVE state.  The QUERY origin flag is set to indicate that
      there was another topology change while in ACTIVE state.  This
      indication is used so new Feasible Successors are compared to
      the metric that made the route go to ACTIVE state with the
      current successor.
 (6)  While a route for a destination is in ACTIVE state and a QUERY
      is received from a neighbor that is not the current successor, a
      REPLY should be sent to the neighbor.  The metric received in
      the QUERY should be recorded.
 (7)  If a link cost changes, or an UPDATE with a metric change is
      received in ACTIVE state from a non-successor, the router stays
      in ACTIVE state for the destination.  The metric information in
      the UPDATE is recorded.  When a route is in the ACTIVE state,
      neither a QUERY nor UPDATE are ever sent.
 (8)  If a REPLY for a destination, in ACTIVE state, is received from
      a neighbor or the link between a router and the neighbor fails,
      the router records that the neighbor replied to the QUERY.  The
      REPLY status flag is set to 0 to indicate this.  The route stays
      in ACTIVE state if there are more replies pending because the
      router has not heard from all neighbors.
 (9)  If a route for a destination is in ACTIVE state, and a link
      fails or a cost increase occurred between a router and its
      successor, the router treats this case like it has received a
      REPLY from its successor.  When this occurs after the router
      originates a QUERY, it sets the QUERY origin flag to indicate
      that another topology change occurred in ACTIVE state.
 (10) If a route for a destination is in ACTIVE state, and a link
      fails or a cost increase occurred between a router and its
      successor, the router treats this case like it has received a
      REPLY from its successor.  When this occurs after a successor
      originated a QUERY, the router sets the QUERY origin flag to
      indicate that another topology change occurred in ACTIVE state.
 (11) If a route for a destination is in ACTIVE state, the cost of the
      link through which the successor increases, and the last REPLY
      was received from all neighbors, but there is no Feasible
      Successor, the route should stay in ACTIVE state.  A QUERY is
      sent to all neighbors.  The QUERY origin flag is set to 1.

Savage, et al. Informational [Page 17] RFC 7868 Cisco's EIGRP May 2016

 (12) If a route for a destination is in ACTIVE state because of a
      QUERY received from the current successor, and the last REPLY
      was received from all neighbors, but there is no Feasible
      Successor, the route should stay in ACTIVE state.  A QUERY is
      sent to all neighbors.  The QUERY origin flag is set to 3.
 (13) Received replies from all neighbors.  Since the QUERY origin
      flag indicates the successor originated the QUERY, it
      transitions to PASSIVE state and sends a REPLY to the old
      successor.
 (14) Received replies from all neighbors.  Since the QUERY origin
      flag indicates a topology change to the successor while in
      ACTIVE state, it need not send a REPLY to the old successor.
      When the Feasibility Condition is met, the route state
      transitions to PASSIVE.
 (15) Received replies from all neighbors.  Since the QUERY origin
      flag indicates either the router itself originated the QUERY or
      FC was not satisfied with the replies received in ACTIVE state,
      FD is reset to infinite value and the minimum of all the
      reported metrics is chosen as FD and route transitions back to
      PASSIVE state.  A REPLY is sent to the old-successor if oij
      flags indicate that there was a QUERY from successor.
 (16) If a route for a destination is in ACTIVE state because of a
      QUERY received from the current successor or there was an
      increase in distance while in ACTIVE state, the last REPLY was
      received from all neighbors, and a Feasible Successor exists for
      the destination, the route can go into PASSIVE state and a REPLY
      is sent to the successor if oij indicates that QUERY was
      received from the successor.

3.6. DUAL Operation – Example Topology

 The following topology (Figure 2) will be used to provide an example
 of how DUAL is used to reroute after a link failure.  Each node is
 labeled with its costs to destination N.  The arrows indicate the
 successor (next hop) used to reach destination N.  The least-cost
 path is selected.

Savage, et al. Informational [Page 18] RFC 7868 Cisco's EIGRP May 2016

                              N
                              |
                           (1)A ---<--- B(2)
                              |         |
                              ^         |
                              |         |
                           (2)D ---<--- C(3)
                      Figure 2: Stable Topology
 In the case where the link between A and D fails (Figure 3);
        N                                   N
        |                                   |
        A ---<--- B                         A ---<--- B
        |         |                         |          |
        X         |                         ^          |
        |         |                         |          |
        D ---<--- C                         D ---<--- C
          Q->                                      <-R
                           N
                           |
                        (1)A ---<--- B(2)
                                     |
                                     ^
                                     |
                        (4)D --->--- C(3)
                Figure 3: Link between A and D Fails
    Only observing the destination provided by node N, D enters the
 ACTIVE state and sends a QUERY to all its neighbors, in this case
 node C.
    C determines that it has a Feasible Successor and replies
 immediately with metric 3.
    C changes its old successor of D to its new single successor B
 and the route to N stays in PASSIVE state.
    D receives the REPLY and can transition out of ACTIVE state
 since it received replies from all its neighbors.
    D now has a viable path to N through C.
    D selects C as its successor to reach node N with a cost of 4.
 Notice that nodes A and B were not involved in the recalculation
 since they were not affected by the change.

Savage, et al. Informational [Page 19] RFC 7868 Cisco's EIGRP May 2016

 Let's consider the situation in Figure 4, where Feasible Successors
 may not exist.  If the link between node A and B fails, B goes into
 ACTIVE state for destination N since it has no Feasible Successors.
 Node B sends a QUERY to node C.  C has no Feasible Successors, so it
 goes active for destination N; and since C has no neighbors, it
 replies to the QUERY, deletes the destination, and returns to the
 PASSIVE state for the unreachable route.  As C removes the (now
 unreachable) destination from its table, C sends REPLY to its old
 successor.  B receives this REPLY from C, and determines this is the
 last REPLY it is waiting on before determining what the new state of
 the route should be; on receiving this REPLY, B deletes the route to
 N from its routing table.
 Since B was the originator of the initial QUERY, it does not have to
 send a REPLY to its old successor (it would not be able to any ways,
 because the link to its old successor is down).  Note that nodes A
 and D were not involved in the recalculation since their successors
 were not affected.
        N                                N
        |                                |
     (1)A ---<--- B(2)                   A ------- B   Q
        |         |                      |         |   |^      ^
        ^         ^                      ^         |   v|      |
        |         |                      |         |      |    |
     (2)D         C(3)                   D         C     ACK   R
      Figure 4: No Feasible Successors When Link between A and B Fails

4. EIGRP Packets

 EIGRP uses five different packet types to handle session management
 and pass DUAL Message types:
     HELLO Packets (includes ACK)
     QUERY Packets (includes SIA-Query)
     REPLY Packets (includes SIA-Reply)
     REQUEST Packets
     UPDATE Packets
 EIGRP packets are directly encapsulated into a network-layer
 protocol, such as IPv4 or IPv6.  While EIGRP is capable of using
 additional encapsulation (such as AppleTalk, IPX, etc.) no further
 encapsulation is specified in this document.

Savage, et al. Informational [Page 20] RFC 7868 Cisco's EIGRP May 2016

 Support for network-layer protocol fragmentation is not supported,
 and EIGRP will attempt to avoid a maximum size packets that exceed
 the interface MTU by sending multiple packets that are less than or
 equal to MTU-sized packets.
 Each packet transmitted will use either multicast or unicast network-
 layer destination addresses.  When multicast addresses are used, a
 mapping for the data link multicast address (when available) must be
 provided.  The source address will be set to the address of the
 sending interface, if applicable.
 The following network-layer multicast addresses and associated data
 link multicast addresses:
    224.0.0.10 for IPv4 "EIGRP Routers" [13]
    FF02:0:0:0:0:0:0:A for IPv6 "EIGRP Routers" [14]
 They will be used on multicast-capable media and will be media
 independent for unicast addresses.  Network-layer addresses will be
 used and the mapping to media addresses will be achieved by the
 native protocol mechanisms.

4.1. UPDATE Packets

 UPDATE packets carry the DUAL UPDATE message type and are used to
 convey information about destinations and the reachability of those
 destinations.  When a new neighbor is discovered, unicast UPDATE
 packets are used to transmit a full table to the new neighbor, so the
 neighbor can build up its topology table.  In normal operation (other
 than neighbor startup such as a link cost changes), UPDATE packets
 are multicast.  UPDATE packets are always transmitted reliably.  Each
 TLV destination will be processed individually through the DUAL FSM.

4.2. QUERY Packets

 A QUERY packet carries the DUAL QUERY message type and is sent by a
 router to advertise that a route is in ACTIVE state and the
 originator is requesting alternate path information from its
 neighbors.  An infinite metric is encoded by setting the delay part
 of the metric to its maximum value.
 If there is a topology change that causes multiple destinations to be
 marked ACTIVE, EIGRP will build one or more QUERY packets for all
 destinations present.  The state of each route is recorded
 individually, so a responding QUERY or REPLY need not contain all the
 same destinations in a single packet.  Since EIGRP uses a reliable
 transport mechanism, route QUERY packets are also guaranteed be
 reliably delivered.

Savage, et al. Informational [Page 21] RFC 7868 Cisco's EIGRP May 2016

 When a QUERY packet is received, each destination will trigger a DUAL
 event, and the state machine will run individually for each route.
 Once the entire original QUERY packet is processed, then a REPLY or
 SIA-REPLY will be sent with the latest information.

4.3. REPLY Packets

 A REPLY packet carries the DUAL REPLY message type and will be sent
 in response to a QUERY or SIA-QUERY packet.  The REPLY packet will
 include a TLV for each destination and the associated vector metric
 in its own topology table.
 The REPLY packet is sent after the entire received QUERY packet is
 processed.  When a REPLY packet is received, there is no reason to
 process the packet before an acknowledgment is sent.  Therefore, an
 acknowledgment is sent immediately and then the packet is processed.
 The sending of the acknowledgment is accomplished either by sending
 an ACK packet or by piggybacking the acknowledgment onto another
 packet already being transmitted.
 Each TLV destination will be processed individually through the DUAL
 FSM.  When a QUERY is received for a route that doesn't exist in our
 topology table, a REPLY with an infinite metric is sent and an entry
 in the topology table is added with the metric in the QUERY if the
 metric is not an infinite value.
 If a REPLY for a designation not in the Active state, or not in the
 topology table, EIGRP will acknowledge the packet and discard the
 REPLY.

4.4. Exception Handling

4.4.1. Active Duration (SIA)

 When an EIGRP router transitions to ACTIVE state for a particular
 destination, a QUERY is sent to a neighbor and the ACTIVE timer is
 started to limit the amount of time a destination may remain in an
 ACTIVE state.
 A route is regarded as SIA when it does not receive a REPLY within a
 preset time.  This time interval is broken into two equal periods
 following the QUERY, and up to three additional "busy" periods in
 which an SIA-QUERY packet is sent for the destination.
 This process is begun when a router sends a QUERY to its neighbor.
 After one-half the SIA time interval (default implementation is 90
 seconds), the router will send an SIA-QUERY; this must be replied to
 with either a REPLY or SIA-REPLY.  Any neighbor that fails to send

Savage, et al. Informational [Page 22] RFC 7868 Cisco's EIGRP May 2016

 either a REPLY or SIA-REPLY with-in one-half the SIA interval will
 result in the neighbor being deemed to be "stuck" in the active
 state.
 Cisco also limits the number of SIA-REPLY messages allowed to three.
 Once the timeout occurs after the third SIA-REPLY with the neighbor
 remaining in an ACTIVE state (as noted in the SIA-Reply message), the
 neighbor being deemed to be "stuck" in the active state.
 If the SIA state is declared, DUAL may take one of two actions;
    a) Delete the route from that neighbor, acting as if the neighbor
       had responded with an unreachable REPLY message from the
       neighbor.
    b) Delete all routes from that neighbor and reset the adjacency
       with that neighbor, acting as if the neighbor had responded
       with an unreachable message for all routes.
 Implementation note: Cisco currently implements option (b).

4.4.1.1. SIA-QUERY

 When a QUERY is still outstanding and awaiting a REPLY from a
 neighbor, there is insufficient information to determine why a REPLY
 has not been received.  A lost packet, congestion on the link, or a
 slow neighbor could cause a lack of REPLY from a downstream neighbor.
 In order to try to ascertain if the neighboring device is still
 attempting to converge on the active route, EIGRP may send an SIA-
 QUERY packet to the active neighbor(s).  This enables an EIGRP router
 to determine if there is a communication issue with the neighbor or
 if it is simply still attempting to converge with downstream routers.
 By sending an SIA-QUERY, the originating router may extend the
 effective active time by resetting the ACTIVE timer that has been
 previously set, thus allowing convergence to continue so long as
 neighbor devices successfully communicate that convergence is still
 underway.
 The SIA-QUERY packet SHOULD be sent on a per-destination basis at
 one-half of the ACTIVE timeout period.  Up to three SIA-QUERY packets
 for a specific destination may be sent, each at a value of one-half
 the ACTIVE time, so long as each are successfully acknowledged and
 met with an SIA-REPLY.

