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rfc:rfc2791

Network Working Group J. Yu Request for Comments: 2791 CoSine Communications Category: Informational July 2000

                 Scalable Routing Design Principles

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

 Routing is essential to a network. Routing scalability is essential
 to a large network. When routing does not scale, there is a direct
 impact on the stability and performance of a network. Therefore,
 routing scalability is an important issue, especially for a large
 network. This document identifies major factors affecting routing
 scalability as well as basic principles of designing scalable routing
 for large networks.

Yu Informational [Page 1] RFC 2791 Scalable Routing Design Principles July 2000

Table of Contents

 1           Introduction  ..................................      2
 2           Common Routing Design Goals  ...................      3
 3           Characteristics of Today's Large Networks  .....      3
 4           Routing Scaling Issues  ..........................    3
 4.1         Router Resource Consumption  .....................    4
 4.2         Routing Complexity  ..............................    5
 5           Routing Protocol Scalability .....................    6
 5.1         IS-IS and OSPF  ..................................    6
 5.2         BGP  .............................................    8
 6           Scalable Routing Design Principles  ..............    9
 6.1         Building Hierarchy  ..............................   10
 6.2         Compartmentalization  ............................   13
 6.3         Making Proper Trade-offs  ........................   13
 6.4         Reduce Burdens of Routing Information Process  ...   14
 6.4.1       Routing Intelligence Placement  ..................   14
 6.4.2       Reduce Routes and Routing Information  ...........   15
 6.4.2.1     CIDR and Route Aggregation  ......................   15
 6.4.2.2     Utilize Default Routing where it's Possible  .....   15
 6.4.2.3     Reduce Alternative Paths  ........................   16
 6.4.3       Use Static Route at Edge  .........................  16
 6.4.4       Minimize the Impact of Route Flapping  ............  16
 6.5         Scalable Routing Policy and Scalable Implementation  17
 6.6         Out-of-band Process  ..............................  19
 7           Conclusion and Discussion  ........................  19
 8           Security Considerations  ..........................  20
 9           Acknowledgement  ..................................  21
 10          References  .......................................  21
 Author's Address ..............................................  22
 Appendix A  Out-of-Band Routing Processes  ....................  23
 Full Copyright Statement  .....................................  26

1. Introduction

 Routing is essential to a network. Without routing, packets cannot be
 delivered to desired destinations and the network would be non-
 functional. The challenge of designing the routing for a large
 network, such as a large ISP backbone network, is not only to make it
 work, but also to make it scale. Without a scalable routing system, a
 network may suffer from severe performance penalties, as
 unfortunately proven by disastrous events in large networks. This
 document attempts to analyze routing scalability issues and define a
 set of principles for designing scalable routing system for large
 networks.
 The organization of this document is as follows: Section 2 describes
 routing functions and design goals. Sections 3 and 4 discuss the

Yu Informational [Page 2] RFC 2791 Scalable Routing Design Principles July 2000

 characteristics of today's large networks and the associated routing
 scaling issues. Section 5 explores routing protocol scalability, and
 Section 6 presents scalable routing design principles. Section 7
 provides a conclusion to the document.

2. Common Routing Design Goals

 The basic goals a routing system should achieve are as follows:
    o Stability
    o Redundancy and robustness
    o Reasonable convergency time
    o Routing information integrity
    o Sensible and manageable routing policy
 The challenge of designing routing in a large network is not only to
 achieve these basic goals but also to make the routing system scale.

3. Characteristics of Today's Large Networks

 Today's large networks typically possess the following features:
    o They are composed of a large number of nodes (routers and/or
      switches), typically in the hundreds. Some provider networks
      include customer CPE routers within their administrative domain,
      which increases the number of nodes to thousands.
    o They have rich connectivity to meet redundancy and robustness
      requirements, and they consequently have complex topologies.
    o They are default-free; that is, they carry all the routes known
      to the entire Internet. Currently, the total number is
      approximately 70,000.
    o The customer aggregation routers inside the large networks
      connect sometimes hundreds of customer routers.
 These characteristics impose a direct challenge to the routing
 scalability of the network.

4. Routing Scaling Issues

 Today, the main issues surrounding routing scaling are: i) excessive
 router resource consumption, which can potentially increase routing
 convergency difficulties thus destabilize a network; and ii) routing
 complexity, resulting in poor management of network, producing low
 service quality.

Yu Informational [Page 3] RFC 2791 Scalable Routing Design Principles July 2000

4.1. Router Resource Consumption

 The routing process puts bursty loads on routers, especially under
 unstable network conditions. In the extreme case, the routing process
 takes all available resources from the routers, which results in slow
 routing convergence or no convergence. A network is paralyzed when it
 cannot converge internal routing information.
 It's worthy noting that routers with internal architectures that
 tightly couple forwarding and routing processes tend to handle the
 excessive routing load poorly. The emerging new generation of routers
 with the architecture of separating resource used for forwarding and
 routing could provide better routing scalability.
 Today, a large network typically employs IS-IS [1,2] or OSPF [3] as
 an Interior Routing Protocol(IGP) and BGP [4] as an Exterior Routing
 Protocol(EGP), respectively. The IGP calculates paths across the
 interior of the network. BGP facilitates routing exchange between
 routing domains, or Autonomous Systems (AS). BGP also processes and
 propagates external routing information within the network. The
 presence of a large number of routers and adjacencies in a network,
 coupled with frequent topology changes due to link instability, will
 contribute to excessive resource consumption by the interior routing.
 In the case of exterior routing, a large quantity of routers in a BGP
 system plus frequent routing updates (route flapping) would put a
 heavy burden on the routers. Section 5 describes scaling issues with
 IS-IS, OSPF and BGP in detail.
 In addition, having many destinations in a routing system, combined
 with multiple paths associated with these routes, impose the
 following scaling issues on BGP:
    o A large number of routes combined with multiple paths for each
      increases the cost of routing processing for route selection,
      routing policy application and filtering.
    o Too many routes combined with multiple paths requires large
      amounts of memory on routers for storage. The demand is even
      higher at InterExchange Points such as NAPs.
    o The larger the number of routes, the greater the chance route
      flapping will occur and the more BGP routing updates will happen
      as a result. Based on statistics collected by [5], thousands of
      BGP updates in a measured 15 minute interval can occur on a
      typical default-free router at a NAP.

