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Network Working Group J. Rekhter Request for Comments: 1092 T. J. Watson Research Center

                                                         February 1989

      EGP and Policy Based Routing in the New NSFNET Backbone

Status of this Memo

 This memo discusses implementation decisions for routing issues in
 the NSFNET, especially in the NSFNET Backbone.  Of special concern is
 the restriction of routing information to advertize the best route as
 established by a policy decision.  Distribution of this memo is
 unlimited.

Introduction

 The NSFNET backbone routes packets between the Regionals Networks to
 which it is connected, (i.e., the packets arriving at a backbone
 entry node are routed to an exit node).  How they travel through the
 network is determined by two components:

   the NSFNET backbone routing protocol/algorithm, and

   additional information about the externally connected networks.

 This paper is concerned with how reachability information between the
 external networks and the NSFNET backbone is exchanged so that
 packets can be routed to the correct destination by using a
 reasonable path.

EGP as reachability protocol

 The EGP (Exterior Gateway Protocol) routing method will be used to
 exchange reachability information between the NSFNET backbone and the
 regional networks.

 There are several problems with using EGP as a reachability protocol
 for routing in a meshed environment.  Some EGP components require
 further definitions for the NSFNET backbone - regional network
 interactions.  It should be noted that the use of EGP is only viewed
 as an interim measure until better inter autonomous system protocols
 are defined and widely deployed for gateways used by regional
 networks.

 The following is a list of some EGP problems and issues:

    The EGP model assumes an engineered spanning tree topology,

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    however, the NSFNET (due to the presence of backdoor routes) does
    not fit into this model.  In the NSFNET the same network may be
    advertized as reachable by more than one regional network.
    Besides the fact that the overall NSFNET does not fit into a
    spanning tree model there are serious concerns with the concept
    of the "core" (central to the EGP) and its obvious deficiencies.

    While EGP is going to isolate intra-Regional routing from the
    intra-NSFNET-Backbone routing, it does not address the issue of
    false information which may be supplied by regional networks.
    EGP by itself does not protect a particular network from unwanted
    and unsolicited representation by some regional network.  As an
    example, if network N1 is reachable through regional network R1
    as well as through regional network R2, EGP has no provisions to
    specify one of these paths as a primary and one as a secondary,
    since there is not generally accepted interpretation of EGP
    metrics today.  Also, there is nothing in EGP which can prevent one
    or more regional networks from advertizing other networks (in
    particular, networks which belong to other regional networks) as
    reachable with zero distance.  This could result in the creation
    of a "black hole" or at least in suboptimal IP routing.

    EGP by itself has no provisions to guarantee that routes through
    the NSFNET Backbone will be preferred over routes through the
    backdoor routers or vice versa.

Policy Based Routing

 Looking at the problems listed above the appearance of the new
 factors like autonomy and mutual trust becomes obvious.  While trying
 to achieve the routing functionality required for the new NSFNET
 backbone we should realize that one of our primary concerns has to be
 the accommodation of those new factors.

 This means that some kind of a rudimentary Policy Based Routing
 method becomes imperative.  We would like to emphasize, however, that
 we are not talking about complete Policy Based Routing, but that we
 are rather concerned about supporting a minimum subset of a policy
 functionality to be an initial solution to the above mentioned
 problems.  This requires support and cooperation between the
 management of each of the networks connected to the NSFNET backbone.

 We need to support the ability of a particular network N, which
 belongs to one of the regional networks, to establish a bilateral
 agreement with one or more regional networks of the type "network N
 can be reached via one or more regional networks (RN1, RN2, ...
 RNx)".  This allows each network to select one or more
 representatives at the regional network level.  Once this agreement

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 is established the information will be available to:

   The network which initiated the agreement.

   The management of the regional network(s) with whom this
   agreement has been established.

   The NSFNET backbone Network Operation Center where it will be
   entered into the Routing Policy Data Base which will be available
   through the NSFNET information services.

 Supporting multiple routes to the NSFNET core requires the guarantee
 that for a certain network N, no regional network other than the
 one(s) selected by N, will advertize N as reachable, which
 necessitates that the NSFNET core will ignore unauthorized
 advertisements for network N.