Savage, et al. Informational [Page 23] RFC 7868 Cisco's EIGRP May 2016

 Upon receipt of an SIA-QUERY packet, an EIGRP router should first
 send an ACK and then continue to process the SIA-QUERY information.
 The QUERY is sent on a per-destination basis at approximately one-
 half the active time.
 If the EIGRP router is still active for the destination specified in
 the SIA-QUERY, the router should respond to the originator with the
 SIA-REPLY indicating that active processing for this destination is
 still underway by setting the ACTIVE flag in the packet upon
 response.
 If the router receives an SIA-QUERY referencing a destination for
 which it has not received the original QUERY, the router should treat
 the packet as though it was a standard QUERY:
    1) Acknowledge the receipt of the packet
    2) Send a REPLY if a successor exists
    3) If the SIA-QUERY is from the successor, transition to the
       ACTIVE state if and only if a Feasibility Condition check fails
       and send an SIA-REPLY with the ACTIVE bit set

4.4.1.2. SIA-REPLY

 An SIA-REPLY packet is the corresponding response upon receipt of an
 SIA-QUERY from an EIGRP neighbor.  The SIA-REPLY packet will include
 a TLV for each destination and the associated vector metric in the
 topology table.  The SIA-REPLY packet is sent after the entire
 received SIA-QUERY packet is processed.
 If the EIGRP router is still ACTIVE for a destination, the SIA-REPLY
 packet will be sent with the ACTIVE bit set.  This confirms for the
 neighbor device that the SIA-QUERY packet has been processed by DUAL
 and that the router is still attempting to resolve a loop-free path
 (likely awaiting responses to its own QUERY to downstream neighbors).
 The SIA-REPLY informs the recipient that convergence is complete or
 still ongoing; it is an explicit notification that the router is
 still actively engaged in the convergence process.  This allows the
 device that sent the SIA-QUERY to determine whether it should
 continue to allow the routes that are not converged to be in the
 ACTIVE state or if it should reset the neighbor relationship and
 flush all routes through this neighbor.

Savage, et al. Informational [Page 24] RFC 7868 Cisco's EIGRP May 2016

5. EIGRP Operation

 EIGRP has four basic components:
      o Finite State Machine
      o Reliable Transport Protocol
      o Neighbor Discovery/Recovery
      o Route Management

5.1. Finite State Machine

 The detail of DUAL, the State Machine used by EIGRP, is covered in
 Section 3.5.

5.2. Reliable Transport Protocol

 The reliable transport is responsible for guaranteed, ordered
 delivery of EIGRP packets to all neighbors.  It supports intermixed
 transmission of multicast and unicast packets.  Some EIGRP packets
 must be transmitted reliably and others need not.  For efficiency,
 reliability is provided only when necessary.
 For example, on a multi-access network that has multicast
 capabilities, such as Ethernet, it is not necessary to send HELLOs
 reliably to all neighbors individually.  EIGRP sends a single
 multicast HELLO with an indication in the packet informing the
 receivers that the packet need not be acknowledged.  Other types of
 packets, such as UPDATE packets, require acknowledgment and this is
 indicated in the packet.  The reliable transport has a provision to
 send multicast packets quickly when there are unacknowledged packets
 pending.  This helps ensure that convergence time remains low in the
 presence of varying speed links.
 DUAL assumes there is lossless communication between devices and thus
 must depend on the transport protocol to guarantee that messages are
 transmitted reliably.  EIGRP implements the reliable transport
 protocol to ensure ordered delivery and acknowledgment of any
 messages requiring reliable transmission.  State variables such as a
 received sequence number, acknowledgment number, and transmission
 queues MUST be maintained on a per-neighbor basis.

Savage, et al. Informational [Page 25] RFC 7868 Cisco's EIGRP May 2016

 The following sequence number rules must be met for the EIGRP
 reliable transport protocol to work correctly:
    o  A sender of a packet includes its global sequence number in the
       sequence number field of the fixed header.  The sequence number
       wraps around to one when the maximum value is exceeded
       (sequence number zero is reserved for unreliable transmission).
       The sender includes the receivers sequence number in the
       acknowledgment number field of the fixed header.
    o  Any packets that do not require acknowledgment must be sent
       with a sequence number of 0.
    o  Any packet that has an acknowledgment number of 0 indicates
       that sender is not expecting to explicitly acknowledge
       delivery.  Otherwise, it is acknowledging a single packet.
    o  Packets that are network-layer multicast must contain
       acknowledgment number of 0.
 When a router transmits a packet, it increments its sequence number
 and marks the packet as requiring acknowledgment by all neighbors on
 the interface for which the packet is sent.  When individual
 acknowledgments are unicast addressed by the receivers to the sender
 with the acknowledgment number equal to the packets sequence number,
 the sender SHALL clear the pending acknowledgment requirement for the
 packet from the respective neighbor.
 If the required acknowledgment is not received for the packet, it
 MUST be retransmitted.  Retransmissions will occur for a maximum of 5
 seconds.  This retransmission for each packet is tried 16 times,
 after which, if there is no ACK, the neighbor relationship is reset
 with the peer that didn't send the ACK.
 The protocol has no explicit windowing support.  A receiver will
 acknowledge each packet individually and will drop packets that are
 received out of order.
 Implementation note: The exception to this occurs if a duplicate
 packet is received, and the acknowledgment for the original packet
 has been scheduled for transmission, but not yet sent.  In this case,
 EIGRP will not send an acknowledgment for the duplicate packet, and
 the queued acknowledgment will acknowledge both the original and
 duplicate packet.
 Duplicate packets are also discarded upon receipt.  Acknowledgments
 are not accumulative.  Therefore, an ACK with a non-zero sequence
 number acknowledges a single packet.

Savage, et al. Informational [Page 26] RFC 7868 Cisco's EIGRP May 2016

 There are situations when multicast and unicast packets are
 transmitted close together on multi-access broadcast-capable
 networks.  The reliable transport mechanism MUST ensure that all
 multicasts are transmitted in order and not mix the order among
 unicast and multicast packets.  The reliable transport provides a
 mechanism to deliver multicast packets in order to some receivers
 quickly, while some receivers have not yet received all unicast or
 previously sent multicast packets.  The SEQUENCE_TYPE TLV in HELLO
 packets achieves this.  This will be explained in more detail in this
 section.
 Figure 5 illustrates the reliable transport protocol on point-to-
 point links.  There are two scenarios that may occur: an UPDATE-
 initiated packet exchange or a QUERY-initiated packet exchange.
 This example will assume no packet loss.

Router A Router B

              An Example UPDATE Exchange
                               <----------------
                               UPDATE (multicast)

A receives packet SEQ=100, ACK=0

                               Add packet to A's retransmit list

—————→ ACK (unicast) SEQ=0, ACK=100 Receive ACK Process UPDATE Delete packet from A's retransmit list

              An Example QUERY Exchange
                               <----------------
                               QUERY (multicast)

A receives packet SEQ=101, ACK=0 Process QUERY Add packet to A's retransmit list

—————→ REPLY (unicast) SEQ=201, ACK=101 Process ACK

                               Delete packet from A's retransmit

list

                               Process REPLY packet
                               <----------------
                               ACK (unicast)

A receives packet SEQ=0, ACK=201

     Figure 5: Reliable Transfer on Point-to-Point Links

Savage, et al. Informational [Page 27] RFC 7868 Cisco's EIGRP May 2016

 The UPDATE exchange sequence requires UPDATE packets sent to be
 delivered reliably.  The UPDATE packet transmitted contains a
 sequence number that is acknowledged by a receipt of an ACK packet.
 If the UPDATE or the ACK packet is lost on the network, the UPDATE
 packet will be retransmitted.
 This example will assume there is heavy packet loss on a network.

Router A Router B

                               <----------------
                               UPDATE (multicast)

A receives packet SEQ=100, ACK=0

                               Add packet to A's retransmit list

—————→ ACK (unicast) SEQ=0, ACK=100 Receive ACK Process UPDATE Delete packet from A's retransmit list

                               <--/LOST/--------------
                               UPDATE (multicast)
                               SEQ=101, ACK=0
                               Add packet to A's retransmit list
                               Retransmit Timer Expires
                               <----------------
                               Retransmit UPDATE (unicast)
                               SEQ=101, ACK=0
                               Keep packet on A's retransmit list

—————→ ACK (unicast) SEQ=0, ACK=101 Receive ACK Process UPDATE Delete packet from A's retransmit list

        Figure 6: Reliable Transfer on Lossy Point-to-Point Links
 Reliable delivery on multi-access LANs works in a similar fashion to
 point-to-point links.  The initial packet is always multicast and
 subsequent retransmissions are unicast addressed.  The
 acknowledgments sent are always unicast addressed.  Figure 7 shows an
 example with four routers on an Ethernet.
         Router B -----------+
                             |
         Router C -----------+------------ Router A
                             |
         Router D -----------+

Savage, et al. Informational [Page 28] RFC 7868 Cisco's EIGRP May 2016

                      An Example UPDATE Exchange
                                <----------------
                                A send UPDATE (multicast)
                                SEQ=100, ACK=0
                                Add packet to B's retransmit list
                                Add packet to C's retransmit list
                                Add packet to D's retransmit list

—————→ B sends ACK (unicast) SEQ=0, ACK=100 Receive ACK Process UPDATE Delete packet from B's retransmit list

—————→ C sends ACK (unicast) SEQ=0, ACK=100 Receive ACK Process UPDATE Delete packet from C's retransmit list

—————→ D sends ACK (unicast) SEQ=0, ACK=100 Receive ACK Process UPDATE Delete packet from D's retransmit list

                       An Example QUERY Exchange
                                <----------------
                                A sends UPDATE (multicast)
                                SEQ=101, ACK=0
                                Add packet to B's retransmit list
                                Add packet to C's retransmit list
                                Add packet to D's retransmit list

—————→ B sends REPLY (unicast) ←————— SEQ=511, ACK=101 A sends ACK (unicast to B) Process UPDATE SEQ=0, ACK=511

                                Delete packet from B's retransmit list

—————→ C sends REPLY (unicast) ←————— SEQ=200, ACK=101 A sends ACK (unicast to C) Process UPDATE SEQ=0, ACK=200

                                Delete packet from C's retransmit list

—————→ D sends REPLY (unicast) ←————— SEQ=11, ACK=101 A sends ACK (unicast to D) Process UPDATE SEQ=0, ACK=11

                                Delete packet from D's retransmit list
       Figure 7: Reliable Transfer on Multi-Access Links

Savage, et al. Informational [Page 29] RFC 7868 Cisco's EIGRP May 2016

 And finally, a situation where numerous multicast and unicast packets
 are sent close together in a multi-access environment is illustrated
 in Figure 8.
      Router B -----------+
                          |
      Router C -----------+------------ Router A
                          |
      Router D -----------+
                              <----------------
                              A sends UPDATE (multicast)
                              SEQ=100, ACK=0

—————/LOST/→ Add packet to B's retransmit list B sends ACK (unicast) Add packet to C's retransmit list SEQ=0, ACK=100 Add packet to D's retransmit list

—————→ C sends ACK (unicast) SEQ=0, ACK=100 Delete packet from C's retransmit list

—————→ D sends ACK (unicast) SEQ=0, ACK=100 Delete packet from D's retransmit list

                              <----------------
                              A sends HELLO (multicast)
                              SEQ=0, ACK=0, SEQ_TLV listing B

B receives Hello, does not set CR-Mode C receives Hello, sets CR-Mode D receives Hello, sets CR-Mode

                              <----------------
                              A sends UPDATE (multicast)
                              SEQ=101, ACK=0, CR-Flag=1

—————/LOST/→ Add packet to B's retransmit list B sends ACK (unicast) Add packet to C's retransmit list SEQ=0, ACK=100 Add packet to D's retransmit list

B ignores UPDATE 101 because the CR-Flag is set and it is not in CR-Mode

—————→ C sends ACK (unicast) SEQ=0, ACK=101

Savage, et al. Informational [Page 30] RFC 7868 Cisco's EIGRP May 2016

—————→ D sends ACK (unicast)

SEQ=0, ACK=101

                              <----------------
                              A resends UPDATE (unicast to B)
                              SEQ=100, ACK=0

B packet duplicate

—————> B sends ACK (unicast) A removes packet from retransmit list SEQ=0, ACK=100

                              <----------------
                              A resends UPDATE (unicast to B)
                              SEQ=101, ACK=0

—————> B sends ACK (unicast) A removes packet from retransmit list SEQ=0, ACK=101

       Figure 8: Reliable Transfer on Multi-Access Links
                    with Conditional Receive
 Initially, Router A sends a multicast addressed UPDATE packet on the
 LAN.  B and C receive it and send acknowledgments.  Router B receives
 the UPDATE, but the acknowledgment sent is lost on the network.
 Before the retransmission timer for Router B's packet expires, there
 is an event that causes a new multicast addressed UPDATE to be sent.
 Router A detects that there is at least one neighbor on the interface
 with a full queue.  Therefore, it MUST signal that neighbor not to
 receive the next packet or it would receive the retransmitted packet
 out of order.  If all neighbors on the interface have a full queue,
 then EIGRP should reschedule the transmission of the UPDATE once the
 queues are no longer full.
 Router A builds a HELLO packet with a SEQUENCE_TYPE TLV indicating
 all the neighbors that have full queues.  In this case, the only
 neighbor address in the list is Router B.  The HELLO packet is sent
 via multicast unreliably out the interface.
 Routers C and D process the SEQUENCE_TYPE TLV by looking for their
 own addresses in the list.  If not found, they put themselves in CR-
 Mode.
 Router B does not find its address in the SEQUENCE TLV peer list, so
 it enters CR-Mode.  Packets received by Router B with the CR-Flag
 MUST be discarded and not acknowledged.

