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rfc:rfc1655

Network Working Group Y. Rekhter Request for Comments: 1655 T.J. Watson Research Center, IBM Corp. Obsoletes: 1268 P. Gross Category: Standards Track MCI

                                                               Editors
                                                             July 1994
     Application of the Border Gateway Protocol in the Internet

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document, together with its companion document, "A Border
 Gateway Protocol 4 (BGP-4)", define an inter-autonomous system
 routing protocol for the Internet.  "A Border Gateway Protocol 4
 (BGP-4)" defines the BGP protocol specification, and this document
 describes the usage of the BGP in the Internet.
 Information about the progress of BGP can be monitored and/or
 reported on the BGP mailing list (bgp@ans.net).

Acknowledgements

 This document was originally published as RFC 1164 in June 1990,
 jointly authored by Jeffrey C. Honig (Cornell University), Dave Katz
 (MERIT), Matt Mathis (PSC), Yakov Rekhter (IBM), and Jessica Yu
 (MERIT).
 The following also made key contributions to RFC 1164 -- Guy Almes
 (ANS, then at Rice University), Kirk Lougheed (cisco Systems), Hans-
 Werner Braun (SDSC, then at MERIT), and Sue Hares (MERIT).
 We like to explicitly thank Bob Braden (ISI) for the review of the
 previous version of this document.
 This updated version of the document is the product of the IETF BGP
 Working Group with Phill Gross (MCI) and Yakov Rekhter (IBM) as
 editors.

Rekhter & Gross [Page 1] RFC 1655 BGP-4 Application July 1994

 John Moy (Proteon) contributed Section 7 "Required set of supported
 routing policies".
 Scott Brim (Cornell University) contributed the basis for Section 8
 "Interaction with other exterior routing protocols".
 Most of the text in Section 9 was contributed by Gerry Meyer
 (Spider).
 Parts of the Introduction were taken almost verbatim from [3].
 We would like to acknowledge Dan Long (NEARNET) and Tony Li (cisco
 Systems) for their review and comments on the current version of the
 document.

1. Introduction

 This memo describes the use of the Border Gateway Protocol (BGP) [1]
 in the Internet environment. BGP is an inter-Autonomous System
 routing protocol. The network reachability information exchanged via
 BGP provides sufficient information to detect routing loops and
 enforce routing decisions based on performance preference and policy
 constraints as outlined in RFC 1104 [2]. In particular, BGP exchanges
 routing information containing full AS paths and enforces routing
 policies based on configuration information.
 As the Internet has evolved and grown over in recent years, it has
 become painfully evident that it is soon to face several serious
 scaling problems. These include:
  1. Exhaustion of the class-B network address space. One

fundamental cause of this problem is the lack of a network

       class of a size which is appropriate for mid-sized
       organization; class-C, with a maximum of 254 host addresses, is
       too small while class-B, which allows up to 65534 addresses, is
       too large to be densely populated.
  1. Growth of routing tables in Internet routers are beyond the

ability of current software (and people) to effectively manage.

  1. Eventual exhaustion of the 32-bit IP address space.
 It has become clear that the first two of these problems are likely
 to become critical within the next one to three years.  Classless
 inter-domain routing (CIDR) attempts to deal with these problems by
 proposing a mechanism to slow the growth of the routing table and the
 need for allocating new IP network numbers. It does not attempt to
 solve the third problem, which is of a more long-term nature, but

Rekhter & Gross [Page 2] RFC 1655 BGP-4 Application July 1994

 instead endeavors to ease enough of the short to mid-term
 difficulties to allow the Internet to continue to function
 efficiently while progress is made on a longer- term solution.
 BGP-4 is an extension of BGP-3 that provides support for routing
 information aggregation and reduction based on the Classless inter-
 domain routing architecture (CIDR) [3].  This memo describes the
 usage of BGP-4 in the Internet.
 All of the discussions in this paper are based on the assumption that
 the Internet is a collection of arbitrarily connected Autonomous
 Systems. That is, the Internet will be modeled as a general graph
 whose nodes are AS's and whose edges are connections between pairs of
 AS's.
 The classic definition of an Autonomous System is a set of routers
 under a single technical administration, using an interior gateway
 protocol and common metrics to route packets within the AS and using
 an exterior gateway protocol to route packets to other AS's. Since
 this classic definition was developed, it has become common for a
 single AS to use several interior gateway protocols and sometimes
 several sets of metrics within an AS. The use of the term Autonomous
 System here stresses the fact that, even when multiple IGPs and
 metrics are used, the administration of an AS appears to other AS's
 to have a single coherent interior routing plan and presents a
 consistent picture of which networks are reachable through it.
 AS's are assumed to be administered by a single administrative
 entity, at least for the purposes of representation of routing
 information to systems outside of the AS.