EGP and Rudimentary Policy Based Routing

 Each network which belongs to the NSFNET will select a specific
 regional network as its primary representative to the NSFNET core by
 bilateral agreement with the management of same regional network as
 well as the NSFNET backbone management.  The same network can
 furthermore select an arbitrary number of other regional networks as
 their secondary, tertiary, etc., representative by establishing
 bilateral agreements with the management of the corresponding
 regional networks as well as the NSFNET backbone management.

 Reachability information supplied by each regional network will be
 distributed to all other NSS nodes of the NSFNET Backbone.  We would
 like to emphasize that we are not going to flood EGP packets
 internally within the backbone, but to rather use the learned
 information for the interior gateway protocol, which uses the ANSI
 IS-IS protocol.

 The implementation allows for a defined regional network to advertize
 a particular leaf network in the EGP NR packets with a distance of
 zero.  Secondary representatives may advertize the same network with
 distance one or higher.  If the path through the primary regional
 representative is available all secondary paths will be ignored.  If
 the path through the primary regional representative goes down (which
 will be discovered via the EGP NR information), the next path with
 the lowest available EGP metric will be used.

 We will also be able to detect and report unsolicited
 representations.  This will be done by examining (on a periodic
 basis) all reachability information obtained via EGP.  The result
 will be compared against the Routing Policy Data Base which will hold

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 information about all bilateral agreements between networks and their
 regional representatives.  Any mismatch will cause an alarm to the
 Network Operations Center.  For example, network N established a
 bilateral agreement with the regional network R1 electing it as its
 primary representative. The EGP NR record received from the regional
 network R5 advertizes the network N as reachable with distance zero.
 By comparing the Routing Policy Data Base entry for the network N
 with the EGP NR record a mismatch will be detected and an alarm is
 forwarded to the Network Operation Center.

 Since the whole scheme is based on a combination of the network
 number and the autonomous system number, to allow for further
 verification, it is also important to insure the correctness of the
 autonomous system numbers as advertized by the regionals networks to
 the NSFNET core.

 The autonomous system number validation for each regional network
 will be performed at the NSS which connects the particular leaf
 network to the NSFNET backbone.  All discrepancies wil be reported to
 the Network Operations Center.

 The NSFNET backbone will be considered as a separate Autonomous
 System with its own autonomous system number.

Backbone versus Backdoor Routes

 There are instances where regional networks prefer paths through some
 backdoor route over paths through the NSFNET backbone.  Therefore,
 the reachability information advertized by the NSFNET core to the
 regional networks (via EGP NR records) will always use a fixed metric
 of 128 for all routes.  This may aid to encourage traffic to flow
 through backdoors, if desired and available.

 The regional networks can use a variety of techniques to determine
 how they route traffic for any particular network at their own
 option.

What do we expect from the Regional Networks

 Each regional network should get its own Autonomous System number.

 The connection between regional networks to NSFNET backbone will be
 done via EGP.  It is the responsibility of the regional backbone to
 provide an EGP functionality via the attachment to the E-PSP
 dedicated to the regional network.

 The EGP functionality may require a translation of network numbers in
 and out of the regional network.  In any case, the NSFNET backbone

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 expects individual network numbers of the leaf networks of the
 regional network, as long as they should be advertised, and will
 announce individual networks known to the NSFNET core to the regional
 network.

 The EGP support should includes the ability to configure EGP metrics
 from some statically definable configuration table.  If the EGP
 metrics cannot be defined or if they are not fixed the metric
 determination will be done by the NSFNET backbone routers, as taken
 from their databases, themselves.  In that case, it is the
 responsibility of the regional network to provide the NSFNET backbone
 management with the metric data to allow for proper use of metrics.

 We also expect each regional network to handle all bilateral
 agreements with its leaf networks regarding Policy Based Routing and
 supply a copy of those agreements to the NSFNET backbone management.

Acknowledgements

 I would like to express my thanks to Barry Appelman (T.J. Watson
 Research Center, IBM Corp.) and Hans-Werner Braun (Merit) for their
 contributions to this document.

Author's Address

 Jacob Rekhter
 T.J. Watson Research Center
 IBM Corporation
 P.O. Box 218
 Yorktown Heights, NY 10598

 Phone: (914) 945-3896

 Email: YAKOV@IBM.COM

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