Yu Informational [Page 4] RFC 2791 Scalable Routing Design Principles July 2000

      Route flapping refers to frequent routing updates occurring due
      to network instability, for example, when the state of a
      physical link in the network is fluctuating, or when a BGP
      session is torn down and re-established numerous time within a
      short period of time.
      To facilitate fast convergence, topology change information must
      be propagated in a timely fashion. When a route becomes
      unavailable and is withdrawn, the information is typically sent
      immediately. If the affected routes have been announced to the
      global Internet, the update information is likely to be
      propagated to the entire Internet.
      Route flapping has a profound impact on routers running BGP. The
      routers have to process routing information frequently and this
      consumes a tremendous amounts of the available resources. When a
      local route or link is oscillating, interior routing is affected
      as well by excessive topology information flooding and
      subsequent shortest path calculations. However, OSPF (or IS-IS)
      imposes rate limits on such activity to reduce the burden on the
      routers. For example, OSPF specifies that an individual SLA can
      be updated at most once every 5 seconds. This essentially
      dampens the flapping.
 Moreover, large numbers of E-BGP sessions processed by a single
 router create another potential scaling issue. Large networks usually
 have huge customer subscriptions and connections. To scale the
 hardware and the number of nodes in the network, providers tend to
 dedicate a group of customer aggregation routers, each connecting as
 many customer CPE routers as possible. As a result, it's not uncommon
 for a customer aggregation router to handle hundreds of E-BGP
 sessions, which imposes potential problems, such as BGP session
 processing and maintenance, route processing, filtering and route
 storage.

4.2. Routing Complexity

 Routing complexity can lead to network management difficulties, which
 will have an impact on trouble shooting and quick problem resolution.
 It can result in a less than desirable service quality across the
 network. Complicated routing policies and special cases or exceptions
 in a routing design can contribute to routing complexity in a large
 system.
 Routing Policy refers to the administrative criteria for handling
 routing information, commonly in the form of routing path selection
 and route filtering. The way routing information is handled has a
 direct impact on traffic flow within a network and across domains. As

Yu Informational [Page 5] RFC 2791 Scalable Routing Design Principles July 2000

 a result, it affects business agreements among different networks.
 Therefore, the determination of routing policy is largely dominated
 by non-technical concerns, such as business considerations. Routing
 policy can be very complex, which would make management and
 configuration an unscalable task.
 The keys to reducing routing complexity are systematic as well as
 consistent routing scheme and a routing policy that is simple but
 meets the requirement of administrative polices.
 Another factor contributing to the complexity of routing management
 is prefix-based route filtering. As is well known, prefix-based
 filtering is necessary in order to protect the integrity of the
 routing system. This becomes a challenge when the number of routes
 known to the Internet is as large as it is today.

5. Routing Protocol Scalability

 Today's commonly deployed routing protocols are IS-IS or OSPF for
 Interior routing (aka IGP) and BGP for exterior routing (aka EGP). In
 terms of scaling and other aspects, these protocols are already an
 improvement over the previous generation of protocols, such as RIP
 and EGP. However, scalability is still a major issue when a network
 is large, when a routing design is insensitive to scaling issues, or
 the protocol implementation is inefficient.

5.1. IS-IS and OSPF

 As described earlier in the document, IS-IS and OSPF are Link State
 routing protocols. The basic components of a link state routing
 protocol are i) generation and maintenance of a Link-State-DataBase
 (LSDB) that describes the routing topology of a given routing area;
 and ii) route calculation based on the topology information in the
 database. Each node in a routing area is responsible for describing
 its local routing topology in a Link State Advertisement or LSA (LSP
 in the case of IS-IS.) Each individually generated LSA will be
 distributed or flooded to all the routers in the area. Each router
 receives LSAs from all the other routers, forming a link-state-
 database that reflects the routing topology of the entire routing
 area.
 The main associated scaling issues are the complexity of the link
 state flooding and routing calculation, plus the size of the LSDB
 which contributes to the cost of routing calculation and router
 memory consumption.

Yu Informational [Page 6] RFC 2791 Scalable Routing Design Principles July 2000

 Flooding is the process by which a router distributes its self-
 originated LSA to the rest of the routers in the area in case of any
 link state change. A router will send the LSA via all its interfaces.
 When receiving an LSA update, a router validates the information and
 updates its local LSDB before sending it out via all its own
 interfaces, except the one from which it received the original LSA
 update. Given the nature of IS-IS or OSPF flooding, a full-mesh
 network with N routers would have O(N^2) of LSAs flooded in the
 network when a single link failure occurs. A single router outage
 would cause LSA in the order of O(N^3) to be flooded in the system.
 In the case of OSPF, the protocol will refresh or flood every 30
 minutes even under stable network conditions, which could increase
 the problem for an already highly loaded router.
 From the above discussion, one can easily observe that the more
 routers and adjacencies in a Link State IGP routing area, the more
 CPU burden there are for each router to bear. When a network is
 unstable, the load will be amplified.
 A link-state protocol typically uses Dijkstra's Shortest Path First
 (SPF) algorithm for route calculation. The Dijkstra algorithm scales
 to the order of O(N^2), where N is the number of nodes. The algorithm
 could be improved to the order of O(l*logN) where l is the number of
 links in the network and N is the number of destinations or routers
 [6].
 Consequently, link state routing protocols do not scale to a network
 topology with many routers and excessive adjacencies in an area. When
 the network topology is unstable, the computation, processing and
 bandwidth costs are magnified, which causes excessive consumption of
 router resources. When the instability prevents IS-IS or OSPF from
 maintaining adjacencies, a network routing meltdown occurs.
 Node adjacencies are discovered and maintained through the exchange
 of HELLO messages sent periodically from each node. When a node fails
 to receive HELLO messages from its neighbor within a certain period
 of time (40 seconds for OSPF and less for IS-IS), it considers the
 neighbor down. When heavy flooding, re-calculation and other
 activities happen that make router CPU a scarce resource, a router
 may not be able to allocate CPU time to send or process HELLO
 packets. Routers in the network then lose adjacency, which magnifies
 the instability. As a result, an isolated instability can escalate to
 a routing failure across the entire network.
 Link-state IGPs also do not scale well to carry a large number of
 routes such as the 70,000 routes known to the Internet today. Since
 external routes are included in the link-state-database and in LSA