Savage, et al. Informational [Page 31] RFC 7868 Cisco's EIGRP May 2016

 Later, Router A will unicast transmit both packets 100 and 101
 directly to Router B.  Router B already has 100, so it discards and
 acknowledges it.
 Router B then accepts and acknowledges packet 101.  Once an
 acknowledgment is received, Router A can remove both packets from
 Router B's transmission list.

5.2.1. Bandwidth on Low-Speed Links

 By default, EIGRP limits itself to using no more than 50% of the
 bandwidth reported by an interface when determining packet-pacing
 intervals.  If the bandwidth does not match the physical bandwidth
 (the network architect may have put in an artificially low or high
 bandwidth value to influence routing decisions), EIGRP may:
    1. Generate more traffic than the interface can handle, possibly
       causing drops, thereby impairing EIGRP performance.
    2. Generate a lot of EIGRP traffic that could result in little
       bandwidth remaining for user data.  To control such
       transmissions, an interface-pacing timer is defined for the
       interfaces on which EIGRP is enabled.  When a pacing timer
       expires, a packet is transmitted out on that interface.

5.3. Neighbor Discovery/Recovery

 Neighbor Discovery/Recovery is the process that routers use to
 dynamically learn of other routers on their directly attached
 networks.  Routers MUST also discover when their neighbors become
 unreachable or inoperative.  This process is achieved with low
 overhead by periodically sending small HELLO packets.  As long as any
 packets are received from a neighbor, the router can determine that
 neighbor is alive and functioning.  Only after a neighbor router is
 considered operational can the neighboring routers exchange routing
 information.

5.3.1. Neighbor Hold Time

 Each router keeps state information about adjacent neighbors.  When
 newly discovered neighbors are learned the address, interface, and
 Hold Time of the neighbor is noted.  When a neighbor sends a HELLO,
 it advertises its Hold Time.  The Hold Time is the amount of time a
 router treats a neighbor as reachable and operational.  In addition
 to the HELLO packet, if any packet is received within the Hold Time
 period, then the Hold Time period will be reset.  When the Hold Time
 expires, DUAL is informed of the topology change.

Savage, et al. Informational [Page 32] RFC 7868 Cisco's EIGRP May 2016

5.3.2. HELLO Packets

 When an EIGRP router is initialized, it will start sending HELLO
 packets out any interface on which EIGRP is enabled.  HELLO packets,
 when used for neighbor discovery, are normally sent multicast
 addressed.  The HELLO packet will include the configured EIGRP metric
 K-values.  Two routers become neighbors only if the K-values are the
 same.  This enforces that the metric usage is consistent throughout
 the Internet.  Also included in the HELLO packet is a Hold Time
 value.  This value indicates to all receivers the length of time in
 seconds that the neighbor is valid.  The default Hold Time will be
 three times the HELLO interval.  HELLO packets will be transmitted
 every 5 seconds (by default).  There may be a configuration command
 that controls this value and therefore changes the Hold Time.  HELLO
 packets are not transmitted reliably, so the sequence number should
 be set to 0.

5.3.3. UPDATE Packets

 A router detects a new neighbor by receiving a HELLO packet from a
 neighbor not presently known.  To ensure unicast and multicast packet
 delivery, the detecting neighbor will send a unicast UPDATE packet to
 the new neighbor with no routing information (the NULL UPDATE
 packet).  The initial NULL UPDATE packet sent MUST have the INIT-Flag
 set and contain no topology information.
 Implementation note: The NULL UPDATE packet is used to ensure
 bidirectional UNICAST packet delivery as the NULL UPDATE and the ACK
 are both sent unicast.  Additional UPDATE packets cannot be sent
 until the initial NULL UPDATE packet is acknowledged.
 The INIT-Flag instructs the neighbor to advertise its routes, and it
 is also useful when a neighbor goes down and comes back up before the
 router detects it went down.  In this case, the neighbor needs new
 routing information.  The INIT-Flag informs the router to send it.
 Implementation note: When a router sends an UPDATE with the INIT-Flag
 set, and without the Restart (RS) flag set in the header, the
 receiving neighbor must also send an UPDATE with the INIT-Flag.
 Failure to do so will result in a Cisco device posting a "stuck in
 INIT state" error and subsequent discards.

Savage, et al. Informational [Page 33] RFC 7868 Cisco's EIGRP May 2016

5.3.4. Initialization Sequence

          Router A                           Router B
        (just booted)                    (up and running)
      (1)---------------->
           HELLO (multicast)           <----------------     (2)
           SEQ=0, ACK=0                 HELLO (multicast)
                                        SEQ=0, ACK=0
                                       <----------------     (3)
                                        UPDATE (unicast)
                                        SEQ=10, ACK=0, INIT
      (4)---------------->              UPDATE 11 is queued
           UPDATE (unicast)
           SEQ=100, ACK=10, INIT       <----------------     (5)
                                       UPDATE (unicast)
                                       SEQ=11, ACK=100
                                       All UPDATES sent
      (6)--------------/lost/->
           ACK (unicast)
           SEQ=0, ACK=11
                                       (5 seconds later)
                                       <----------------     (7)
           Duplicate received,         UPDATE (unicast)
           packet discarded            SEQ=11, ACK=100
      (8)--------------->
           ACK (unicast)
           SEQ=0, ACK=11
                  Figure 9: Initialization Sequence
 (1) Router A sends a multicast HELLO and Router B discovers it.
 (2) Router B sends an expedited HELLO and starts the process of
     sending its topology table to Router A.  In addition, Router B
     sends the NULL UPDATE packet with the INIT-Flag.  The second
     packet is queued, but it cannot be sent until the first is
     acknowledged.
 (3) Router A receives the first UPDATE packet and processes it as a
     DUAL event.  If the UPDATE contains topology information, the
     packet will be processed and stored in a topology table.  Router
     B sends its first and only UPDATE packet with an accompanied ACK.

Savage, et al. Informational [Page 34] RFC 7868 Cisco's EIGRP May 2016

 (4) Router B receives UPDATE packet 100 from Router A.  Router B can
     dequeue packet 10 from A's transmission list since the UPDATE
     acknowledged 10.  It can now send UPDATE packet 11 and with an
     acknowledgment of Router A's UPDATE.
 (5) Router A receives the last UPDATE packet from Router B and
     acknowledges it.  The acknowledgment gets lost.
 (6) Router B later retransmits the UPDATE packet to Router A.
 (7) Router A detects the duplicate and simply acknowledges the
     packet.  Router B dequeues packet 11 from A's transmission list,
     and both routers are up and synchronized.

5.3.5. Neighbor Formation

 To prevent packets from being sent to a neighbor prior to verifying
 multicast and unicast packet delivery is reliable, a three-way
 handshake is utilized.
 During normal adjacency formation, multicast HELLOs cause the EIGRP
 process to place new neighbors into the neighbor table.  Unicast
 packets are then used to exchange known routing information and
 complete the neighbor relationship (Section 5.2).
 To prevent EIGRP from sending sequenced packets to neighbors that
 fail to have bidirectional unicast/multicast, or one neighbor
 restarts while building the relationship, EIGRP MUST place the newly
 discovered neighbor in a "pending" state as follows:
    when Router A receives the first multicast HELLO from Router B, it
    places Router B in the pending state and transmits a unicast
    UPDATE containing no topology information and SHALL set the
    initialization bit.  While Router B is in this state, A will send
    it neither a QUERY nor an UPDATE.  When Router A receives the
    unicast acknowledgment from Router B, it will change the state
    from "pending" to "up".

5.3.6. QUERY Packets during Neighbor Formation

 As described above, during the initial formation of the neighbor
 relationship, EIGRP uses a form of three-way handshake to verify both
 unicast and multicast connectivity are working successfully.  During
 this period of neighbor creation, the new neighbor is considered to
 be in the pending state, and it is not eligible to be included in the
 convergence process.

Savage, et al. Informational [Page 35] RFC 7868 Cisco's EIGRP May 2016

 Because of this, any QUERY received by an EIGRP router would not
 cause a QUERY to be sent to the new (and pending) neighbor.  It would
 perform the DUAL process without the new peer in the conversation.
 To do this, when a router in the process of establishing a new
 neighbor receives a QUERY from a fully established neighbor, it
 performs the normal DUAL Feasible Successor check to determine
 whether it needs to REPLY with a valid path or whether it needs to
 enter the ACTIVE process on the prefix.
 If it determines that it must go active, each fully established
 neighbor that participates in the convergence process will be sent a
 QUERY packet, and REPLY packets are expected from each.  Any pending
 neighbor will not be expected to REPLY and will not be sent a QUERY
 directly.  If it resides on an interface containing a mix of fully
 established neighbors and pending neighbors, it might receive the
 QUERY, but it will not be expected to REPLY to it.

5.4. Topology Table

 The topology table is populated by the Protocol-Dependent Modules
 (PDMs) (IPv4/IPv6), and it is acted upon by the DUAL finite state
 machine.  Associated with each entry are the destination address, a
 list of neighbors that have advertised this destination, and the
 metric associated with the destination.  The metric is referred to as
 the "CD".
 The CD is the best-advertised RD from all neighbors, plus the link
 cost between the receiving router and the neighbor.
 The "RD" is the CD as advertised by the Feasible Successor for the
 destination.  In other words, the Computed Distance, when sent by a
 neighbor, is referred to as the "Reported Distance" and is the metric
 that the neighboring router uses to reach the destination (its CD as
 described above).
 If the router is advertising a destination route, it MUST be using
 the route to forward packets; this is an important rule that distance
 vector protocols MUST follow.

5.4.1. Route Management

 Within the topology table, EIGRP has the notion of internal and
 external routes.  Internal routes MUST be preferred over external
 routes, independent of the metric.  In practical terms, if an
 internal route is received, the diffusing computation will be run
 considering only the internal routes.  Only when no internal routes
 for a given destination exist will EIGRP choose the successor from
 the available external routes.

Savage, et al. Informational [Page 36] RFC 7868 Cisco's EIGRP May 2016

5.4.1.1. Internal Routes

 Internal routes are destinations that have been originated within the
 same EIGRP AS.  Therefore, a directly attached network that is
 configured to run EIGRP is considered an internal route and is
 propagated with this information throughout the network topology.
 Internal routes are tagged with the following information:
    o Router ID of the EIGRP router that originated the route.
    o Configurable administrator tag.

