path: root/doc/rfc/rfc1863.txt

Network Working Group                                          D. Haskin
Request For Comments: 1863                            Bay Networks, Inc.
Category: Experimental                                      October 1995


       A BGP/IDRP Route Server alternative to a full mesh routing

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Abstract

   This document describes the use and detailed design of Route Servers
   for dissemination of routing information among BGP/IDRP speaking
   routers.

   The intention of the proposed technique is to reduce overhead and
   management complexity of maintaining numerous direct BGP/IDRP
   sessions which otherwise might be required or desired among routers
   within a single routing domain as well as among routers in different
   domains that are connected to a common switched fabric (e.g. an ATM
   cloud).

1. Overview

   Current deployments of Exterior Routing protocols, such as the Border
   Gateway Protocol [BGP4] and the adaptation of the ISO Inter-Domain
   Routing Protocol [IDRP], require that all BGP/IDRP routers, which
   participate in inter-domain routing (border routers) and belong to
   the same routing domain, establish a full mesh connectivity with each
   other for purpose of exchanging routing information acquired from
   other routing domains. In large routing domains the number of intra-
   domain connections that needs to be maintained by each border route
   can be significant.

   In addition, it may be desired for a border router to establish
   routing sessions with all border routers in other domains which are
   reachable via a shared communication media. We refer to routers that
   are directly reachable via a shared media as adjacent routers.  Such
   direct peering allows a router to acquire "first hand" information
   about destinations which are directly reachable through adjacent
   routers and select the optimum direct paths to these destinations.
   Establishment of BGP/IDRP sessions among all adjacent border routers
   would result in a full mesh routing connectivity.  Unfortunately for



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   a switched media as ATM, SMDS or Frame Relay network which may
   inter-connect a large number of routers,  due to the number of
   connections that would be needed to maintain a full mesh direct
   peering between the routers,  makes this approach impractical.

   In order to alleviate the "full mesh" problem, this paper proposes to
   use IDRP/BGP Route Servers which would relay external routes with all
   of their attributes between client routers. The clients would
   maintain IDRP/BGP sessions only with the assigned route servers
   (sessions with more than one server would be needed if redundancy is
   desired).  All routes that are received from a client router would be
   propagated to other clients by the Route Server.  Since all external
   routes and their attributes are relayed unmodified between the client
   routers, the client routers would acquire the same routing
   information as they would via direct peering.  We refer to such
   arrangement as virtual peering.  Virtual peering allows client
   routers independently apply selection criteria to the acquired
   external routes according to their local policies as they would if a
   direct peering were established.

   The routing approach described in this paper assumes that border
   routers possess a mechanism to resolve the media access address of
   the next hop router for any route acquired from a virtual peer.

   It is fair to note that the approach presented in this paper only
   reduces the number of routing connection each border router needs to
   maintain. It does not reduce the volume of routing information that
   needs to maintained at each border router.

   Besides addressing the "full mesh" problems,  the proposal attempts
   to achieve the following goals:

   - to minimize BGP/IDRP changes that need to be implemented in client
     routers in order to inter-operate with route servers;

   - to provide for redundancy of distribution of routing information to
     route server clients;

   - to minimize the amount of routing updates that have to be sent to
     route server clients;

   - to provide load distribution between route servers;

   - to avoid an excessive complexity of the interactions between Route
     Servers themselves.






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2. Terms And Acronyms

   The following terms and acronyms are used in this paper:

  Routing Domain     -  a collection of routers with the same set of
                        routing policies.  For IPv4 it can be identified
                        with an Autonomous System Number, for IPv6
                        it can be identified with a Routing Domain
                        Identifier.

  Border Router (BR) -  a router that acquires external routes, i.e.
                        routes to internet points outside its routing
                        domain.

  Route Server (RS)  -  a process that collects routing information
                        from border routers and distributes this
                        information to 'client routers'.

  RS Client (RC)     -  a router than peers with an RS in order to
                        acquire routing information.  A server's client
                        can be a router or another route server.

  RS Cluster (RSC)   -  two or more of route servers that share the same
                        subset of clients.  A RS Cluster provides
                        redundancy of routing information to its
                        clients,  i.e. routing information is provided
                        to all RS Cluster clients as long as there is
                        at least one functional route server in the RS
                        Cluster.

  RCID             -    Cluster ID

3. RS Model

   In the proposed scheme a Route Server (RS) does not apply any
   selection criteria to the routes received from border routers for the
   purpose of distributing these routes to its clients.  All routes
   acquired from border routers or other Route Servers are relayed to
   the client border routers.

   There can be two classes of Route Servers: Route Servers that relay
   external routes between routers in a single routing domain and Route
   Servers that relay external routes between border routers in
   different routing domains.  The former are Intra-Domain Route Servers
   and the latter are Inter-Domain Route Servers.

   In the RS model proposed in this document there is no routing
   exchange between Intra-Domain Route Servers and Inter-Domain Route



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   Servers.  Routes that cross a domain boundary must always pass
   through a border router of such a domain which may apply
   administrative filters to such routes.

   Operations of Intra-Domain Route Servers and Inter-Domain Route
   Servers are identical.

   One or more Route Servers form an RS Cluster (RSC).  For redundancy's
   sake two or more RSs can be configured to operate in an RS Cluster.
   All route servers in an RSC share the same clients,  i.e. cluster
   clients establish connections to all route servers in such an RSC for
   the purpose of exchanging routing information. Each cluster is
   assigned an unique RSC Identifier (RCID) represented by a 2-octet
   unsigned integer.