Yu Informational [Page 7] RFC 2791 Scalable Routing Design Principles July 2000

 (LSP for IS-IS) updates, the link bandwidth and router memory
 consumption will be tremendous. Moreover, due to the large size of
 LSA updates, it would aggravate router resource consumption in the
 process of LSA flooding, especially under unstable network condition.
 To summarize, a scalable design should avoid inclusion of too many
 routers in an IGP routing area, a large external routes carried by
 IGP and, more important, excessive adjacencies in the area.

5.2. BGP

 BGP is an inter-domain routing protocol allowing the exchange of
 routing or reachability information between different Autonomous-
 System networks. Functionally, BGP is composed of External BGP(E-BGP)
 and Internal BGP(I-BGP). E-BGP is used for exchanging external routes
 while I-BGP is typically used for distributing externally learned
 routes within an AS.
 The general costs of BGP are as follows:
    o CPU consumption in BGP session establishment, route selection,
      routing information processing, and handling of routing updates
    o Router memory to install routes and multiple paths associated
      with the routes.
 The major scaling issue associated with BGP lie in the full mesh I-
 BGP connections. Since it does not scale for an IGP to carry
 externally learned prefixes, as mentioned in the previous section,
 I-BGP assumes this duty. In order to prevent routing loops, prefixes
 learned via I-BGP are prohibited from being advertised to another I-
 BGP speaker. As a result, a full mesh of I-BGP sessions among the
 routers within an AS is required. In an AS with N routers, each
 router will have to establish I-BGP sessions with N-1 routers, and
 the system complexity is in the order of O(N^2). Therefore, BGP
 scales poorly when the number of routers involved in I-BGP mesh is
 large.
 A large network normally learns all the routes known to the Internet,
 which is approximately 70,000. I-BGP will need to carry all these
 routes.
 The large number of I-BGP sessions and routes consumes tremendous
 resources from each router, especially during BGP session
 establishment and during periods of heavy route flapping.

Yu Informational [Page 8] RFC 2791 Scalable Routing Design Principles July 2000

 Frequent routing updates are another potential scaling problem in
 large networks. BGP uses incremental updates and sends out routing
 information about unreachable routes quickly for fast convergence.
 This is a great improvement from EGP, in which the whole routing
 table is updated at a fixed time interval. However, when a network is
 unstable the updates, especially those containing route withdrawals,
 are sent immediately, causing global BGP updates. As a result,
 network instability initiated anywhere in a network triggers updates
 all over the Internet. This effect is magnified when large amounts of
 routes are visible to the Internet, putting a heavy load on routers
 that participate in BGP.
 The introduction of a routing hierarchy in BGP, through I-BGP Route
 Reflectors [7] and BGP Confederations [8], for example, will help
 alleviate the scaling problem caused by the requirement of full mesh
 I-BGP establishment.
 Another potential solution is to avoid the requirement of full mesh
 pairwise I-BGP connections. This will change the way that BGP
 distributes routing information among the I-BGP peers. Mechanisms
 worth considering are using multicast to distribute information or
 adopting flooding mechanisms similar to those used in IS-IS or OSPF.
 Further investigation of the implication of using such mechanism for
 BGP route distribution is needed.
 Route dampening [9] is one way to reduce excessive updates triggered
 by route flapping. The trade-off between fast convergence and
 stability of the network should be considered, as discussed in
 section 6.3.

6. Scalable Routing Design Principles

 The routing design for a large-scale network should achieve the basic
 goals of accuracy, stability, redundancy and convergence as described
 in Section 2 and moreover should achieve it in a scalable fashion.
 How routing scales is influenced by protocol design decisions,
 protocol implementation decisions, and network design decisions. A
 network engineer has direct control over network design decisions and
 can have substantial influence over protocol design and
 implementation. The focus of this document is network design
 decisions.

Yu Informational [Page 9] RFC 2791 Scalable Routing Design Principles July 2000

 Following is a set of design principles for making a large network
 routing system more scalable:
    o Building hierarchy
    o Compartmentalization
    o Making proper trade-offs
    o Reducing route processing burdens
    o Defining scalable routing policies and implementation
    o Utilizing out-of-band routing assistance

6.1. Building Hierarchy

 As discussed in Section 5.1, OSPF and IS-IS scale poorly when a
 network has a large number of routers and in particular, a large
 quantity of adjacencies. This has unfortunately been proven by
 networks that deploy IP over ATM with full mesh adjacencies among the
 routers. The full mesh overlay design combined with the inefficient
 protocol implementation led to disastrous network outages. A lesson
 learned from this is to avoid full mesh overlay topology in a large
 network with a large, flat network routing structure.
 Building hierarchical routing structures in the network is the key to
 achieving routing scalability in a large network. As discussed
 earlier in this document, large networks are usually composed of many
 routers with a complex topology, which results in a large number of
 adjacencies. As also discussed earlier, currently available routing
 protocols scale poorly for handling a large number of routers in a
 routing domain or many adjacencies among the routers. Therefore, it
 is sensible to build a routing hierarchy to reduce the number of
 routers as well as the number of adjacencies in a routing domain.
 The current common practice is to build a two-tiered hierarchy in a
 network with a center component (or transit core network) to which a
 number of outskirt components (or access networks) attach. The
 transit core network covers the entire geographical area the network
 serves; each access network (aka regional network) covers one region.
 There are usually no direct link connections among the regional
 components. Traffic from one regional network to another traverses
 the transit core. Customer networks connect only to access or
 regional networks. There are a number of ways to build a routing
 hierarchy in the above described hierarchical network topology.
    1) Completely Separate Routing Domains
    This design treats the transit core network and each regional
    network as completely independent ASs with respect to routing, and
    each AS runs an independent IGP. Each regional network E-BGP with
    the transit core for exchanging routing knowledge. Full I-BGP