2. BGP Topological Model

 When we say that a connection exists between two AS's, we mean two
 things:
    Physical connection:  There is a shared network between the two
    AS's, and on this shared network each AS has at least one border
    gateway belonging to that AS. Thus the border gateway of each AS
    can forward packets to the border gateway of the other AS without
    resorting to Inter-AS or Intra-AS routing.
    BGP connection:  There is a BGP session between BGP speakers in
    each of the AS's, and this session communicates those routes that
    can be used for specific networks via the advertising AS.
    Throughout this document we place an additional restriction on the
    BGP speakers that form the BGP connection: they must themselves
    share the same network that their border gateways share. Thus, a

Rekhter & Gross [Page 3] RFC 1655 BGP-4 Application July 1994

    BGP session between adjacent AS's requires no support from either
    Inter-AS or Intra-AS routing. Cases that do not conform to this
    restriction fall outside the scope of this document.
 Thus, at each connection, each AS has one or more BGP speakers and
 one or more border gateways, and these BGP speakers and border
 gateways are all located on a shared network. Note that BGP speakers
 do not need to be a border gateway, and vice versa. Paths announced
 by a BGP speaker of one AS on a given connection are taken to be
 feasible for each of the border gateways of the other AS on the same
 shared network, i.e. indirect neighbors are allowed.
 Much of the traffic carried within an AS either originates or
 terminates at that AS (i.e., either the source IP address or the
 destination IP address of the IP packet identifies a host on a
 network internal to that AS).  Traffic that fits this description is
 called "local traffic". Traffic that does not fit this description is
 called "transit traffic". A major goal of BGP usage is to control the
 flow of transit traffic.
 Based on how a particular AS deals with transit traffic, the AS may
 now be placed into one of the following categories:
    stub AS: an AS that has only a single connection to one other AS.
    Naturally, a stub AS only carries local traffic.
    multihomed AS: an AS that has connections to more than one other
    AS, but refuses to carry transit traffic.
    transit AS: an AS that has connections to more than one other AS,
    and is designed (under certain policy restrictions) to carry both
    transit and local traffic.
 Since a full AS path provides an efficient and straightforward way of
 suppressing routing loops and eliminates the "count-to-infinity"
 problem associated with some distance vector algorithms, BGP imposes
 no topological restrictions on the interconnection of AS's.

3. BGP in the Internet

3.1 Topology Considerations

 The overall Internet topology may be viewed as an arbitrary
 interconnection of transit, multihomed, and stub AS's.  In order to
 minimize the impact on the current Internet infrastructure, stub and
 multihomed AS's need not use BGP.  These AS's may run other protocols
 (e.g., EGP) to exchange reachability information with transit AS's.
 Transit AS's using BGP will tag this information as having been

Rekhter & Gross [Page 4] RFC 1655 BGP-4 Application July 1994

 learned by some method other than BGP. The fact that BGP need not run
 on stub or multihomed AS's has no negative impact on the overall
 quality of inter-AS routing for traffic that either destined to or
 originated from the stub or multihomed AS's in question.
 However, it is recommended that BGP be used for stub and multihomed
 AS's as well. In these situations, BGP will provide an advantage in
 bandwidth and performance over some of the currently used protocols
 (such as EGP).  In addition, this would reduce the need for the use
 of default routes and in better choices of Inter-AS routes for
 multihomed AS's.

3.2 Global Nature of BGP

 At a global level, BGP is used to distribute routing information
 among multiple Autonomous Systems. The information flows can be
 represented as follows:
                  +-------+         +-------+
            BGP   |  BGP  |   BGP   |  BGP  |   BGP
         ---------+       +---------+       +---------
                  |  IGP  |         |  IGP  |
                  +-------+         +-------+
                  <-AS A-->         <--AS B->
 This diagram points out that, while BGP alone carries information
 between AS's, both BGP and an IGP may carry information across an AS.
 Ensuring consistency of routing information between BGP and an IGP
 within an AS is a significant issue and is discussed at length later
 in Appendix A.