5.4.1.2. External Routes

 External routes are destinations that have been learned from another
 source, such as a different routing protocol or static route.  These
 routes are marked individually with the identity of their
 origination.  External routes are tagged with the following
 information:
    o Router ID of the EIGRP router that redistributed the route.
    o AS number where the destination resides.
    o Configurable administrator tag.
    o Protocol ID of the external protocol.
    o Metric from the external protocol.
    o Bit flags for default routing.
 As an example, suppose there is an AS with three border routers: BR1,
 BR2, and BR3.  A border router is one that runs more than one routing
 protocol.  The AS uses EIGRP as the routing protocol.  Two of the
 border routers, BR1 and BR2, also use Open Shortest Path First (OSPF)
 [10] and the other, BR3, also uses the Routing Information Protocol
 (RIP).
 Routes learned by one of the OSPF border routers, BR1, can be
 conditionally redistributed into EIGRP.  This means that EIGRP
 running in BR1 advertises the OSPF routes within its own AS.  When it
 does so, it advertises the route and tags it as an OSPF-learned route
 with a metric equal to the routing table metric of the OSPF route.
 The router-id is set to BR1.  The EIGRP route propagates to the other
 border routers.
 Let's say that BR3, the RIP border router, also advertises the same
 destinations as BR1.  Therefore, BR3, redistributes the RIP routes
 into the EIGRP AS.  BR2, then, has enough information to determine
 the AS entry point for the route, the original routing protocol used,
 and the metric.

Savage, et al. Informational [Page 37] RFC 7868 Cisco's EIGRP May 2016

 Further, the network administrator could assign tag values to
 specific destinations when redistributing the route.  BR2 can utilize
 any of this information to use the route or re-advertise it back out
 into OSPF.
 Using EIGRP route tagging can give a network administrator flexible
 policy controls and help customize routing.  Route tagging is
 particularly useful in transit ASes where EIGRP would typically
 interact with an inter-domain routing protocol that implements global
 policies.

5.4.2. Split Horizon and Poison Reverse

 In some circumstances, EIGRP will suppress or poison QUERY and UPDATE
 information to prevent routing loops as changes propagate though the
 network.
 Within Cisco, the split horizon rule suggests: "Never advertise a
 route out of the interface through which it was learned".  EIGRP
 implements this to mean, "if you have a successor route to a
 destination, never advertise the route out the interface on which it
 was learned".
 The poison reverse rule states: "A route learned through an interface
 will be advertised as unreachable through that same interface".  As
 with the case of split horizon, EIGRP applies this rule only to
 interfaces it is using for reaching the destination.  Routes learned
 though interfaces that EIGRP is NOT using to reach the destination
 may have the route advertised out those interfaces.
 In EIGRP, split horizon suppresses a QUERY, where as poison reverse
 advertises a destination as unreachable.  This can occur for a
 destination under any of the following conditions:
    o two routers are in startup or restart mode
    o advertising a topology table change
    o sending a query

5.4.2.1. Startup Mode

 When two routers first become neighbors, they exchange topology
 tables during startup mode.  For each destination a router receives
 during startup mode, it advertises the same destination back to its
 new neighbor with a maximum metric (Poison Route).

Savage, et al. Informational [Page 38] RFC 7868 Cisco's EIGRP May 2016

5.4.2.2. Advertising Topology Table Change

 If a router uses a neighbor as the successor for a given destination,
 it will send an UPDATE for the destination with a metric of infinity.

5.4.2.3. Sending a QUERY/UPDATE

 In most cases, EIGRP follows normal split-horizon rules.  When a
 metric change is received from the successor via QUERY or UPDATE that
 causes the route to go ACTIVE, the router will send a QUERY to
 neighbors on all interfaces except the interface toward the
 successor.
 In other words, the router does not send the QUERY out of the inbound
 interface through which the information causing the route to go
 ACTIVE was received.
 An exception to this can occur if a router receives a QUERY from its
 successor while already reacting to an event that did not cause it to
 go ACTIVE, for example, a metric change from the successor that did
 not cause an ACTIVE transition, but was followed by the UPDATE/QUERY
 that does result the router to transition to ACTIVE.

5.5. EIGRP Metric Coefficients

 EIGRP allows for modification of the default composite metric
 calculation (see Section 5.6) through the use of coefficients (K-
 values).  This adjustment allows for per-deployment tuning of network
 behavior.  Setting K-values up to 254 scales the impact of the scalar
 metric on the final composite metric.
 EIGRP default coefficients have been carefully selected to provide
 optimal performance in most networks.  The default K-values are as
 follows:
             K1 == K3 == 1
             K2 == K4 == K5 == 0
             K6 == 0
 If K5 is equal to 0, then reliability quotient is defined to be 1.

Savage, et al. Informational [Page 39] RFC 7868 Cisco's EIGRP May 2016

5.5.1. Coefficients K1 and K2

 K1 is used to allow path selection to be based on the bandwidth
 available along the path.  EIGRP can use one of two variations of
 Throughput-based path selection.
 o  Maximum Theoretical Bandwidth: paths chosen based on the highest
    reported bandwidth
 o  Network Throughput: paths chosen based on the highest "available"
    bandwidth adjusted by congestion-based effects (interface reported
    load)
 By default, EIGRP computes the Throughput using the maximum
 theoretical Throughput expressed in picoseconds per kilobyte of data
 sent.  This inversion results in a larger number (more time)
 ultimately generating a worse metric.
 If K2 is used, the effect of congestion as a measure of load reported
 by the interface will be used to simulate the "available Throughput"
 by adjusting the maximum Throughput.

5.5.2. Coefficient K3

 K3 is used to allow delay or latency-based path selection.  Latency
 and delay are similar terms that refer to the amount of time it takes
 a bit to be transmitted to an adjacent neighbor.  EIGRP uses one-way-
 based values either provided by the interface or computed as a factor
 of the link s bandwidth.

5.5.3. Coefficients K4 and K5

 K4 and K5 are used to allow for path selection based on link quality
 and packet loss.  Packet loss caused by network problems results in
 highly noticeable performance issues or Jitter with streaming
 technologies, voice over IP, online gaming and videoconferencing, and
 will affect all other network applications to one degree or another.
 Critical services should pass with less than 1% packet loss.  Lower
 priority packet types might pass with less than 5% and then 10% for
 the lowest of priority of services.  The final metric can be weighted
 based on the reported link quality.
 The handling of K5 is conditional.  If K5 is equal to 0, then
 reliability quotient is defined to be 1.

Savage, et al. Informational [Page 40] RFC 7868 Cisco's EIGRP May 2016

5.5.4. Coefficient K6

 K6 has been introduced with Wide Metric support and is used to allow
 for Extended Attributes, which can be used to reflect in a higher
 aggregate metric than those having lower energy usage.  Currently
 there are two Extended Attributes, Jitter and energy, defined in the
 scope of this document.

5.5.4.1. Jitter

 Use of Jitter-based Path Selection results in a path calculation with
 the lowest reported Jitter.  Jitter is reported as the interval
 between the longest and shortest packet delivery and is expressed in
 microseconds.  Higher values result in a higher aggregate metric when
 compared to those having lower Jitter calculations.
 Jitter is measured in microseconds and is accumulated along the path,
 with each hop using an averaged 3-second period to smooth out the
 metric change rate.
 Presently, EIGRP does not have the ability to measure Jitter, and, as
 such, the default value will be zero (0).  Performance-based
 solutions such as PfR could be used to populate this field.

5.5.4.2. Energy

 Use of Energy-based Path Selection results in paths with the lowest
 energy usage being selected in a loop-free and deterministic manner.
 The amount of energy used is accumulative and has results in a higher
 aggregate metric than those having lower energy.
 Presently, EIGRP does not report energy usage, and as such the
 default value will be zero (0).

5.6. EIGRP Metric Calculations

5.6.1. Classic Metrics

 The composite metric is based on bandwidth, delay, load, and
 reliability.  MTU is not an attribute for calculating the composite
 metric, but carried in the vector metrics.
 One of the original goals of EIGRP was to offer and enhance routing
 solutions for IGRP.  To achieve this, EIGRP used the same composite
 metric as IGRP, with the terms multiplied by 256 to change the metric
 from 24 bits to 32 bits.

Savage, et al. Informational [Page 41] RFC 7868 Cisco's EIGRP May 2016

5.6.1.1. Classic Composite Formulation

 EIGRP calculates the composite metric with the following formula:
 metric = 256 * ({(K1*BW) + [(K2*BW)/(256-LOAD)] + (K3*DELAY)} *
          (K5/(REL+K4)))
 In this formula, Bandwidth (BW) is the lowest interface bandwidth
 along the path, and delay (DELAY) is the sum of all outbound
 interface delays along the path.  Load (LOAD) and reliability (REL)
 values are expressed percentages with a value of 1 to 255.
 Implementation note: Cisco IOS routers display reliability as a
 fraction of 255.  That is, 255/255 is 100% reliability or a perfectly
 stable link; a value of 229/255 represents a 90% reliable link.  Load
 is a value between 1 and 255.  A load of 255/255 indicates a
 completely saturated link.  A load of 127/255 represents a 50%
 saturated link.  These values are not dynamically measured; they are
 only measured at the time a link changes.
 Bandwidth is the inverse minimum bandwidth (in kbps) of the path in
 bits per second scaled by a factor of 10^7.  The formula for
 bandwidth is as follows:
                   (10^7)/BWmin
 Implementation note: When converting the real bandwidth to the
 composite bandwidth, truncate before applying the scaling factor.
 When converting the composite bandwidth to the real bandwidth, apply
 the scaling factor before the division and only then truncate.
 The delay is the sum of the outgoing interface delay (in tens of
 microseconds) to the destination.  A delay set to it maximum value
 (hexadecimal 0xFFFFFFFF) indicates that the network is unreachable.
 The formula for delay is as follows:
                   [sum of delays]
 The default composite metric, adjusted for scaling factors, for EIGRP
 is:
           metric = 256 * { [(10^7)/ BWmin] + [sum of delays]}
 Minimum Bandwidth (BWmin) is represented in kbps, and the "sum of
 delays" is represented in tens of microseconds.  The bandwidth and
 delay for an Ethernet interface are 10 Mbps and 1 ms, respectively.

Savage, et al. Informational [Page 42] RFC 7868 Cisco's EIGRP May 2016

 The calculated EIGRP bandwidth (BW) metric is then:
             256 * (10^7)/BW = 256 * {(10^7)/10,000}
                             = 256 * 1000
                             = 256,000
 And the calculated EIGRP delay metric is then:
          256 * sum of delay = 256 * 100 * 10 microseconds
                             = 25,600 (in tens of microseconds)

5.6.1.2. Cisco Interface Delay Compatibility

 For compatibility with Cisco products, the following table shows the
 times in nanoseconds EIGRP uses for bandwidth and delay.
 Bandwidth        Classic     Wide Metrics     Interface
 (kbps)           Delay       Delay            Type
 ---------------------------------------------------------
 9               500000000   500000000         Tunnel
 56               20000000    20000000         56 kbps
 64               20000000    20000000         DS0
 1544             20000000    20000000         T1
 2048             20000000    20000000         E1
 10000             1000000     1000000         Ethernet
 16000              630000      630000         TokRing16
 45045            20000000    20000000         HSSI
 100000             100000      100000         FDDI
 100000             100000      100000         FastEthernet
 155000             100000      100000         ATM 155 Mbps
 1000000             10000       10000         GigaEthernet
 2000000             10000        5000         2 Gig
 5000000             10000        2000         5 Gig
 10000000            10000        1000         10 Gig
 20000000            10000          500        20 Gig
 50000000            10000          200        50 Gig
 100000000           10000          100        100 Gig
 200000000           10000           50        200 Gig
 500000000           10000           20        500 Gig

5.6.2. Wide Metrics

 To enable EIGRP to perform the path selection for interfaces with
 high bandwidths, both the EIGRP packet and composite metric formula
 have been modified.  This change allows EIGRP to choose paths based
 on the computed time (measured in picoseconds) information takes to
 travel though the links.