Yu Informational [Page 10] RFC 2791 Scalable Routing Design Principles July 2000

    connections need to be established only within each component
    network. With this design, the maximum number of routers in an IGP
    domain is the total number of routers in each component. As a
    result, the IGP processing load is reduced, and the number of
    routers in an I-BGP mesh in the network routing system is
    decreased dramatically.
    Another advantage of this design is that it compartmentalizes the
    routing system so that instability in one such component has less
    impact on the entire system. See the discussion in section 6.2.
    The main disadvantage of this scheme is that it inserts one extra
    AS in the routing path when routes are advertised to the Internet
    via BGP. This extra AS in the path may cause route selection
    difficulties for other providers.
    2) One Domain with IGP and BGP Hierarchy
    This method includes the transit core and each regional network
    into one AS domain. The routing hierarchy is realized by utilizing
    multi-level IS-IS or OSPF areas and either BGP Confederation or
    I-BGP Reflector or a combination of the two.
    This mechanism avoids the introduction of an extra AS in the
    routing path, which is an advantage over the method described in
    Point 1).  However, multi-area hierarchical IGP is rarely used
    now-a-days in large networks since most of them are using IS-IS
    for internal routing, which does not have sufficient multi-level
    support. Although IS-IS supports multi-area routing, it imposes a
    strict hierarchy between backbone and sub-areas and allows only
    the advertisement of a default route from the backbone area to the
    sub-areas instead of specific prefixes. This restriction may be
    suitable for a network with a simple sub-area topology. A sub-area
    in a large network, typically a regional or access network, itself
    has a complicated topology. Receiving highly abstract routing
    information, such as a default route, would affect the sub-area's
    ability to make route selections required for traffic engineering.
    It would also limit the information passed to external ASs, for
    example, IGP-derived BGP Multi-Exit-Discriminator (MED)
    information.
    Efforts are being made to modify the IS-IS protocol to allow the
    distribution of specific route from backbone area to sub-areas. A
    mechanism facilitates such distribution is specified in [15]. When
    implementation of such mechanism become available, implementing
    multi-level IGP will be an attractive option for building routing
    hierarchy within a large network.

Yu Informational [Page 11] RFC 2791 Scalable Routing Design Principles July 2000

    3) One IGP Area with BGP Hierarchy
    In lieu of multi-area IS-IS, the routing hierarchy could be
    achieved by defining one IGP domain for the entire network while
    employing a BGP hierarchy. Fortunately, the hierarchical topology
    of the network in this case helps reduce adjacencies in the
    routing domain (recall there are no connections among the second-
    level network components). In addition, improvements could be made
    to further reduce the adjacency by carefully arranging the
    adjacencies to keep them at a minimum but still achieve good
    redundancy. However, this is less than ideal since the number of
    routers remains unchanged, which increases the load on the SPF
    calculation. Moreover, instability within any regional network
    would still affect the entire network (that is, there would be no
    fault isolation).
    Even with one IGP domain, it is possible to build BGP hierarchy to
    make I-BGP more scalable in the network. BGP Reflectors and BGP
    Confederations are existing mechanisms to address the scaling
    problem of full-mesh I-BGP.
    Further, a BGP reflector provides the ability to build more than
    two levels of hierarchy, as long as the interactions among the
    different levels of the hierarchy are carefully arranged to avoid
    the possibility of creating routing loops.
 Questions worth asking are: "Are two levels of routing hierarchy
 sufficient for handling scaling issues?" "Is there really a need for
 more than two levels of hierarchy?"
 When a second-tier sub-domain of a large network, such as a regional
 network, grows too big for routing protocols to handle, either
 another layer of hierarchy needs to be introduced or the sub-domain
 needs to be split into multiple second-tiered sub-domains.
 Keeping two levels of hierarchy and adding more sub-domains appears
 to be more manageable than adding another level to the hierarchy.
 However, one concern is to avoid adding more nodes to the top-level
 or transit core network to make it less scalable. Connecting the
 split sub-areas to the same core router would eliminate the need to
 add more nodes in the core area than is recommended.
 Having more than two levels of hierarchy would exceed the capability
 of IGPs as they are defined today. In OSPF, for example, all the
 areas must be connected via the backbone area, which eliminates the
 possibility of having more than two levels of hierarchy. IS-IS has
 the same limitation. Therefore, the protocols need to be redefined
 should more than two hierarchical layers in IGP be desirable.

Yu Informational [Page 12] RFC 2791 Scalable Routing Design Principles July 2000

 The complexity of protocols and management will increase with the
 number of levels added to the hierarchy. According to [6], most of
 the OSPF protocol bugs found over the years are related to routing
 area support. Because the interaction among the multiple levels
 increases management and debugging complexity, it is desirable to
 keep the levels within a hierarchy to a minimum.