3.3 BGP Neighbor Relationships

 The Internet is viewed as a set of arbitrarily connected AS's. BGP
 speakers in each AS communicate with each other to exchange network
 reachability information based on a set of policies established
 within each AS. Routers that communicate directly with each other via
 BGP are known as BGP neighbors. BGP neighbors can be located within
 the same AS or in different AS's. For the sake of discussion, BGP
 communications with neighbors in different AS's will be referred to
 as External BGP, and with neighbors in the same AS as Internal BGP.
 There can be as many BGP speakers as deemed necessary within an AS.
 Usually, if an AS has multiple connections to other AS's, multiple
 BGP speakers are needed. All BGP speakers representing the same AS
 must give a consistent image of the AS to the outside. This requires

Rekhter & Gross [Page 5] RFC 1655 BGP-4 Application July 1994

 that the BGP speakers have consistent routing information among them.
 These gateways can communicate with each other via BGP or by other
 means. The policy constraints applied to all BGP speakers within an
 AS must be consistent. Techniques such as using a tagged IGP (see
 A.2.2) may be employed to detect possible inconsistencies.
 In the case of External BGP, the BGP neighbors must belong to
 different AS's, but share a common network. This common network
 should be used to carry the BGP messages between them. The use of BGP
 across an intervening AS invalidates the AS path information. An
 Autonomous System number must be used with BGP to specify which
 Autonomous System the BGP speaker belongs to.

4. Requirements for Route Aggregation

 A conformant BGP-4 implementation is required to have the ability to
 specify when an aggregated route may be generated out of partial
 routing information. For example, a BGP speaker at the border of an
 autonomous system (or group of autonomous systems) must be able to
 generate an aggregated route for a whole set of destination IP
 addresses (in BGP-4 terminology such a set is called the Network
 Layer Reachability Information or NLRI) over which it has
 administrative control (including those addresses it has delegated),
 even when not all of them are reachable at the same time.
 A conformant implementation may provide the capability to specify
 when an aggregated NLRI may be generated.
 A conformant implementation is required to have the ability to
 specify how NLRI may be de-aggregated.
 A conformant implementation is required to support the following
 options when dealing with overlapping routes:
  1. Install both the less and the more specific routes
  1. Install the more specific route only
  1. Install the less specific route only
  1. Install neither route
 By default a BGP speaker should aggregate NLRI representing subnets
 to the corresponding network.
 Injecting NLRI representing arbitrary subnets into BGP without
 aggregation to the corresponding network shall be controlled via
 configuration parameters.

Rekhter & Gross [Page 6] RFC 1655 BGP-4 Application July 1994

 Certain routing policies may depend on the NLRI (e.g., "research"
 versus "commercial"). Therefore, a BGP speaker that performs route
 aggregation should be cognizant, if possible, of potential
 implications on routing policies when aggregating NLRI.

5. Policy Making with BGP

 BGP provides the capability for enforcing policies based on various
 routing preferences and constraints. Policies are not directly
 encoded in the protocol. Rather, policies are provided to BGP in the
 form of configuration information.
 BGP enforces policies by affecting the selection of paths from
 multiple alternatives and by controlling the redistribution of
 routing information.  Policies are determined by the AS
 administration.
 Routing policies are related to political, security, or economic
 considerations. For example, if an AS is unwilling to carry traffic
 to another AS, it can enforce a policy prohibiting this. The
 following are examples of routing policies that can be enforced with
 the use of BGP:
   1.  A multihomed AS can refuse to act as a transit AS for other
       AS's.  (It does so by only advertising routes to networks
       internal to the AS.)
   2.  A multihomed AS can become a transit AS for a restricted set of
       adjacent AS's, i.e., some, but not all, AS's can use the
       multihomed AS as a transit AS. (It does so by advertising its
       routing information to this set of AS's.)
   3.  An AS can favor or disfavor the use of certain AS's for
       carrying transit traffic from itself.
 A number of performance-related criteria can be controlled with the
 use of BGP:
   1.  An AS can minimize the number of transit AS's. (Shorter AS
       paths can be preferred over longer ones.)
   2.  The quality of transit AS's. If an AS determines that two or
       more AS paths can be used to reach a given destination, that AS
       can use a variety of means to decide which of the candidate AS
       paths it will use. The quality of an AS can be measured by such
       things as diameter, link speed, capacity, tendency to become
       congested, and quality of operation. Information about these
       qualities might be determined by means other than BGP.