Savage, et al. Informational [Page 43] RFC 7868 Cisco's EIGRP May 2016

5.6.2.1. Wide Metric Vectors

 EIGRP uses five "vector metrics": minimum Throughput, latency, load,
 reliability, and MTU.  These values are calculated from destination
 to source as follows:
            o Throughput    - Minimum value
            o Latency       - accumulative
            o Load          - maximum
            o Reliability   - minimum
            o MTU           - minimum
            o Hop count     - accumulative
 There are two additional values: Jitter and energy.  These two values
 are accumulated from destination to source:
         o Jitter - accumulative
         o Energy - accumulative
 These Extended Attributes, as well as any future ones, will be
 controlled via K6.  If K6 is non-zero, these will be additive to the
 path's composite metric.  Higher Jitter or energy usage will result
 in paths that are worse than those that either do not monitor these
 attributes or that have lower values.
 EIGRP will not send these attributes if the router does not provide
 them.  If the attributes are received, then EIGRP will use them in
 the metric calculation (based on K6) and will forward them with those
 routers values assumed to be "zero" and the accumulative values are
 forwarded unchanged.
 The use of the vector metrics allows EIGRP to compute paths based on
 any of four (bandwidth, delay, reliability, and load) path selection
 schemes.  The schemes are distinguished based on the choice of the
 key-measured network performance metric.
 Of these vector metric components, by default, only minimum
 Throughput and latency are traditionally used to compute the best
 path.  Unlike most metrics, minimum Throughput is set to the minimum
 value of the entire path, and it does not reflect how many hops or
 low Throughput links are in the path, nor does it reflect the
 availability of parallel links.  Latency is calculated based on one-
 way delays and is a cumulative value, which increases with each
 segment in the path.
 Network Designer note: When trying to manually influence EIGRP path
 selection though interface bandwidth/delay configuration, the
 modification of bandwidth is discouraged for following reasons:

Savage, et al. Informational [Page 44] RFC 7868 Cisco's EIGRP May 2016

 The change will only affect the path selection if the configured
 value is the lowest bandwidth over the entire path.  Changing the
 bandwidth can have impact beyond affecting the EIGRP metrics.  For
 example, Quality of Service (QoS) also looks at the bandwidth on an
 interface.
 EIGRP throttles its packet transmissions so it will only use 50% of
 the configured bandwidth.  Lowering the bandwidth can cause EIGRP to
 starve an adjacency, causing slow or failed convergence and control-
 plane operation.
 Changing the delay does not impact other protocols, nor does it cause
 EIGRP to throttle back; changing the delay configured on a link only
 impacts metric calculation.

5.6.2.2. Wide Metric Conversion Constants

 EIGRP uses a number of defined constants for conversion and
 calculation of metric values.  These numbers are provided here for
 reference
         EIGRP_BANDWIDTH                    10,000,000
         EIGRP_DELAY_PICO                    1,000,000
         EIGRP_INACCESSIBLE       0xFFFFFFFFFFFFFFFFLL
         EIGRP_MAX_HOPS                            100
         EIGRP_CLASSIC_SCALE                       256
         EIGRP_WIDE_SCALE                        65536
 When computing the metric using the above units, all capacity
 information will be normalized to kilobytes and picoseconds before
 being used.  For example, delay is expressed in microseconds per
 kilobyte, and would be converted to kilobytes per second; likewise,
 energy would be expressed in power per kilobytes per second of usage.

5.6.2.3. Throughput Calculation

 The formula for the conversion for Max-Throughput value directly from
 the interface without consideration of congestion-based effects is as
 follows:
                                (EIGRP_BANDWIDTH * EIGRP_WIDE_SCALE)
      Max-Throughput = K1 *     ------------------------------------
                                     Interface Bandwidth (kbps)

Savage, et al. Informational [Page 45] RFC 7868 Cisco's EIGRP May 2016

 If K2 is used, the effect of congestion as a measure of load reported
 by the interface will be used to simulate the "available Throughput"
 by adjusting the maximum Throughput according to the formula:
                                         K2 * Max-Throughput
      Net-Throughput = Max-Throughput + ---------------------
                                            256 - Load
 K2 has the greatest effect on the metric occurs when the load
 increases beyond 90%.

5.6.2.4. Latency Calculation

 Transmission times derived from physical interfaces MUST be n units
 of picoseconds, converted to picoseconds prior to being exchanged
 between neighbors, or used in the composite metric determination.
 This includes delay values present in configuration-based commands
 (i.e., interface delay, redistribute, default-metric, route-map,
 etc.).
 The delay value is then converted to a "latency" using the formula:
                        Delay * EIGRP_WIDE_SCALE
      Latency = K3 *   --------------------------
                           EIGRP_DELAY_PICO

5.6.2.5. Composite Calculation

                                                              K5
    metric =[(K1*Net-Throughput) + Latency)+(K6*ExtAttr)] * ------
                                                            K4+Rel
 By default, the path selection scheme used by EIGRP is a combination
 of Throughput and Latency where the selection is a product of total
 latency and minimum Throughput of all links along the path:
    metric = (K1 * min(Throughput)) + (K3 * sum(Latency)) }

6. EIGRP Packet Formats

6.1. Protocol Number

 The IPv6 and IPv4 protocol identifier number spaces are common and
 will both use protocol identifier 88 [8] [9].

Savage, et al. Informational [Page 46] RFC 7868 Cisco's EIGRP May 2016

 EIGRP IPv4 will transmit HELLO packets using either the unicast
 destination of a neighbor or using a multicast host group address [7]
 with a source address EIGRP IPv4 multicast address [13].
 EIGRP IPv6 will transmit HELLO packets with a source address being
 the link-local address of the transmitting interface.  Multicast
 HELLO packets will have a destination address of EIGRP IPv6 multicast
 address [14].  Unicast packets directed to a specific neighbor will
 contain the destination link-local address of the neighbor.
 There is no requirement that two EIGRP IPv6 neighbors share a common
 prefix on their connecting interface.  EIGRP IPv6 will check that a
 received HELLO contains a valid IPv6 link-local source address.
 Other HELLO processing will follow common EIGRP checks, including
 matching AS number and matching K-values.

6.2. Protocol Assignment Encoding

 The External Protocol field is an informational assignment to
 identify the originating routing protocol that this route was learned
 by.  The following values are assigned:
         Protocols             Value
         IGRP                    1
         EIGRP                   2
         Static                  3
         RIP                     4
         HELLO                   5
         OSPF                    6
         ISIS                    7
         EGP                     8
         BGP                     9
         IDRP                   10
         Connected              11

6.3. Destination Assignment Encoding

 Destinations types are encoded according to the IANA address family
 number assignments.  Currently only the following types are used:
       AFI Description            AFI Number
      --------------------------------------
       IP (IP version 4)                 1
       IP6 (IP version 6)                2
       EIGRP Common Service Family   16384
       EIGRP IPv4 Service Family     16385
       EIGRP IPv6 Service Family     16386

Savage, et al. Informational [Page 47] RFC 7868 Cisco's EIGRP May 2016

6.4. EIGRP Communities Attribute

 EIGRP supports communities similar to the BGP Extended Communities
 RFC 4360 [4] extended type with Type field composed of 2 octets and
 Value field composed of 6 octets.  Each Community is encoded as an
 8-octet quantity, as follows:
  1. Type field: 2 octets
  2. Value field: Remaining octets
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type high     | Type low      |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+          Value                |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 In addition to well-known communities supported by BGP (such as Site
 of Origin), EIGRP defines a number of additional Community values in
 the "Experimental Use" [5] range as follows:
   Type high: 0x88
   Type low:
     Value       Name               Description
     ---------------------------------------------------------------
       00        EXTCOMM_EIGRP      EIGRP route information appended
       01        EXTCOMM_DAD        Data: AS + Delay
       02        EXTCOMM_VRHB       Vector: Reliability + Hop + BW
       03        EXTCOMM_SRLM       System: Reserve + Load + MTU
       04        EXTCOMM_SAR        System: Remote AS + Remote ID
       05        EXTCOMM_RPM        Remote: Protocol + Metric
       06        EXTCOMM_VRR        Vecmet: Rsvd + RouterID

Savage, et al. Informational [Page 48] RFC 7868 Cisco's EIGRP May 2016

6.5. EIGRP Packet Header

 The basic EIGRP packet payload format is identical for both IPv4 and
 IPv6, although there are some protocol-specific variations.  Packets
 consist of a header, followed by a set of variable-length fields
 consisting of Type/Length/Value (TLV) triplets.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Header Version | Opcode        |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             Flags                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Sequence Number                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Acknowledgment Number                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Virtual Router ID             |   Autonomous System Number    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Header Version: EIGRP Packet Header Format version.  Current Version
    is 2.  This field is not the same as the TLV Version field.
 Opcode: Indicates the type of the message.  It will be one of the
    following values:
         EIGRP_OPC_UPDATE              1
         EIGRP_OPC_REQUEST             2
         EIGRP_OPC_QUERY               3
         EIGRP_OPC_REPLY               4
         EIGRP_OPC_HELLO               5
         Reserved                      6      (EIGRP_OPC_IPXSAP)
         Reserved                      7      (EIGRP_OPC_PROBE)
         Reserved                      8      (EIGRP_OPC_ACK)
         Reserved                      9
         EIGRP_OPC_SIAQUERY           10
         EIGRP_OPC_SIAREPLY           11
 Checksum: Each packet will include a checksum for the entire contents
    of the packet.  The checksum will be the standard ones' complement
    of the ones' complement sum.  For purposes of computing the
    checksum, the value of the checksum field is zero.  The packet is
    discarded if the packet checksum fails.
 Flags: Defines special handling of the packet.  There are currently
    four defined flag bits.

Savage, et al. Informational [Page 49] RFC 7868 Cisco's EIGRP May 2016

 INIT-Flag (0x01): This bit is set in the initial UPDATE sent to a
    newly discovered neighbor.  It instructs the neighbor to advertise
    its full set of routes.
 CR-Flag (0x02): This bit indicates that receivers should only accept
    the packet if they are in Conditionally Received mode.  A router
    enters Conditionally Received mode when it receives and processes
    a HELLO packet with a SEQUENCE TLV present.
 RS-Flag (0x04): The Restart flag is set in the HELLO and the UPDATE
    packets during the restart period.  The router looks at the RS-
    Flag to detect if a neighbor is restarting.  From the restarting
    routers perspective, if a neighboring router detects the RS-Flag
    set, it will maintain the adjacency, and will set the RS-Flag in
    its UPDATE packet to indicated it is doing a soft restart.
 EOT-Flag (0x08): The End-of-Table flag marks the end of the startup
    process with a neighbor.  If the flag is set, it indicates the
    neighbor has completed sending all UPDATEs.  At this point, the
    router will remove any stale routes learned from the neighbor
    prior to the restart event.  A stale route is any route that
    existed before the restart and was not refreshed by the neighbor
    via and UPDATE.
 Sequence Number: Each packet that is transmitted will have a 32-bit
    sequence number that is unique with respect to a sending router.
    A value of 0 means that an acknowledgment is not required.
 Acknowledgment Number: The 32-bit sequence number that is being
    acknowledged with respect to the receiver of the packet.  If the
    value is 0, there is no acknowledgment present.  A non-zero value
    can only be present in unicast-addressed packets.  A HELLO packet
    with a non-zero ACK field should be decoded as an ACK packet
    rather than a HELLO packet.
 Virtual Router Identifier (VRID): A 16-bit number that identifies the
    virtual router with which this packet is associated.  Packets
    received with an unknown, or unsupported, value will be discarded.
           Value Range       Usage
             0x0000            Unicast Address Family
             0x0001            Multicast Address Family
             0x0002-0x7FFF     Reserved
             0x8000            Unicast Service Family
             0x8001-0xFFFF     Reserved

Savage, et al. Informational [Page 50] RFC 7868 Cisco's EIGRP May 2016

 Autonomous System Number: 16-bit unsigned number of the sending
    system.  This field is indirectly used as an authentication value.
    That is, a router that receives and accepts a packet from a
    neighbor must have the same AS number or the packet is ignored.
    The range of valid AS numbers is 1 through 65,535.

6.6. EIGRP TLV Encoding Format

 The contents of each packet can contain a variable number of fields.
 Each field will be tagged and include a length field.  This allows
 for newer versions of software to add capabilities and coexist with
 old versions of software in the same configuration.  Fields that are
 tagged and not recognized can be skipped over.  Another advantage of
 this encoding scheme is that it allows multiple network-layer
 protocols to carry independent information.  Therefore, if it is
 later decided to implement a single "integrated" protocol, this can
 be done.
 The format of a {type, length, value} (TLV) is encoded as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type high     | Type low      |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Value (variable length)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The type values are the ones defined below.  The length value
 specifies the length in octets of the type, length, and value fields.
 TLVs can appear in a packet in any order, and there are no
 interdependencies among them.
 Malformed TLVs contained in EIGRP messages are handled by silently
 discarding the containing message.  A TLV is malformed if the TLV
 Length is invalid or if the TLV extends beyond the end of the
 containing message.

Savage, et al. Informational [Page 51] RFC 7868 Cisco's EIGRP May 2016

6.6.1. Type Field Encoding

 The type field is structured as follows: Type High: 1 octet that
 defines the protocol classification:
          Protocol            ID   VERSION
          General            0x00    1.2
          IPv4               0x01    1.2
          IPv6               0x04    1.2
          SAF                0x05    3.0
          Multiprotocol      0x06    2.0
 Type Low: 1 octet that defines the TLV Opcode; see TLV Definitions in
    Section 3.

6.6.2. Length Field Encoding

 The Length field is a 2-octet unsigned number, which indicates the
 length of the TLV.  The value includes the Type and Length fields.