6.2. Compartmentalization

 A scalable routing design of a large network should be able to
 localize problems or failures, thus preventing them from spreading to
 the entire network, consuming resources of network routers, and
 causing network wide instability. This is compartmentalization.
 Network compartmentalization makes fault isolation possible which
 contributes the stability of a large network.
 To achieve compartmentalization in routing design for a large
 network, one needs to avoid a design where the whole large network is
 one flat routing system or routing domain. This is the reason for the
 architecture of dividing interior and exterior routing in the global
 routing system. Within a network, it is best to divide the network
 into multiple routing domains or multiple routing areas. For example,
 in OSPF, only summary route SLAs, rather than individual area routes,
 are flooded beyond the area. When an area border router aggregates
 the routes in its sub-area, instability of any route included in the
 summary route would not cause flooding of SLAs to other areas. As a
 result, router resources in other areas would not be consumed for
 handling flooding and the SPF recalculation. In other words,
 instability within each individual area would be prevented from
 spreading to the entire routing domain.
 Since building a routing hierarchy essentially divides a big routing
 area into smaller areas or domains, it help achieve the goal of
 compartmentalization.

6.3. Making Proper Trade-offs

 When designing routing for a large network, the overall goal should
 be set with considerations of routing scalability and stability. The
 trade-offs between conflicting goals should be taken into account.
 Examples of such trade-offs are redundancy vs. scalability and
 convergence vs. stability.
 Redundancy introduces complexity and increased adjacencies to the
 network topology. Redundancy also imposes the need for as many
 alternative paths as possible for each route, which increases route

Yu Informational [Page 13] RFC 2791 Scalable Routing Design Principles July 2000

 processing and storage burdens. Because of these problems, it may be
 necessary to sacrifice absolute redundancy in favor of a reasonable
 level that scales better for the routing system.
 Fast convergence requires that changes in network topology be
 propagated to the network as quickly as possible. Such action
 increases routing updates and, consequently, the route processing
 burden. The burden is aggravated when a network carries full Internet
 routing information, as large networks usually do, and topology
 changes happen frequently. Route dampening may be necessary to
 achieve stability at the expense of absolute fast convergence.

6.4. Reduce Burdens of Routing Information Processing

 The tasks of reducing routing processing burdens includes: i)
 strategically place the routing intelligence within the network, ii)
 avoid carrying unnecessary routing information and iii) reduce the
 impact of route flapping.

6.4.1. Routing Intelligence Placement

 A router that executes routing policies, performs route filtering and
 dampening is said to posses routing intelligence. Routing
 intelligence is needed for a network i) to enforce the business
 agreement between network entities in the form of routing policies;
 ii) to protect the integrity of the routing information within the
 network and sometimes iii) to shield a network from instability
 happening elsewhere in the Internet.
 The more routing intelligence a router has, the more resources of the
 router are needed to perform those tasks. It is logical, then, to
 place as little routing intelligence as possible on routers that
 already are heavily burdened with other tasks.
 Usually, traffic is heavily concentrated in the core of the network.
 Because traffic aggregates from the edge of the network toward the
 core, traffic is less concentrated near the edge of the network.
 Consequently, to build a scalable routing system, it is wise to place
 routing intelligence at the edge of the network, especially in the
 networks deployed with routers that do not sufficiently decouple
 forwarding and routing. In addition, pushing routing intelligency as
 close to the edge of the network as possible also serves the purpose
 of distributing computational and configuration burdens across all
 routers.
 It is also desirable to move the heavy burden of processing routes to
 out-of-band processors, freeing more resources in network routers for
 packet forwarding and handling.

Yu Informational [Page 14] RFC 2791 Scalable Routing Design Principles July 2000

6.4.2. Reduce Routes and Routing Information

 As discussed in Section 4.1, a large number of routes in the system
 is one of the major culprits in route scaling problems. Therefore, it
 is best to reduce the number of routes in the system without losing
 necessary routing information.

6.4.2.1. CIDR and Route Aggregation

 CIDR as specified in [10] provides a mechanism to aggregate routes
 for efficiently utilizing IP address space as well as reducing the
 number of routes in the global routing table. CIDR offers a way to
 summarize routing information, which is one of the keys for routing
 scalability in today's Internet.
 Route aggregation would not only help global Internet scalability but
 would also contribute to scalability in local networks. The overall
 goal is to keep the routes in the backbone to a minimum.
 To achieve better aggregation within the network; that is, to reduce
 the number of routes in the network, a block of consecutive IP
 addresses should be allocated to each access or regional network so
 that when a regional network announces its routes to the transit core
 network, they can be aggregated. This way, the core and other
 regional networks would not need to know the specific prefixes of any
 particular access network. Although assignment of customer addresses
 from a provider block would have to be planned to support
 aggregation, the effort would be worthwhile.

6.4.2.2. Utilize Default Routing When Possible

 The use of a default route achieves ultimate route summarization,
 which reduces routing information to minimum. Route summarization
 also masks the instability associated with an individual route, for
 example, in the case of route flapping. It's beneficial for a network
 to utilize default routing when appropriate. For example, if a
 second-tiered regional network is a stub and there is no connected
 customer requesting full Internet routing information, the regional
 network can simply point default to its connected core network.
 However, over-summarization of routing information has the danger of
 losing routing granularity and as a result, management of network
 such as traffic engineering would be adversely affected. Therefore,
 caution needs to be exercised when using default routing.

Yu Informational [Page 15] RFC 2791 Scalable Routing Design Principles July 2000

6.4.2.3. Reduce Alternative Paths

 Due to the requirement of reliability, the connectivity in the
 Internet is rich, resulting in many paths toward a particular
 destination. In other words, there are many alternate paths in the
 BGP routing table towards the same destination, which consumes router
 memory and adds to the routing processing burden.
 To make routing scale, it is desirable to reduce alternate paths
 while preserving reasonable redundancy. For example, on a given
 border router (such as a NAP router), one primary path plus an
 alternate path should provide reasonable redundancy. In this case, a
 third or a fourth alternate route could be discarded for the sake of
 scaling.  This is a trade-off decision every network administrator
 needs to make based on the particular needs of her network.