Rekhter & Gross [Page 7] RFC 1655 BGP-4 Application July 1994

   3.  Preference of internal routes over external routes.
 For consistency within an AS, equal cost paths, resulting from
 combinations of policies and/or normal route selection procedures,
 must be resolved in a consistent fashion.
 Fundamental to BGP is the rule that an AS advertises to its
 neighboring AS's only those routes that it uses. This rule reflects
 the "hop-by-hop" routing paradigm generally used by the current
 Internet.

6. Path Selection with BGP

 One of the major tasks of a BGP speaker is to evaluate different
 paths to a destination network from its border gateways at that
 network, select the best one, apply appropriate policy constraints,
 and then advertise it to all of its BGP neighbors. The key issue is
 how different paths are evaluated and compared.  In traditional
 distance vector protocols (e.g., RIP) there is only one metric (e.g.,
 hop count) associated with a path. As such, comparison of different
 paths is reduced to simply comparing two numbers. A complication in
 Inter-AS routing arises from the lack of a universally agreed-upon
 metric among AS's that can be used to evaluate external paths.
 Rather, each AS may have its own set of criteria for path evaluation.
 A BGP speaker builds a routing database consisting of the set of all
 feasible paths and the list of networks reachable through each path.
 For purposes of precise discussion, it's useful to consider the set
 of feasible paths for a given destination network. In most cases, we
 would expect to find only one feasible path. However, when this is
 not the case, all feasible paths should be maintained, and their
 maintenance speeds adaptation to the loss of the primary path. Only
 the primary path at any given time will ever be advertised.
 The path selection process can be formalized by defining a complete
 order over the set of all feasible paths to a given destination
 network. One way to define this complete order is to define a
 function that maps each full AS path to a non-negative integer that
 denotes the path's degree of preference. Path selection is then
 reduced to applying this function to all feasible paths and choosing
 the one with the highest degree of preference.
 In actual BGP implementations, the criteria for assigning degree of
 preferences to a path are specified as configuration information.
 The process of assigning a degree of preference to a path can be
 based on several sources of information:

Rekhter & Gross [Page 8] RFC 1655 BGP-4 Application July 1994

   1.  Information explicitly present in the full AS path.
   2.  A combination of information that can be derived from the full
       AS path and information outside the scope of BGP (e.g., policy
       routing constraints provided as configuration information).
 Possible criteria for assigning a degree of preference to a path are:
  1. AS count. Paths with a smaller AS count are generally better.
  1. Policy considerations. BGP supports policy-based routing based

on the controlled distribution of routing information. A BGP

       speaker may be aware of some policy constraints (both within
       and outside of its own AS) and do appropriate path selection.
       Paths that do not comply with policy requirements are not
       considered further.
  1. Presence or absence of a certain AS or AS's in the path. By

means of information outside the scope of BGP, an AS may know

       some performance characteristics (e.g., bandwidth, MTU, intra-
       AS diameter) of certain AS's and may try to avoid or prefer
       them.
  1. Path origin. A path learned entirely from BGP (i.e., whose

endpoint is internal to the last AS on the path) is generally

       better than one for which part of the path was learned via EGP
       or some other means.
  1. AS path subsets. An AS path that is a subset of a longer AS

path to the same destination should be preferred over the

       longer path.  Any problem in the shorter path (such as an
       outage) will also be a problem in the longer path.
  1. Link dynamics. Stable paths should be preferred over unstable

ones. Note that this criterion must be used in a very careful

       way to avoid causing unnecessary route fluctuation. Generally,
       any criteria that depend on dynamic information might cause
       routing instability and should be treated very carefully.

7. Required set of supported routing policies

 Policies are provided to BGP in the form of configuration
 information.  This information is not directly encoded in the
 protocol. Therefore, BGP can provide support for very complex routing
 policies. However, it is not required that all BGP implementations
 support such policies.