6.6.3. Value Field Encoding

 The Value field is a multi-octet field containing the payload for the
 TLV.

6.7. EIGRP Generic TLV Definitions

                               Ver 1.2   Ver 2.0
 PARAMETER_TYPE                0x0001    0x0001
 AUTHENTICATION_TYPE           0x0002    0x0002
 SEQUENCE_TYPE                 0x0003    0x0003
 SOFTWARE_VERSION_TYPE         0x0004    0x0004
 MULTICAST_SEQUENCE_TYPE       0x0005    0x0005
 PEER_INFORMATION_TYPE         0x0006    0x0006
 PEER_TERMINATION_TYPE         0x0007    0x0007
 PEER_TID_LIST_TYPE             ---      0x0008

Savage, et al. Informational [Page 52] RFC 7868 Cisco's EIGRP May 2016

6.7.1. 0x0001 - PARAMETER_TYPE

 This TLV is used in HELLO packets to convey the EIGRP metric
 coefficient values: noted as "K-values" as well as the Hold Time
 values.  This TLV is also used in an initial UPDATE packet when a
 neighbor is discovered.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            0x0001             |            0x000C             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       K1      |       K2      |       K3      |       K4      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       K5      |       K6      |           Hold Time           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 K-values: The K-values associated with the EIGRP composite metric
    equation.  The default values for weights are:
              K1 - 1
              K2 - 0
              K3 - 1
              K4 - 0
              K5 - 0
              K6 - 0
 Hold Time: The amount of time in seconds that a receiving router
    should consider the sending neighbor valid.  A valid neighbor is
    one that is able to forward packets and participates in EIGRP.  A
    router that considers a neighbor valid will store all routing
    information advertised by the neighbor.

6.7.2. 0x0002 - AUTHENTICATION_TYPE

 This TLV may be used in any EIGRP packet and conveys the
 authentication type and data used.  Routers receiving a mismatch in
 authentication shall discard the packet.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             0x0002            |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Auth Type    | Auth Length  |      Auth Data (Variable)     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Savage, et al. Informational [Page 53] RFC 7868 Cisco's EIGRP May 2016

 Authentication Type: The type of authentication used.
 Authentication Length: The length, measured in octets, of the
    individual authentication.
 Authentication Data: Variable-length field reflected by "Auth
    Length", which is dependent on the type of authentication used.
    Multiple authentication types can be present in a single
    AUTHENTICATION_TYPE TLV.

6.7.2.1. 0x02 - MD5 Authentication Type

 MD5 Authentication will use Auth Type code 0x02, and the Auth Data
 will be the MD5 Hash value.

6.7.2.2. 0x03 - SHA2 Authentication Type

 SHA2-256 Authentication will use Type code 0x03, and the Auth Data
 will be the 256-bit SHA2 [6] Hash value.

6.7.3. 0x0003 - SEQUENCE_TYPE

 This TLV is used for a sender to tell receivers to not accept packets
 with the CR-Flag set.  This is used to order multicast and unicast
 addressed packets.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            0x0003             |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Address Length |                 Protocol Address              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Address Length and Protocol Address will be repeated one or more
 times based on the Length field.
 Address Length: Number of octets for the address that follows.  For
    IPv4, the value is 4.  For IPv6, it is 16.  For AppleTalk, the
    value is 4; for Novell IPX, the value is 10 (both are no longer in
    use).
 Protocol Address: Neighbor address on interface in which the HELLO
    with SEQUENCE TLV is sent.  Each address listed in the HELLO
    packet is a neighbor that should not enter Conditionally Received
    mode.

Savage, et al. Informational [Page 54] RFC 7868 Cisco's EIGRP May 2016

6.7.4. 0x0004 - SOFTWARE_VERSION_TYPE

         Field                        Length
         Vender OS major version        1
         Vender OS minor version        1
         EIGRP major revision           1
         EIGRP minor revision           1
 The EIGRP TLV Version fields are used to determine TLV format
 versions.  Routers using Version 1.2 TLVs do not understand Version
 2.0 TLVs, therefore Version 2.0 routers must send the packet with
 both TLV formats in a mixed network.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            0x0004             |            0x000C             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Vendor Major V.|Vendor Minor V.| EIGRP Major V.| EIGRP Minor V.|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.7.5. 0x0005 - MULTICAST_SEQUENCE_TYPE

 The next multicast SEQUENCE TLV.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            0x0005             |             0x0008            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Sequence Number                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.7.6. 0x0006 - PEER_INFORMATION_TYPE

 This TLV is reserved, and not part of this document.

Savage, et al. Informational [Page 55] RFC 7868 Cisco's EIGRP May 2016

6.7.7. 0x0007 - PEER_ TERMINATION_TYPE

 This TLV is used in HELLO packets to notify the list of neighbor(s)
 the router has reset the adjacency.  This TLV is used in HELLO
 packets to notify the list of neighbors that the router has reset the
 adjacency.  This is used anytime a router needs to reset an
 adjacency, or signal an adjacency it is going down.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            0x0007             |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Address List (variable)                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Implementation note: Older Cisco routers implement this using the
 "Parameters TLV" with all K-values set to 255 (except K6).

6.7.8. 0x0008 - TID_LIST_TYPE

 List of sub-topology identifiers, including the Base Topology,
 supported by the router.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            0x0008             |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Topology Identification List (variable)            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 If this information changes from the last state, it means either a
 new topology was added or an existing topology was removed.  This TLV
 is ignored until the three-way handshake has finished
 When the TID list is received, it compares the list to the previous
 list sent.  If a TID is found that does not previously exist, the TID
 is added to the neighbor's topology list, and the existing sub-
 topology is sent to the peer.
 If a TID that was in a previous list is not found, the TID is removed
 from the neighbor's topology list and all routes learned though that
 neighbor for that sub-topology are removed from the topology table.

Savage, et al. Informational [Page 56] RFC 7868 Cisco's EIGRP May 2016

6.8. Classic Route Information TLV Types

6.8.1. Classic Flag Field Encoding

 EIGRP transports a number of flags with in the TLVs to indicate
 addition route state information.  These bits are defined as follows:
 Flags Field
 -----------
 Source Withdraw (Bit 0) - Indicates if the router that is the
 original source of the destination is withdrawing the route from the
 network or if the destination is lost due as a result of a network
 failure.
 Candidate Default (CD) (Bit 1) - Set to indicate the destination
 should be regarded as a candidate for the default route.  An EIGRP
 default route is selected from all the advertised candidate default
 routes with the smallest metric.
 ACTIVE (Bit 2) - Indicates if the route is in the ACTIVE State.

6.8.2. Classic Metric Encoding

 The handling of bandwidth and delay for Classic TLVs is encoded in
 the packet "scaled" form relative to how they are represented on the
 physical link.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Scaled Delay                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Scaled Bandwidth                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   MTU                         | Hop Count     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Reliability   |       Load    | Internal Tag  | Flags Field   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Scaled Delay: An administrative parameter assigned statically on a
    per-interface-type basis to represent the time it takes along an
    unloaded path.  This is expressed in units of tens of microseconds
    divvied by 256.  A delay of 0xFFFFFFFF indicates an unreachable
    route.
 Scaled Bandwidth: The path bandwidth measured in bits per second.  In
    units of 2,560,000,000/kbps.

Savage, et al. Informational [Page 57] RFC 7868 Cisco's EIGRP May 2016

 MTU: The minimum MTU size for the path to the destination.
 Hop Count: The number of router traversals to the destination.
 Reliability: The current error rate for the path, measured as an
    error percentage.  A value of 255 indicates 100% reliability
 Load: The load utilization of the path to the destination, measured
    as a percentage.  A value of 255 indicates 100% load.
 Internal-Tag: A tag assigned by the network administrator that is
    untouched by EIGRP.  This allows a network administrator to filter
    routes in other EIGRP border routers based on this value.
 Flags Field: See Section 6.8.1.

6.8.3. Classic Exterior Encoding

 Additional routing information so provided for destinations outside
 of the EIGRP AS as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Router Identifier (RID)                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               External Autonomous System (AS) Number          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Administrative Tag                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    External Protocol Metric                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Reserved            |Extern Protocol|  Flags Field  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Router Identifier (RID): A 32-bit number provided by the router
    sourcing the information to uniquely identify it as the source.
 External Autonomous System (AS) Number: A 32-bit number indicating
    the external AS of which the sending router is a member.  If the
    source protocol is EIGRP, this field will be the [VRID, AS] pair.
    If the external protocol does not have an AS, other information
    can be used (for example, Cisco uses process-id for OSPF).
 Administrative Tag: A tag assigned by the network administrator that
    is untouched by EIGRP.  This allows a network administrator to
    filter routes in other EIGRP border routers based on this value.

Savage, et al. Informational [Page 58] RFC 7868 Cisco's EIGRP May 2016

 External Protocol Metric: 32-bit value of the composite metric that
    resides in the routing table as learned by the foreign protocol.
    If the External Protocol is IGRP or another EIGRP routing process,
    the value can optionally be the composite metric or 0, and the
    metric information is stored in the metric section.
 External Protocol: Contains an enumerated value defined in Section
    6.2 to identify the routing protocol (external protocol)
    redistributing the route.
 Flags Field: See Section 6.8.1

6.8.4. Classic Destination Encoding

 EIGRP carries destination in a compressed form, where the number of
 bits significant in the variable-length address field are indicated
 in a counter.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Subnet Mask   |    Destination Address (variable length)      |
 | Bit Count     |         ((Bit Count - 1) / 8) + 1             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Subnet Mask Bit Count: 8-bit value used to indicate the number of
    bits in the subnet mask.  A value of 0 indicates the default
    network, and no address is present.
 Destination Address: A variable-length field used to carry the
    destination address.  The length is determined by the number of
    consecutive bits in the destination address.  The formula to
    calculate the length is address-family dependent:
    IPv4: ((Bit Count - 1) / 8) + 1
    IPv6: (Bit Count == 128) ? 16 : ((x / 8) + 1)

6.8.5. IPv4-Specific TLVs

    INTERNAL_TYPE       0x0102
    EXTERNAL_TYPE       0x0103
    COMMUNITY_TYPE      0x0104

Savage, et al. Informational [Page 59] RFC 7868 Cisco's EIGRP May 2016

6.8.5.1. IPv4 INTERNAL_TYPE

 This TLV conveys IPv4 destination and associated metric information
 for IPv4 networks.  Routes advertised in this TLV are network
 interfaces that EIGRP is configured on as well as networks that are
 learned via other routers running EIGRP.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      0x01     |       0x02    |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Next-Hop Forwarding Address                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Vector Metric Section (see Section 6.8.2)          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
 |                       Destination Section                     |
 |                 IPv4 Address (variable length)                |
 |                       (see Section 6.8.4)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next-Hop Forwarding Address: IPv4 address represented by four 8-bit
    values (total 4 octets).  If the value is zero (0), the IPv4
    address from the received IPv4 header is used as the next hop for
    the route.  Otherwise, the specified IPv4 address will be used.
 Vector Metric Section: The vector metrics for destinations contained
    in this TLV.  See the description of "metric encoding" in Section
    6.8.2.
 Destination Section: The network/subnet/host destination address
    being requested.  See the description of "destination" in Section
    6.8.4.

6.8.5.2. IPv4 EXTERNAL_TYPE

 This TLV conveys IPv4 destination and metric information for routes
 learned by other routing protocols that EIGRP injects into the AS.
 Available with this information is the identity of the routing
 protocol that created the route, the external metric, the AS number,
 an indicator if it should be marked as part of the EIGRP AS, and a
 network-administrator tag used for route filtering at EIGRP AS
 boundaries.

Savage, et al. Informational [Page 60] RFC 7868 Cisco's EIGRP May 2016

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      0x01     |       0x03    |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Next-Hop Forwarding Address                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Exterior Section (see Section 6.8.3)          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Vector Metric Section (see Section 6.8.2)          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
 |                       Destination Section                     |
 |                 IPv4 Address (variable length)                |
 |                       (see Section 6.8.4)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next-Hop Forwarding Address: IPv4 address represented by four 8-bit
    values (total 4 octets).  If the value is zero (0), the IPv4
    address from the received IPv4 header is used as the next hop for
    the route.  Otherwise, the specified IPv4 address will be used.
 Exterior Section: Additional routing information provided for a
    destination that is outside of the AS and that has been
    redistributed into the EIGRP.  See the description of "exterior
    encoding" in Section 6.8.3.
 Vector Metric Section: Vector metrics for destinations contained in
    this TLV.  See the description of "metric encoding" in Section
    6.8.2.
 Destination Section: The network/subnet/host destination address
    being requested.  See the description of "destination" in Section
    6.8.4.