6.4.3. Use Static Route at Edges

 As mentioned earlier, one of the scaling issues in large networks is
 that a single router may fan out to hundreds of customer routers. As
 a result, resource consumption will be very intensive if all the
 customer routers communicate via BGP with the edge router. Is it
 necessary for the edge router to BGP with all of its attached
 customer routers?
 At first glance, it seems necessary for a customer network in a
 different Autonomous System(AS) to exchange routing information with
 the provider network via BGP. However, this is not necessarily the
 case. When a customer network is single-homed (that is, if the sole
 network connection for a customer is via its provider network), BGP
 is not necessary and static routing can work. Since the customer
 network is single-homed, static routing will not have any negative
 impact on services. The advantages are that the customer aggregation
 router will have fewer E-BGP sessions to handle, and no route
 flapping can result from the statically configured customer routes.
 Configuration of the customer's static routes on the provider's
 aggregation router may add management overhead, especially if a
 customer advertises a large number of routes. On the other hand, the
 set of routes a customer announces to the provider usually changes
 infrequently; thus it requires low maintenance once it is configured.

6.4.4. Minimize the Impact of Route Flapping

 As discussed earlier, route flapping is largely caused by link
 instability and/or BGP session instability that results in excessive
 routing updates across the Internet. Route flapping can originate
 anywhere in the global Internet and affect every network in the

Yu Informational [Page 16] RFC 2791 Scalable Routing Design Principles July 2000

 Internet routing mesh (BGP mesh). Given that there are over 70,000
 routes known to the Internet and there is little isolation for route
 flapping, handling route flapping could be overwhelming to routers in
 any network.
 One way to reduce the effect of route flapping is to turn on route
 dampening as specified in [10]. Essentially, dampening suppresses an
 unstable route until it becomes stable. The current practice is for
 each ISP to enable route dampening on its border routers. This way,
 excessive routing updates can be stopped at the border.
 An ideal model is to suppress the announcement of a flapping route
 right at the source. One way to implement this is to have a router
 recognize instability associated with its directly connected links
 and suppress the announcement of the route. So far, there is no such
 implementation. This approach should be explored.
 Route aggregation often masks route flapping since components of an
 aggregated route (more specific routes) would not cause the
 aggregated route to flap. Therefore using CIDR can also help to
 alleviate route flapping.

6.5. Scalable Routing Policy and Scalable Implementation

 Routing policy involves routing decisions about acceptance and
 advertisement of certain routes to or from other networks and about
 routing preference when more than one route becomes available.
 Routing policy enforces business agreements between network entities
 and is largely governed by non-technical criteria. In essence,
 routing policy involves defining criteria for route filtering and
 route selection.
 One aspect of route filtering has to do with traffic control between
 routing domains or between different provider networks. Making policy
 based on individual prefixes should be avoided in this case because,
 with the large number of prefixes in the Internet, it does not scale.
 Making policy based on ASs that administratively represent a set of
 prefixes scales better.
 Another purpose of route filtering is to protect the integrity of
 routing information by preventing the acceptance of falsely
 advertised routing information that would lead traffic to 'black
 holes'. In this case, only prefix-based filtering will sufficiently
 achieve the goal. Prefix-based filtering needs to occur at the
 borders between a network and its direct customers or peer networks.
 The filtering is harder to manage at the boundary of the peer
 networks since a peer network usually advertises a large amount of
 prefixes. As mentioned earlier, there are about 70,000 routes known

Yu Informational [Page 17] RFC 2791 Scalable Routing Design Principles July 2000

 to the Internet. This means a large default-free network would need
 to filter on the order of hundred of thousands of prefixes or even
 more since a route could be advertised by more than one sources. The
 sheer amount of the prefixes to be filtered imposes challenges for
 router configuration memory and configuration management. To make it
 scale, one would need to rely on the help from an out-of-band process
 to sort out which prefixes should be accepted or denied from which
 source. IRR [11] and DNS [12] are among the current proposed
 mechanisms for implementing prefix-based filtering.
 Route selection policy determines which path should be used to send
 traffic toward a certain destination. This is important, for example,
 when a network has two connections to another network and learns
 routes from both connections. The decision involves which path to
 select to send traffic to the customers behind the other network. The
 choices are typically:
    o Directing traffic to the closest interconnection point for
      traffic to exit the network. This policy is also known as Hot-
      Potato-Routing
    o Directing traffic to the optimal network exit point. The optimal
      exit point is determined based on certain criteria by the
      network administrator and is not necessary the closest exit
      point
    o Always preferring routes advertised by directly connected
      customers
    o Allowing other network or customer to determine the path
 When a policy is defined, its implications for scalable
 implementation need to be considered. For example, if the policy
 allows customers to determine which paths traffic follows, customers,
 not the provider, should be required to set routing parameters to
 make the routing favor their preferred path. Customers can use the
 BGP community or mechanisms such as MED to set routing preferences in
 a much more scalable way. This avoids putting such routing management
 burdens solely on the provider. Distributing the routing management
 burden makes the policy implementation more scalable.
 Another scaling measure is to avoid making complex policy. When
 routing policy is complex, management, such as configuration of the
 router and debugging, would be a problem. The ultimate goal is to
 make the network manageable.
 The following basic principles would help scale the routing policy
 management.