Rekhter & Gross [Page 9] RFC 1655 BGP-4 Application July 1994

 We are not attempting to standardize the routing policies that must
 be supported in every BGP implementation; we strongly encourage all
 implementors to support the following set of routing policies:
   1.  BGP implementations should allow an AS to control announcements
       of BGP-learned routes to adjacent AS's.  Implementations should
       also support such control with at least the granularity of a
       single network.  Implementations should also support such
       control with the granularity of an autonomous system, where the
       autonomous system may be either the autonomous system that
       originated the route, or the autonomous system that advertised
       the route to the local system (adjacent autonomous system).
       Care must be taken when a BGP speaker selects a new route that
       can't be announced to a particular external peer, while the
       previously selected route was announced to that peer.
       Specifically, the local system must explicitly indicate to the
       peer that the previous route is now infeasible.
   2.  BGP implementations should allow an AS to prefer a particular
       path to a destination (when more than one path is available).
       At the minimum an implementation shall support this
       functionality by allowing to administratively assign a degree
       of preference to a route based solely on the IP address of the
       neighbor the route is received from. The allowed range of the
       assigned degree of preference shall be between 0 and 2^(31) -
       1.
   3.  BGP implementations should allow an AS to ignore routes with
       certain AS's in the AS_PATH path attribute.  Such function can
       be implemented by using the technique outlined in [2], and by
       assigning "infinity" as "weights" for such AS's. The route
       selection process must ignore routes that have "weight" equal
       to "infinity".

8. Interaction with other exterior routing protocols

 The guidelines suggested in this section are consistent with the
 guidelines presented in [3].
 An AS should advertise a minimal aggregate for its internal networks
 with respect to the amount of address space that it is actually
 using.  This can be used by administrators of non-BGP 4 AS's to
 determine how many routes to explode from a single aggregate.
 A route that carries the ATOMIC_AGGREGATE path attribute shall not be
 exported into either BGP-3 or EGP2, unless such an exportation can be
 accomplished without exploding the NLRI of the route.

Rekhter & Gross [Page 10] RFC 1655 BGP-4 Application July 1994

8.1 Exchanging information with EGP2

 This document suggests the following guidelines for exchanging
 routing information between BGP-4 and EGP2.
 To provide for graceful migration, a BGP speaker may participate in
 EGP2, as well as in BGP-4. Thus, a BGP speaker may receive IP
 reachability information by means of EGP2 as well as by means of
 BGP-4.  The information received by EGP2 can be injected into BGP-4
 with the ORIGIN path attribute set to 1.  Likewise,  the information
 received via BGP-4 can be injected into EGP2 as well. In the latter
 case, however, one needs to be aware of the potential information
 explosion when a given IP prefix received from BGP-4 denotes a set of
 consecutive A/B/C class networks.  Injection of BGP-4 received NLRI
 that denotes IP subnets requires the BGP speaker to inject the
 corresponding network into EGP2.  The local system shall provide
 mechanisms to control the exchange of reachability information
 between EGP2 and BGP-4.  Specifically, a conformant implementation is
 required to support all of the following options when injecting BGP-4
 received reachability information into EGP2:
  1. inject default only (0.0.0.0); no export of any other NLRI
  1. allow controlled deaggregation, but only of specific routes;

allow export of non-aggregated NLRI

  1. allow export of only non-aggregated NLRI
 The exchange of routing information via EGP2 between a BGP speaker
 participating in BGP-4 and a pure EGP2 speaker may occur  only at the
 domain (autonomous system) boundaries.

8.2 Exchanging information with BGP-3

 This document suggests the following guidelines for exchanging
 routing information between BGP-4 and BGP-3.
 To provide for graceful migration, a BGP speaker may participate in
 BGP-3, as well as in BGP-4. Thus, a BGP speaker may receive IP
 reachability information by means of BGP-3, as well as by means of
 BGP-4.
 A BGP speaker may inject the information received by BGP-4 into BGP-3
 as follows.
 If an AS_PATH attribute of a BGP-4 route carries AS_SET path
 segments, then the AS_PATH attribute of the BGP-3 route shall be
 constructed by treating the AS_SET segments as AS_SEQUENCE segments,