Savage, et al. Informational [Page 61] RFC 7868 Cisco's EIGRP May 2016

6.8.5.3. IPv4 COMMUNITY_TYPE

 This TLV is used to provide community tags for specific IPv4
 destinations.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      0x01     |       0x04    |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          IPv4 Destination                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Reserved           |       Community Length        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Community List                        |
 |                        (variable length)                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IPv4 Destination: The IPv4 address with which the community
    information should be stored.
 Community Length: A 2-octet unsigned number that indicates the length
    of the Community List.  The length does not include the IPv4
    Address, Reserved, or Length fields.
 Community List: One or more 8-octet EIGRP communities, as defined in
    Section 6.4.

6.8.6. IPv6-Specific TLVs

    INTERNAL_TYPE                 0x0402
    EXTERNAL_TYPE                 0x0403
    COMMUNITY_TYPE                0x0404

Savage, et al. Informational [Page 62] RFC 7868 Cisco's EIGRP May 2016

6.8.6.1. IPv6 INTERNAL_TYPE

 This TLV conveys the IPv6 destination and associated metric
 information for IPv6 networks.  Routes advertised in this TLV are
 network interfaces that EIGRP is configured on as well as networks
 that are learned via other routers running EIGRP.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      0x04     |       0x02    |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                   Next-Hop Forwarding Address                 |
 |                            (16 octets)                        |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Vector Metric Section (see Section 6.8.2)          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
 |                       Destination Section                     |
 |                 IPv6 Address (variable length)                |
 |                       (see Section 6.8.4)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next-Hop Forwarding Address: This IPv6 address is represented by
    eight groups of 16-bit values (total 16 octets).  If the value is
    zero (0), the IPv6 address from the received IPv6 header is used
    as the next hop for the route.  Otherwise, the specified IPv6
    address will be used.
 Vector Metric Section: Vector metrics for destinations contained in
    this TLV.  See the description of "metric encoding" in Section
    6.8.2.
 Destination Section: The network/subnet/host destination address
    being requested.  See the description of "destination" in Section
    6.8.4.

6.8.6.2. IPv6 EXTERNAL_TYPE

 This TLV conveys IPv6 destination and metric information for routes
 learned by other routing protocols that EIGRP injects into the
 topology.  Available with this information is the identity of the
 routing protocol that created the route, the external metric, the AS
 number, an indicator if it should be marked as part of the EIGRP AS,
 and a network administrator tag used for route filtering at EIGRP AS
 boundaries.

Savage, et al. Informational [Page 63] RFC 7868 Cisco's EIGRP May 2016

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      0x04     |        0x03   |           Length              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                   Next-Hop Forwarding Address                 |
 |                             (16 octets)                       |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               Exterior Section (see Section 6.8.3)            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Vector Metric Section (see Section 6.8.2)          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
 |                        Destination Section                    |
 |                 IPv6 Address (variable length)                |
 |                       (see Section 6.8.4)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next-Hop Forwarding Address: IPv6 address is represented by eight
    groups of 16-bit values (total 16 octets).  If the value is zero
    (0), the IPv6 address from the received IPv6 header is used as the
    next hop for the route.  Otherwise, the specified IPv6 address
    will be used.
 Exterior Section: Additional routing information provided for a
    destination that is outside of the AS and that has been
    redistributed into the EIGRP.  See the description of "exterior
    encoding" in Section 6.8.3.
 Vector Metric Section: vector metrics for destinations contained in
    this TLV.  See the description of "metric encoding" in Section
    6.8.2.
 Destination Section: The network/subnet/host destination address
    being requested.  See the description of "destination" in Section
    6.8.4.

Savage, et al. Informational [Page 64] RFC 7868 Cisco's EIGRP May 2016

6.8.6.3 IPv6 COMMUNITY_TYPE

 This TLV is used to provide community tags for specific IPv4
 destinations.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      0x04     |       0x04    |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                            Destination                        |
 |                            (16 octets)                        |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Reserved           |       Community Length        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Community List                        |
 |                        (variable length)                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Destination: The IPv6 address with which the community information
    should be stored.
 Community Length: A 2-octet unsigned number that indicates the length
    of the Community List.  The length does not include the IPv6
    Address, Reserved, or Length fields.
 Community List: One or more 8-octet EIGRP communities, as defined in
    Section 6.4.

Savage, et al. Informational [Page 65] RFC 7868 Cisco's EIGRP May 2016

6.9. Multiprotocol Route Information TLV Types

 This TLV conveys topology and associated metric information.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Header Version |    Opcode     |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                              Flags                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Sequence Number                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Acknowledgment Number                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Virtual Router ID             |   Autonomous System Number    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      TLV Header Encoding                      |
 |                      (see Section 6.9.1)                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Wide Metric Encoding                    |
 |                       (see Section 6.9.2)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Destination Descriptor                  |
 |                         (variable length)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.9.1. TLV Header Encoding

 There has been a long-standing requirement for EIGRP to support
 routing technologies, such as multi-topologies, and to provide the
 ability to carry destination information independent of the
 transport.  To accomplish this, a Vector has been extended to have a
 new "Header Extension Header" section.  This is a variable-length
 field and, at a minimum, it will support the following fields:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type High     | Type Low      |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               AFI             |             TID               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Router Identifier (RID)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Value (variable length)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Savage, et al. Informational [Page 66] RFC 7868 Cisco's EIGRP May 2016

 The available fields are:
 TYPE - Topology TLVs have the following TYPE codes:
     Type High: 0x06
     Type Low:
         REQUEST_TYPE                 0x01
         INTERNAL_TYPE                0x02
         EXTERNAL_TYPE                0x03
 Router Identifier (RID): A 32-bit number provided by the router
    sourcing the information to uniquely identify it as the source.

6.9.2. Wide Metric Encoding

 Multiprotocol TLVs will provide an extendable section of metric
 information, which is not used for the primary routing compilation.
 Additional per-path information is included to enable per-path cost
 calculations in the future.  Use of the per-path costing along with
 the VID/TID will prove a complete solution for multidimensional
 routing.
  0                   1                     2                 3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Offset     |   Priority    | Reliability   |        Load   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               MTU                             |   Hop Count   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               Delay                           |
 |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
 |                             Bandwidth                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               Reserved        |         Opaque Flags          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Extended Attributes                      |
 |                        (variable length)                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The fields are as follows:
 Offset: Number of 16-bit words in the Extended Attribute section that
    are used to determine the start of the destination information.  A
    value of zero indicates no Extended Attributes are attached.

Savage, et al. Informational [Page 67] RFC 7868 Cisco's EIGRP May 2016

 Priority: Priority of the prefix when processing a route.  In an AS
    using priority values, a destination with a higher priority
    receives preferential treatment and is serviced before a
    destination with a lower priority.  A value of zero indicates no
    priority is set.
 Reliability: The current error rate for the path.  Measured as an
    error percentage.  A value of 255 indicates 100% reliability
 Load: The load utilization of the path to the destination, measured
    as a percentage.  A value of 255 indicates 100% load.
 MTU: The minimum MTU size for the path to the destination.  Not used
    in metric calculation but available to underlying protocols
 Hop Count: The number of router traversals to the destination.
 Delay: The one-way latency along an unloaded path to the destination
    expressed in units of picoseconds per kilobit.  This number is not
    scaled; a value of 0xFFFFFFFFFFFF indicates an unreachable route.
 Bandwidth: The path bandwidth measured in kilobit per second as
    presented by the interface.  This number is not scaled; a value of
    0xFFFFFFFFFFFF indicates an unreachable route.
 Reserved: Transmitted as 0x0000.
 Opaque Flags: 16-bit protocol-specific flags.  Values currently
    defined by Cisco are:
        OPAQUE_SRCWD    0x01   Route Source Withdraw
        OPAQUE_CD       0x02   Candidate default route
        OPAQUE_ACTIVE   0x04   Route is currently in active state
        OPAQUE_REPL     0x08   Route is replicated from another VRF
 Extended Attributes (Optional): When present, defines extendable per-
    destination attributes.  This field is not normally transmitted.

6.9.3. Extended Metrics

 Extended metrics allow for extensibility of the vector metrics in a
 manner similar to RFC 6390 [11].  Each Extended metric shall consist
 of a header identifying the type (Opcode) and the length (Offset)
 followed by application-specific information.  Extended metric values
 not understood must be treated as opaque and passed along with the
 associated route.

Savage, et al. Informational [Page 68] RFC 7868 Cisco's EIGRP May 2016

 The general formats for the Extended Metric fields are:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Opcode    |      Offset   |              Data             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Opcode: Indicates the type of Extended Metric.
 Offset: Number of 16-bit words in the application-specific
    information.  Offset does not include the length of the Opcode or
    Offset.
 Data: Zero or more octets of data as defined by Opcode.

6.9.3.1. 0x00 - NoOp

 This is used to pad the attribute section to ensure 32-bit alignment
 of the metric encoding section.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     0x00      |      0x00     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The fields are:
 Opcode: Transmitted as zero (0).
 Offset: Transmitted as zero (0) indicating no data is present.
 Data: No data is present with this attribute.

Savage, et al. Informational [Page 69] RFC 7868 Cisco's EIGRP May 2016

6.9.3.2. 0x01 - Scaled Metric

 If a route is received from a back-rev neighbor, and the route is
 selected as the best path, the scaled metric received in the older
 UPDATE may be attached to the packet.  If received, the value is for
 informational purposes and is not affected by K6.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       0x01    |       0x04    |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Scaled Bandwidth                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Scaled Delay                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Reserved: Transmitted as 0x0000
 Scaled Bandwidth: The minimum bandwidth along a path expressed in
    units of 2,560,000,000/kbps.  A bandwidth of 0xFFFFFFFF indicates
    an unreachable route.
 Scaled Delay: An administrative parameter assigned statically on a
    per-interface-type basis to represent the time it takes along an
    unloaded path.  This is expressed in units of tens of microseconds
    divvied by 256.  A delay of 0xFFFFFFFF indicates an unreachable
    route.

6.9.3.3. 0x02 - Administrator Tag

 EIGRP administrative tag does not alter the path decision-making
 process.  Routers can set a tag value on a route and use the flags to
 apply specific routing polices within their network.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       0x02    |       0x02    |       Administrator Tag       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Administrator Tag (cont.)     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Administrator Tag: A tag assigned by the network administrator that
    is untouched by EIGRP.  This allows a network administrator to
    filter routes in other EIGRP border routers based on this value.

Savage, et al. Informational [Page 70] RFC 7868 Cisco's EIGRP May 2016

6.9.3.4. 0x03 - Community List

 EIGRP communities themselves do not alter the path decision-making
 process, communities can be used as flags in order to mark a set of
 routes.  Upstream routers can then use these flags to apply specific
 routing polices within their network.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       0x03    |      Offset   |          Community List       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
 |                          (variable length)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Offset: Number of 16-bit words in the sub-field.
 Community List: One or more 8-octet EIGRP communities, as defined in
    Section 6.4.

6.9.3.5. 0x04 - Jitter

 (Optional) EIGRP can carry one-way Jitter in networks that carry UDP
 traffic if the node is capable of measuring UDP Jitter.  The Jitter
 reported to will be averaged with any existing Jitter data and
 include in the route updates.  If no Jitter value is reported by the
 peer for a given destination, EIGRP will use the locally collected
 value.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        0x04    |      0x03    |             Jitter            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Jitter: The measure of the variability over time of the latency
    across a network measured in measured in microseconds.

6.9.3.6. 0x05 - Quiescent Energy

 (Optional) EIGRP can carry energy usage by nodes in networks if the
 node is capable of measuring energy.  The Quiescent Energy reported
 will be added to any existing energy data and include in the route
 updates.  If no energy data is reported by the peer for a given
 destination, EIGRP will use the locally collected value.

Savage, et al. Informational [Page 71] RFC 7868 Cisco's EIGRP May 2016

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        0x05    |        0x02  |        Q-Energy (high)        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Q-Energy (low)       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Q-Energy: Paths with higher idle (standby) energy usage will be
    reflected in a higher aggregate metric than those having lower
    energy usage.  If present, this number will represent the idle
    power consumption expressed in milliwatts per kilobit.

6.9.3.7. 0x06 - Energy

 (Optional) EIGRP can carry energy usage by nodes in networks if the
 node is capable of measuring energy.  The active Energy reported will
 be added to any existing energy data and include in the route
 updates.  If no energy data is reported by the peer for a given
 destination, EIGRP will use the locally collected value.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        0x06    |      0x02    |          Energy (high)        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Energy (low)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Energy: Paths with higher active energy usage will be reflected in a
    higher aggregate metric than those having lower energy usage.  If
    present, this number will represent the power consumption
    expressed in milliwatts per kilobit.