Yu Informational [Page 18] RFC 2791 Scalable Routing Design Principles July 2000

    o Making policies as simple as possible but meet the requirements
    o Automating as much as possible to avoid error-prone manual work
    o Avoiding policy based on individual prefixes as much as possible
      with the exception of prefix-based route filtering for
      protecting routing integrity
    o Avoiding making exceptions
    o Using out-of-band routing policy processing where possible

6.6. Out-of-Band Process

 A typical router assumes both routing and forwarding functions.
 However, conceptually, routing and forwarding are two separate
 processes. A router's ultimate task is to forward packets based on
 its forwarding table, which is derived from routing information. One
 of the main causes of route scaling problems is that routers run out
 of processing power because routing requires too much processing.
 While a router has to forward packets, it does not necessarily have
 to exchange and process routing information or execute routing
 policy; these tasks can be performed elsewhere. Thus the question
 should be: Would it be possible to remove the routing process from a
 router to reduce its burden? Moving the routing process from the
 routers to other systems is referred to as out-of-band route
 processing.
 Out-of-band route processes would, in short, perform the heavy-duty
 routing tasks. They would build a forwarding table for the router,
 select routes based on pre-defined policy, filter routes, and shield
 the router from route flapping attacks.
 The shortcomings of out-of-band route processing are the possible
 introduction of delays in routing changes; the de-coupling of routing
 and forwarding paths, which could introduce inaccurate routing
 information; and the cost of extra equipment.
 Appendix A presents a current example of out-of-band route
 processing. It also suggests other possible solutions.

7. Conclusion and Discussion

 How routing scales has a direct impact on network stability and
 performance. With the fast growth of the Internet and consequent
 expansion of providers' networks, routing scaling become increasingly

Yu Informational [Page 19] RFC 2791 Scalable Routing Design Principles July 2000

 an important issue to address. This document identifies the major
 factors that affect route scalability and establishes basic
 principles for designing scalable routing in large networks.
 The major routing scaling issues we are facing today are excessive
 router resource consumption due to routing processing burdens causing
 routing convergency difficulties thus introducing network
 instability; and routing complexity resulting in difficulties of
 management and trouble shooting causing degradation of service.
 The outlined principles for designing a scalable routing system are
 building routing hierarchy; introducing fault isolation; reducing
 routing processing burden where possible; defining manageable routing
 policies and using the assistance of available out-of-band routing
 process.
 The use of out-of-band resources to assist routing processing is a
 concept only been used in the Internet Exchange Points (IXPs).
 However, it could potentially be used to advantage within a network
 to help addressing routing scaling issues. This is a topic worthy of
 further exploration.
 Routing protocols and/or their implementations can still be improved
 or enhanced for better handling of the scaling issues. For example,
 the IS-IS multiple level mechanism is needed in order to scale the
 IGP in large network. Also, using multicast or a reliable flooding
 mechanism for I-BGP updates instead of pairwise full mesh peering is
 something worth investigating.
 It is our belief that even with the deployment of new technologies
 such as DWDM, MPLS and others in the future, the fundamental routing
 scheme will remain the current IGP/BGP paradigm.  Therefore, the
 scalable routing design principles outlined in this document should
 still apply with the deployment of new technologies.

8. Security Considerations

 This document deals with routing scaling issues and thus is unlikely
 to have a direct impact on security.
 However, certain routing scaling improvement mechanisms suggested in
 the document, such as network compartmentalization, will possibly
 alleviate network outages caused by denial-of-service attacks since
 it would help prevent such outages from spreading to the entire
 network.

Yu Informational [Page 20] RFC 2791 Scalable Routing Design Principles July 2000

 Although the mechanisms described in this document do not enhance or
 weaken the security aspect of routing protocols, it is worth
 indicating here that security enhancement of routing protocols or
 routing mechanisms may impact routing scalability. Therefore, when
 applying security enhancement in routing, one has to be aware of the
 implications on scalability.
 For example, TCP MD5 signature option is proposed to be a mechanism
 to protect BGP sessions from being spoofed [13]. It is done on a
 per-session basis and the overhead of MD-5 extensions are minimal
 thus has no direct impact on scalability. There have been concerns
 about doing per-prefix AS path verification as any one ISP along a
 path could have forged or modified information (maliciously or not).
 One extreme solution is to have a signature for each prefix which
 gives very strong security but presents enormous scaling issues in
 terms of processing, memory and administrative overhead.

9. Acknowledgement

 Special thanks to Curtis Villamizar and Dave Katz for the extensive
 review of the document and many helpful comments. Many thanks to
 Yakov Rekhter, Noel Chiappa and Rob Coltun for their insightful
 comments. The author also like to thank Susan R. Harris for the much
 needed polishing of English language in the document.
 The author was made aware after the publication of this document that
 there is a relevant and independent presentation made by Enke Chen on
 the subject. The presentation is thus referenced in [14].

10. References

 [1]  "Intermediate System to Intermediate System Intra-Domain
      Routeing Exchange Protocol for use in Conjunction with the
      Protocol for Providing the Connectionless-mode Network Service
      (ISO 8473)", ISO DP 10589, February 1990.
 [2]  Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual
      Environments", RFC 1195, December 1990.
 [3]  Moy, J., "OSPF Version 2", RFC 2328, April 1998.
 [4]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
      RFC 1771, March 1995.
 [5]  C. Labovitz, R. Malan, F. Jahanian, "Origins of Internet Routing
      Instability," in the Proceedings of INFOCOM99, New York, NY,
      June, 1999

Yu Informational [Page 21] RFC 2791 Scalable Routing Design Principles July 2000

 [6]  J. Moy, "OSPF-Anatomy of an Internet Routing Protocol",
      Addison-Wesley, January 1998.
 [7]  Bates, T., Chandra, R. and E. Chen, "BGP Route Reflection - An
      alternative to full mesh IBGP", RFC 2796, April 2000.
 [8]  Traina, P., "Autonomous System Confederation Approach to Solving
      the I-BGP Scaling Problem", RFC 1965, June 1996.
 [9]  Curtis, V., Chandra, R. and R. Govindan, "BGP Route Flap
      Damping", RFC 2439, November 1998.
 [10] Fuller, V., Li, T., Yu, J. and K. Varadhan "Classless Inter-
      Domain Routing (CIDR): an Address Assignment and Aggregation
      Strategy", RFC 1519, September 1993.
 [11] Villamizar, C., Alaettinoglu, C., Govindan, R. and D. Meyer,
      "Routing Policy System Replication", RFC 2769, February 2000.
 [12] Bates, T., Bush, R., Li, T. and Y. Rekhter, "DNS-based NLRI
      origin AS verification in BGP", Work in Progress.
 [13] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
      Signature Option", RFC 2385, August 1998.
 [14] E. Chen, "Routing Scalability in Backbone Networks." Nanog
      Presentation: http://www.nanog.org/mtg-9901/ppt/enke/index.htm
 [15] T. Li, T. Przygienda, H. Smit,  "Domain-wide Prefix Distribution
      with Two-Level IS-IS", Work in Progress.