Rekhter & Gross [Page 11] RFC 1655 BGP-4 Application July 1994

 with the resulting AS_PATH being a single AS_SEQUENCE. While this
 procedure loses set/sequence information, it doesn't affect
 protection for routing loops suppression, but may affect policies, if
 the policies are based on the content or ordering of the AS_PATH
 attribute.
 While injecting BGP-4 derived NLRI into BGP-3, one needs to be aware
 of the potential information explosion when a given IP prefix denotes
 a set of consecutive A/B/C class networks. Injection of BGP-4 derived
 NLRI that denotes IP subnets requires the BGP speaker to inject the
 corresponding network into BGP-3. The local system shall provide
 mechanisms to control the exchange of routing information between
 BGP-3 and BGP-4.  Specifically, a conformant implementation is
 required to support all of the following options when injecting BGP-4
 received routing information into BGP-3:
  1. inject default only (0.0.0.0), no export of any other NLRI
  1. allow controlled deaggregation, but only of specific routes;

allow export of non-aggregated NLRI

  1. allow export of only non-aggregated NLRI
 The exchange of routing information via BGP-3 between a BGP speaker
 participating in BGP-4 and a pure BGP-3 speaker may occur  only at
 the autonomous system boundaries. Within a single autonomous system
 BGP conversations between all the BGP speakers of that autonomous
 system have to be either BGP-3 or BGP-4, but not a mixture.

9. Operations over Switched Virtual Circuits

 When using BGP over Switched Virtual Circuit (SVC) subnetworks it may
 be desirable to minimize traffic generated by BGP. Specifically, it
 may be desirable to eliminate traffic associated with periodic
 KEEPALIVE messages.  BGP includes a mechanism for operation over
 switched virtual circuit (SVC) services which avoids keeping SVCs
 permanently open and allows it to eliminates periodic sending of
 KEEPALIVE messages.
 This section describes how to operate without periodic KEEPALIVE
 messages to minimise SVC usage when using an intelligent SVC circuit
 manager.  The proposed scheme may also be used on "permanent"
 circuits, which support a feature like link quality monitoring or
 echo request to determine the status of link connectivity.
 The mechanism described in this section is suitable only between the
 BGP speakers that are directly connected over a common virtual
 circuit.

Rekhter & Gross [Page 12] RFC 1655 BGP-4 Application July 1994

9.1 Establishing a BGP Connection

 The feature is selected by specifying zero Hold Time in the OPEN
 message.

9.2 Circuit Manager Properties

 The circuit manager must have sufficient functionality to be able to
 compensate for the lack of periodic KEEPALIVE messages:
  1. It must be able to determine link layer unreachability in a

predictable finite period of a failure occurring.

  1. On determining unreachability it should:
  1. start a configurable dead timer (comparable to a

typical Hold timer value).

  1. attempt to re-establish the Link Layer connection.
  1. If the dead timer expires it should:
  1. send an internal circuit DEAD indication to TCP.
  1. If the connection is re-established it should:
  1. cancel the dead timer.
  1. send an internal circuit UP indication to TCP.

9.3 TCP Properties

 A small modification must be made to TCP to process internal
 notifications from the circuit manager:
  1. DEAD: Flush transmit queue and abort TCP connection.
  1. UP: Transmit any queued data or allow an outgoing TCP call to

proceed.

9.4 Combined Properties

 Some implementations may not be able to guarantee that the BGP
 process and the circuit manager will operate as a single entity; i.e.
 they can have a separate existence when the other has been stopped or
 has crashed.

Rekhter & Gross [Page 13] RFC 1655 BGP-4 Application July 1994

 If this is the case, a periodic two-way poll between the BGP process
 and the circuit manager should be implemented.  If the BGP process
 discovers the circuit manager has gone away it should close all
 relevant TCP connections.  If the circuit manager discovers the BGP
 process has gone away it should close all its connections associated
 with the BGP process and reject any further incoming connections.

10. Conclusion

 The BGP protocol provides a high degree of control and flexibility
 for doing interdomain routing while enforcing policy and performance
 constraints and avoiding routing loops. The guidelines presented here
 will provide a starting point for using BGP to provide more
 sophisticated and manageable routing in the Internet as it grows.

Appendix A. The Interaction of BGP and an IGP

 This section outlines methods by which BGP can exchange routing
 information with an IGP. The methods outlined here are not proposed
 as part of the standard BGP usage at this time.  These methods are
 outlined for information purposes only.  Implementors may want to
 consider these methods when importing IGP information.
 This is general information that applies to any generic IGP.
 Interaction between BGP and any specific IGP is outside the scope of
 this section.  Methods for specific IGP's should be proposed in
 separate documents.  Methods for specific IGP's could be proposed for
 standard usage in the future.