6.9.3.8. 0x07 - AddPath

 The Add Path enables EIGRP to advertise multiple best paths to
 adjacencies.  There will be up to a maximum of four AddPaths
 supported, where the format of the field will be as follows.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       0x07    |       Offset  |     AddPath (Variable Length) |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Offset: Number of 16-bit words in the sub-field.

Savage, et al. Informational [Page 72] RFC 7868 Cisco's EIGRP May 2016

 AddPath: Length of this field will vary in length based on whether it
    contains IPv4 or IPv6 data.

6.9.3.8.1. AddPath with IPv4 Next Hop

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       0x07    |       Offset  | Next-Hop Addr. (Upper 2 bytes)|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | IPv4 Address (Lower 2 bytes)  |       RID (Upper 2 bytes)     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        RID (Upper 2 bytes)    | Admin Tag (Upper 2 bytes)     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Admin Tag (Upper 2 bytes)     |Extern Protocol| Flags Field   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next-Hop Address: An IPv4 address represented by four 8-bit values
    (total 4 octets).  If the value is zero (0), the IPv6 address from
    the received IPv4 header is used as the next hop for the route.
    Otherwise, the specified IPv4 address will be used.
 Router Identifier (RID): A 32-bit number provided by the router
    sourcing the information to uniquely identify it as the source.
 Admin Tag: A 32-bit administrative tag assigned by the network.  This
    allows a network administrator to filter routes based on this
    value.
 If the route is of type external, then two additional bytes will be
 added as follows:
 External Protocol: Contains an enumerated value defined in Section
    6.2 to identify the routing protocol (external protocol)
    redistributing the route.
 Flags Field: See Section 6.8.1.

Savage, et al. Informational [Page 73] RFC 7868 Cisco's EIGRP May 2016

6.9.3.8.2. AddPath with IPv6 Next Hop

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       0x07     |       Offset |         Next-Hop Address      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
 |                                                               |
 |                            (16 octets)                        |
 |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
 |                               |       RID (Upper 2 byes)      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        RID (Upper 2 byes)     | Admin Tag (Upper 2 byes)      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Admin Tag (Upper 2 byes)      | Extern Protocol | Flags Field |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next-Hop Address: An IPv6 address represented by eight groups of
    16-bit values (total 16 octets).  If the value is zero (0), the
    IPv6 address from the received IPv6 header is used as the next hop
    for the route.  Otherwise, the specified IPv6 address will be
    used.
 Router Identifier (RID): A 32-bit number provided by the router
    sourcing the information to uniquely identify it as the source.
 Admin Tag: A 32-bit administrative tag assigned by the network.  This
    allows a network administrator to filter routes based on this
    value.  If the route is of type external, then two addition bytes
    will be added as follows:
 External Protocol: Contains an enumerated value defined in Section
    6.2 to identify the routing protocol (external protocol)
    redistributing the route.
 Flags Field: See Section 6.8.1.

Savage, et al. Informational [Page 74] RFC 7868 Cisco's EIGRP May 2016

6.9.4. Exterior Encoding

 Additional routing information provided for destinations outside of
 the EIGRP AS as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Router Identifier (RID)                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            External Autonomous System (AS) Number             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     External Protocol Metric                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Reserved             |Extern Protocol| Flags Field |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Router Identifier (RID): A 32-bit number provided by the router
    sourcing the information to uniquely identify it as the source.
 External Autonomous System (AS) Number: A 32-bit number indicating
    the external AS of which the sending router is a member.  If the
    source protocol is EIGRP, this field will be the [VRID, AS] pair.
    If the external protocol does not have an AS, other information
    can be used (for example, Cisco uses process-id for OSPF).
 External Protocol Metric: A 32-bit value of the metric used by the
    routing table as learned by the foreign protocol.  If the External
    Protocol is IGRP or EIGRP, the value can (optionally) be 0, and
    the metric information is stored in the metric section.
 External Protocol: Contains an enumerated value defined in Section
    6.2 to identify the routing protocol (external protocol)
    redistributing the route.
 Flags Field: See Section 6.8.1.

Savage, et al. Informational [Page 75] RFC 7868 Cisco's EIGRP May 2016

6.9.5. Destination Encoding

 Destination information is encoded in Multiprotocol packets in the
 same manner used by Classic TLVs.  This is accomplished by using a
 counter to indicate how many significant bits are present in the
 variable-length address field.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Subnet Mask   |    Destination Address (variable length       |
 | Bit Count     |         ((Bit Count - 1) / 8) + 1             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Subnet Mask Bit Count: 8-bit value used to indicate the number of
    bits in the subnet mask.  A value of 0 indicates the default
    network and no address is present.
 Destination Address: A variable-length field used to carry the
    destination address.  The length is determined by the number of
    consecutive bits in the destination address.  The formula to
    calculate the length is address-family dependent:
    IPv4: ((Bit Count - 1) / 8) + 1
    IPv6: (Bit Count == 128) ? 16 : ((x / 8) + 1)

6.9.6. Route Information

6.9.6.1. INTERNAL TYPE

 This TLV conveys destination information based on the IANA AFI
 defined in the TLV Header (see Section 6.9.1), and associated metric
 information.  Routes advertised in this TLV are network interfaces
 that EIGRP is configured on as well as networks that are learned via
 other routers running EIGRP.

6.9.6.2. EXTERNAL TYPE

 This TLV conveys destination information based on the IANA AFI
 defined in the TLV Header (see Section 6.9.1), and metric information
 for routes learned by other routing protocols that EIGRP injects into
 the AS.  Available with this information is the identity of the
 routing protocol that created the route, the external metric, the AS
 number, an indicator if it should be marked as part of the EIGRP AS,
 and a network administrator tag used for route filtering at EIGRP AS
 boundaries.

Savage, et al. Informational [Page 76] RFC 7868 Cisco's EIGRP May 2016

7. Security Considerations

 Being promiscuous, EIGRP will neighbor with any router that sends a
 valid HELLO packet.  Due to security considerations, this
 "completely" open aspect requires policy capabilities to limit
 peering to valid routers.
 EIGRP does not rely on a PKI or a heavyweight authentication system.
 These systems challenge the scalability of EIGRP, which was a primary
 design goal.
 Instead, Denial-of-Service (DoS) attack prevention will depend on
 implementations rate-limiting packets to the control plane as well as
 authentication of the neighbor through the use of MD5 or SHA2-256
 [6].

8. IANA Considerations

 This document serves as the sole reference for two multicast
 addresses: 224.0.0.10 for IPv4 "EIGRP Routers" [13] and
 FF02:0:0:0:0:0:0:A for IPv6 "EIGRP Routers" [14].  It also serves as
 assignment for protocol number 88 (EIGRP) [15].

9. References

9.1. Normative References

 [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
      <http://www.rfc-editor.org/info/rfc2119>.
 [2]  Garcia-Luna-Aceves, J.J., "A Unified Approach to Loop-Free
      Routing Using Distance Vectors or Link States", SIGCOMM '89,
      Symposium proceedings on Communications architectures &
      protocols, Volume 19, pages 212-223, ACM
      089791-332-9/89/0009/0212, DOI 10.1145/75247.75268, 1989.
 [3]  Garcia-Luna-Aceves, J.J., "Loop-Free Routing using Diffusing
      Computations", Network Information Systems Center, SRI
      International, appeared in IEEE/ACM Transactions on Networking,
      Vol. 1, No. 1, DOI 10.1109/90.222913, 1993.
 [4]  Rosen, E. and Y. Rekhter, "IANA Registries for BGP Extended
      Communities", RFC 7153, DOI 10.17487/RFC7153, March 2014,
      <http://www.rfc-editor.org/info/rfc7153>.

Savage, et al. Informational [Page 77] RFC 7868 Cisco's EIGRP May 2016

 [5]  Narten, T., "Assigning Experimental and Testing Numbers
      Considered Useful", BCP 82, RFC 3692, DOI 10.17487/RFC3692,
      January 2004, <http://www.rfc-editor.org/info/rfc3692>.
 [6]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-384, and
      HMAC-SHA-512 with IPsec", RFC 4868, DOI 10.17487/RFC4868, May
      2007, <http://www.rfc-editor.org/info/rfc4868>.
 [7]  Deering, S., "Host extensions for IP multicasting", STD 5,
      RFC 1112, DOI 10.17487/RFC1112, August 1989,
      <http://www.rfc-editor.org/info/rfc1112>.
 [8]  Postel, J., "Internet Protocol", STD 5, RFC 791,
      DOI 10.17487/RFC0791, September 1981,
      <http://www.rfc-editor.org/info/rfc791>.
 [9]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
      Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998,
      <http://www.rfc-editor.org/info/rfc2460>.

9.2. Informative References

 [10] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
      DOI 10.17487/RFC2328, April 1998,
      <http://www.rfc-editor.org/info/rfc2328>.
 [11] Clark, A. and B. Claise, "Guidelines for Considering New
      Performance Metric Development", BCP 170, RFC 6390,
      DOI 10.17487/RFC6390, October 2011,
      <http://www.rfc-editor.org/info/rfc6390>.
 [12] IANA, "Address Family Numbers",
      <http://www.iana.org/assignments/address-family-numbers>.
 [13] IANA, "IPv4 Multicast Address Space Registry",
      <http://www.iana.org/assignments/multicast-addresses>.
 [14] IANA, "IPv6 Multicast Address Space Registry",
      <http://www.iana.org/assignments/ipv6-multicast-addresses>.
 [15] IANA, "Protocol Numbers",
      <http://www.iana.org/assignments/protocol-numbers>.

Savage, et al. Informational [Page 78] RFC 7868 Cisco's EIGRP May 2016

Acknowledgments

 Thank you goes to Dino Farinacci, Bob Albrightson, and Dave Katz.
 Their significant accomplishments towards the design and development
 of the EIGRP provided the bases for this document.
 A special and appreciative thank you goes to the core group of Cisco
 engineers whose dedication, long hours, and hard work led the
 evolution of EIGRP over the past decade.  They are Donnie Savage,
 Mickel Ravizza, Heidi Ou, Dawn Li, Thuan Tran, Catherine Tran, Don
 Slice, Claude Cartee, Donald Sharp, Steven Moore, Richard Wellum, Ray
 Romney, Jim Mollmann, Dennis Wind, Chris Van Heuveln, Gerald Redwine,
 Glen Matthews, Michael Wiebe, and others.
 The authors would like to gratefully acknowledge many people who have
 contributed to the discussions that lead to the making of this
 proposal.  They include Chris Le, Saul Adler, Scott Van de Houten,
 Lalit Kumar, Yi Yang, Kumar Reddy, David Lapier, Scott Kirby, David
 Prall, Jason Frazier, Eric Voit, Dana Blair, Jim Guichard, and Alvaro
 Retana.
 In addition to the tireless work provided by the Cisco engineers over
 the years, we would like to personally recognize the teams that
 created open source versions of EIGRP:
 o  Linux implementation developed by the Quagga team: Jan Janovic,
    Matej Perina, Peter Orsag, and Peter Paluch.
 o  BSD implementation developed and released by Renato Westphal.

Savage, et al. Informational [Page 79] RFC 7868 Cisco's EIGRP May 2016

Authors' Addresses

 Donnie V. Savage
 Cisco Systems, Inc.
 7025 Kit Creek Rd., RTP,
 Morrisville, NC 27560
 United States
 Phone: 919-392-2379
 Email: dsavage@cisco.com
 James Ng
 Cisco Systems, Inc.
 7025 Kit Creek Rd., RTP,
 Morrisville, NC 27560
 United States
 Phone: 919-392-2582
 Email: jamng@cisco.com
 Steven Moore
 Cisco Systems, Inc.
 7025 Kit Creek Rd., RTP,
 Morrisville, NC 27560
 United States
 Phone: 408-895-2031
 Email: smoore@cisco.com
 Donald Slice
 Cumulus Networks
 Apex, NC
 United States
 Email: dslice@cumulusnetworks.com
 Peter Paluch
 University of Zilina
 Univerzitna 8215/1, Zilina 01026
 Slovakia
 Phone: 421-905-164432
 Email: Peter.Paluch@fri.uniza.sk
 Russ White
 LinkedIn
 Apex, NC
 United States
 Phone: 1-877-308-0993
 Email: russw@riw.us

Savage, et al. Informational [Page 80]

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