Author's Address

 Jieyun (Jessica) Yu
 CoSine Communications
 1200 Bridge Parkway
 Redwood City, CA  94065
 EMail: jyy@cosinecom.com

Yu Informational [Page 22] RFC 2791 Scalable Routing Design Principles July 2000

Appendix A. Out-of-Band Routing Processes

 The use of a Route Server(RS) at NAPs is an example of achieving
 routing scalability through an out-of-band routing process. A NAP is
 a public inter-connection point where ISP networks exchange traffic.
 ISP routers at a NAP establish BGP peer sessions with each other. The
 result is full mesh E-BGP peering with a complexity of O(N^2) system
 wide. When the RS is in place, each router peers only with the RS
 (and its backup) to obtain necessary routing information (or more
 precisely, the necessary forwarding information). In addition, the RS
 also filters routes and executes policy for each provider's router,
 which further reduces the burden on all routers involved.
 The concept of the Route Server can also be used to help address
 routing scalability in a large network.
 1) RS Assisted Peering between Customer Aggregation Router and
 Customer Routers
 Currently, in a typical large provider network, it's not unusual that
 a customer aggregation router connects up to hundreds of customer
 routers. That means the router has to handle hundreds of E-BGP
 sessions and filter a large number of prefixes. These tasks impose a
 heavy burden on the aggregation router. Reducing the number of
 customer routers per aggregation router is not an optimal option,
 since this would introduce more routers in the routing system of the
 whole network, which is neither scalable for backbone routing, nor
 cost efficient. Using an RS between customers and the providers'
 customer aggregation router become an attractive option to reduce the
 burden on the router.
 Figure 1 shows one way of incorporating an RS router between a
 provider's customer aggregation router and customer routers.
  1. ————————– LAN Media in a POP

| |

  1. —- —–

|CR | |RS |

  1. —- —–

/ | \

                   /  |  \
                  C1  C2..Cn
       Figure 1: RS serving customer aggregation router connecting
                 customer routers

Yu Informational [Page 23] RFC 2791 Scalable Routing Design Principles July 2000

 In a scenario without an RS, the customer aggregation router(CR) has
 to peer with customer routers C1, C2 ... Cn (where n could be in the
 hundreds). When an RS router is introduced, CR, C1, C2 ... Cn peer
 with the RS router instead, and the RS passes the processed routing
 information (or forwarding information) to all of them, according to
 policy and filters.
 The advantages are obvious:
    o The customer aggregation router peers only with the RS router
      instead of with hundreds of customer routers.
    o The customer aggregation router does not need to filter prefixes
      or process routing policies, which frees resources for packet
      forwarding and handling.
 One general concern with the use of an RS router is the possibility
 of a mismatch of routing connectivity and the physical connectivity.
 For example, if the link between the CR and C1 is down and if the RS
 router is not aware of the outage, it will continue to pass routes
 from C1 to the CR, and the traffic following these routes will be
 black holed. However, this is not a problem in the specific
 application described here. This is because the RS router has to go
 through the CR to peer with C1, C2 ... Cn. When the link is down, C1
 is inaccessible from the RS router, and no routing information can be
 exchanged between the two. Consequently, the RS will announce no
 routes related to C1.
 Another concern is the creation of single point of failure. If the RS
 router is down, no routing information can be exchanged between the
 customer aggregation router and C1, C2 ... Cn, and no traffic will
 flow between them. This problem could be addressed by adding a second
 RS router as a backup.
 In this scenario, since RS peers with C1 ... Cn via CR, it requires
 that when the RS router passes routing information to C1...Cn, it
 designates the IP address of the CR as the next hop. Likewise, when
 the RS router passes routes from each customer router to the customer
 aggregation router, it needs to place the correct next hop on the
 route. Modifications need to be made to the RS code to include this
 function.
 2) Private RS Router at InterExchange Point
 A large provider network often has many BGP peers at the
 Interexchange Point, NAP or private interconnection. This means a
 border router has to handle many E-BGP sessions. Since an

Yu Informational [Page 24] RFC 2791 Scalable Routing Design Principles July 2000

 Interconnect points is usually the administrative boundary between
 ISPs, policy and route filtering are very demanding. This imposes a
 scaling problem on the border router.
 Deploying many routers to distribute the load among them is an
 expensive solution: extra hardware and extra ports cost money.
 Shifting the routing burden to an RS router is a promising
 alternative solution. In the case of using RS for multiple peers at a
 private interexchange point, the scenario is similar to RS used
 between customer aggregation router and customer routers as described
 in 1) above. In the case of such peering at a NAP, the private RS
 could be placed either on the same NAP media or a private media
 between the ISP's NAP router and the RS.
 3) RS Routers at Each POP in a Large Network
 Even in a network with a hierarchical routing structure, a sub-area
 may become too large, and I-BGP full meshing may impose a scaling
 problem. One way to address this would be to split the sub-area or
 add yet another tier of I-BGP reflector structure. Another possible
 solution would be to use an RS router as an I-BGP Server. Depending
 on the topology of a POP, this solution may or may not be suitable.
 The use of RS routers at network POPs need to be investigated
 further.

Yu Informational [Page 25] RFC 2791 Scalable Routing Design Principles July 2000

Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
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 or assist in its implementation may be prepared, copied, published
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 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
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 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
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 The limited permissions granted above are perpetual and will not be
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 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Yu Informational [Page 26]

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