Overview

 By definition, all transit AS's must be able to carry traffic which
 originates from and/or is destined to locations outside of that AS.
 This requires a certain degree of interaction and coordination
 between BGP and the Interior Gateway Protocol (IGP) used by that
 particular AS. In general, traffic originating outside of a given AS
 is going to pass through both interior gateways (gateways that
 support the IGP only) and border gateways (gateways that support both
 the IGP and BGP). All interior gateways receive information about
 external routes from one or more of the border gateways of the AS via
 the IGP.
 Depending on the mechanism used to propagate BGP information within a
 given AS, special care must be taken to ensure consistency between
 BGP and the IGP, since changes in state are likely to propagate at
 different rates across the AS. There may be a time window between the
 moment when some border gateway (A) receives new BGP routing

Rekhter & Gross [Page 14] RFC 1655 BGP-4 Application July 1994

 information which was originated from another border gateway (B)
 within the same AS, and the moment the IGP within this AS is capable
 of routing transit traffic to that border gateway (B). During that
 time window, either incorrect routing or "black holes" can occur.
 In order to minimize such routing problems, border gateway (A) should
 not advertise a route to some exterior network X via border gateway
 (B) to all of its BGP neighbors in other AS's until all the interior
 gateways within the AS are ready to route traffic destined to X via
 the correct exit border gateway (B). In other words, interior routing
 should converge on the proper exit gateway before/advertising routes
 via that exit gateway to other AS's.

A.2 Methods for Achieving Stable Interactions

 The following discussion outlines several techniques capable of
 achieving stable interactions between BGP and the IGP within an
 Autonomous System.

A.2.1 Propagation of BGP Information via the IGP

 While BGP can provide its own mechanism for carrying BGP information
 within an AS, one can also use an IGP to transport this information,
 as long as the IGP supports complete flooding of routing information
 (providing the mechanism to distribute the BGP information) and one
 pass convergence (making the mechanism effectively atomic). If an IGP
 is used to carry BGP information, then the period of
 desynchronization described earlier does not occur at all, since BGP
 information propagates within the AS synchronously with the IGP, and
 the IGP converges more or less simultaneously with the arrival of the
 new routing information. Note that the IGP only carries BGP
 information and should not interpret or process this information.

A.2.2 Tagged Interior Gateway Protocol

 Certain IGPs can tag routes exterior to an AS with the identity of
 their exit points while propagating them within the AS. Each border
 gateway should use identical tags for announcing exterior routing
 information (received via BGP) both into the IGP and into Internal
 BGP when propagating this information to other border gateways within
 the same AS. Tags generated by a border gateway must uniquely
 identify that particular border gateway--different border gateways
 must use different tags.
 All Border Gateways within a single AS must observe the following two
 rules:

Rekhter & Gross [Page 15] RFC 1655 BGP-4 Application July 1994

   1.  Information received via Internal BGP by a border gateway A
       declaring a network to be unreachable must immediately be
       propagated to all of the External BGP neighbors of A.
   2.  Information received via Internal BGP by a border gateway A
       about a reachable network X cannot be propagated to any of the
       External BGP neighbors of A unless/until A has an IGP route to
       X and both the IGP and the BGP routing information have
       identical tags.
 These rules guarantee that no routing information is announced
 externally unless the IGP is capable of correctly supporting it. It
 also avoids some causes of "black holes".
 One possible method for tagging BGP and IGP routes within an AS is to
 use the IP address of the exit border gateway announcing the exterior
 route into the AS. In this case the "gateway" field in the BGP UPDATE
 message is used as the tag.
 An alternate method for tagging BGP and IGP routes is to have BGP and
 the IGP agree on a router ID.  In this case, the router ID is
 available to all BGP (version 3 or higher) speakers.  Since this ID
 is already unique it can be used directly as the tag in the IGP.

A.2.3 Encapsulation

 Encapsulation provides the simplest (in terms of the interaction
 between the IGP and BGP) mechanism for carrying transit traffic
 across the AS. In this approach, transit traffic is encapsulated
 within an IP datagram addressed to the exit gateway. The only
 requirement imposed on the IGP by this approach is that it should be
 capable of supporting routing between border gateways within the same
 AS.
 The address of the exit gateway A for some exterior network X is
 specified in the BGP identifier field of the BGP OPEN message
 received from gateway A via Internal BGP by all other border gateways
 within the same AS. In order to route traffic to network X, each
 border gateway within the AS encapsulates it in datagrams addressed
 to gateway A. Gateway A then performs decapsulation and forwards the
 original packet to the proper gateway in another AS.
 Since encapsulation does not rely on the IGP to carry exterior
 routing information, no synchronization between BGP and the IGP is
 required.

Rekhter & Gross [Page 16] RFC 1655 BGP-4 Application July 1994

 Some means of identifying datagrams containing encapsulated IP, such
 as an IP protocol type code, must be defined if this method is to be
 used.
 Note that, if a packet to be encapsulated has length that is very
 close to the MTU, that packet would be fragmented at the gateway that
 performs encapsulation.

A.2.4 Pervasive BGP

 If all routers in an AS are BGP speakers, then there is no need to
 have any interaction between BGP and an IGP.  In such cases, all
 routers in the AS already have full information of all BGP routes.
 The IGP is then only used for routing within the AS, and no BGP
 routes are imported into the IGP.
 For routers to operate in this fashion, they must be able to perform
 a recursive lookup in their routing table.  The first lookup will use
 a BGP route to establish the exit router, while the second lookup
 will determine the IGP path to the exit router.
 Since the IGP carries no external information in this scenario, all
 routers in the AS will have converged as soon as all BGP speakers
 have new information about this route.  Since there is no need to
 delay for the IGP to converge, an implementation may advertise these
 routes without further delay due to the IGP.

A.2.5 Other Cases

 There may be AS's with IGPs which can neither carry BGP information
 nor tag exterior routes (e.g., RIP). In addition, encapsulation may
 be either infeasible or undesirable. In such situations, the
 following two rules must be observed:
   1.  Information received via Internal BGP by a border gateway A
       declaring a network to be unreachable must immediately be
       propagated to all of the External BGP neighbors of A.
   2.  Information received via Internal BGP by a border gateway A
       about a reachable network X cannot be propagated to any of the
       External BGP neighbors of A unless A has an IGP route to X and
       sufficient time has passed for the IGP routes to have
       converged.
 The above rules present necessary (but not sufficient) conditions for
 propagating BGP routing information to other AS's. In contrast to
 tagged IGPs, these rules cannot ensure that interior routes to the
 proper exit gateways are in place before propagating the routes to

Rekhter & Gross [Page 17] RFC 1655 BGP-4 Application July 1994

 other AS's.
 If the convergence time of an IGP is less than some small value X,
 then the time window during which the IGP and BGP are unsynchronized
 is less than X as well, and the whole issue can be ignored at the
 cost of transient periods (of less than length X) of routing
 instability. A reasonable value for X is a matter for further study,
 but X should probably be less than one second.
 If the convergence time of an IGP cannot be ignored, a different
 approach is needed. Mechanisms and techniques which might be
 appropriate in this situation are subjects for further study.

References

 [1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4), RFC
     1654, cisco Systems, T.J. Watson Research Center, IBM Corp., July
     1994.
 [2] Braun, H-W., "Models of Policy Based Routing", RFC 1104,
     Merit/NSFNET, July 1989.
 [3] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting:  an
     Address Assignment and Aggregation Strategy", RFC 1519, BARRNet,
     cisco, MERIT, OARnet, September 1993.

Rekhter & Gross [Page 18] RFC 1655 BGP-4 Application July 1994

Security Considerations

 Security issues are not discussed in this memo.

Authors' Addresses

 Yakov Rekhter
 T.J. Watson Research Center IBM Corporation
 P.O. Box 218
 Yorktown Heights, NY 10598
 Phone:  (914) 945-3896
 EMail: yakov@watson.ibm.com
 Phill Gross
 Director of Broadband Engineering
 MCI Data Services Division
 2100 Reston Parkway, Room 6001
 Reston, VA 22091
 Phone: +1 703 715 7432
 Fax: +1 703 715 7436
 EMail: 0006423401@mcimail.com
 IETF BGP WG mailing list: bgp@ans.net
 To be added: bgp-request@ans.net

Rekhter & Gross [Page 19]

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