   Clusters which provide virtual connectivity between their clients
   would be normally exchanging routing information among themselves so
   that all external routes are propagated to all participating clients.

   Though a Route Server Client (RC) can be associated with multiple
   RSC, it seems that there is no real advantage of doing so except for
   a short transition period to provide a graceful re-assignment from
   one RSC to another or, if for some reason, there are multiple RS
   groups that don't exchange routing information with each other.

   The inter-cluster route exchange can be accomplished by forming a
   full mesh routing adjacency between clusters.  In this approach,
   illustrated in the diagram below,  each RS in each RSC would maintain
   a routing connection with every RS in other RS clusters.  Only routes
   that are acquired from border routers are propagated to RSs in other
   RS clusters.

         BR11   BR12   BR1n     BR21  BR22  BR2n
           |     |  ... |        |     | ...  |
          -----------------     ------------------
          !  RS11  RS12   ! --- !  RS21    RS22  !
          -----------------     ------------------
               <RSC#1>  \           /    <RSC#2>
                         \         /
                       -----------------
                       !  RS31  RS32   !   <RSC#3>
                       -----------------
                         |     | ... |
                        BR31  BR32  BR3n

   Another way to propagate routing information between clusters would
   be to form a cluster hierarchy in which an RS in one cluster
   maintains sessions only with RSs in designated clusters.  In this



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   approach an RS must advertise all acquired routes to an RS in another
   cluster except the routes that are acquired from that cluster.
   Nevertheless,  it allows for minimizing the number of routing
   sessions which can be highly desirable in some network.  It is
   important for the hierarchical scheme that the inter-cluster route
   exchange links form a tree, i.e. there is only one route propagation
   path between any two clusters, otherwise routing loops may result.
   For detection and pruning of routing loops in a hierarchical cluster
   topology, it is advisable to include the "RCID Path" attribute (see
   4.3.4) in all routing updates sent between route servers. This
   attribute lists IDs of all clusters in the route propagation path.
   When a duplicate ID is detected in this attribute an offending route
   needs to be discarded.

   The diagram below which illustrates the hierarchical approach is
   created from the diagram above by removing the route exchange link
   between clusters 2 and 3.

         BR11   BR12   BR1n     BR21  BR22  BR2n
           |     |  ... |        |     | ...  |
          -----------------     ------------------
          !  RS11  RS12   ! --- !  RS21    RS22  !
          -----------------     ------------------
               <RSC#1>  \                <RSC#2>
                         \
                       -----------------
                       !  RS31  RS32   !   <RSC#3>
                       -----------------
                         |     | ... |
                        BR31  BR32  BR3n

   It seems that the only disadvantage of the hierarchical model, is the
   management headache of avoiding routing loops and redundant
   information flow by insuring that inter-cluster links always form a
   tree.  But more study is needed to fully evaluate the comparative
   merits of the full-mesh and hierarchical models.

   Since RSs in the same cluster maintain routing sessions with the same
   set of clients, it may seem that there is no need to exchange routing
   information between RSs in the same cluster. Nevertheless, such a
   route exchange may help to maintain identical routing databases in
   the servers during client acquisition periods and when a partial
   failure may affect some routing sessions.

   Route servers in the same RS cluster exchange control messages in
   attempt to subdivide the responsibilities of providing routing
   information to their clients.  In order to simplify the RS design,
   the RS messaging is implemented on top of exterior protocol which is



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   used by route servers for the routing information exchange.

4. Operation

4.1 ADVERTISER Path Attribute

   Route servers act as concentrators for routes acquired by border
   routers so that the border routers need to maintain routing
   connections with only one or two designated route servers.  Route
   Servers distribute routing information that is provided to them by
   the border routers to all their client.

   If routing information were relayed to RS clients in UPDATE messages
   with only those path attribute that are currently defined in the
   BGP-4/IDRP specification, the RS clients would not be able to
   associate external routes they receive with the border routers which
   submitted that routes to route servers. Such an association is
   necessary for making a correct route selection decision. Therefore,
   the new path attribute, ADVERTISER, is defined.

   The ADVERTISER is an optional non-transitive attribute that defines
   the identifying address of the border router which originally
   submitted the route to a router server in order for it to be relayed
   to other RS clients. Type Code of the ADVERTISER attribute is 255.
   This attribute must be included in every UPDATE message that is
   relayed by route servers and must be recognized by RS clients.

4.2 Route Client Operation

   An RS client establishes an BGP/IDRP connection to every route server
   in the RS cluster to which the route client is assigned.

   RS clients must be able to recognize the ADVERTISER path attribute
   that is included in all UPDATE messages received from route servers.
   Routes received in UPDATE messages from route servers are processed
   as if they were received directly from the border routers specified
   in the ADVERTISER attributes of the respective updates.

   If an RS client receives a route from a Intra-Domain Route Server, is
   assumed that the border router identified in the ADVERTISER attribute
   is located in the receiving client's own routing domain.

   If an RS client receives a route from a Inter-Domain Route Server,
   the locality of the border router identified in the ADVERTISER
   attribute can be determined from the BGP's AS_PATH attribute or
   IDRP's RD_PATH attribute respectively.





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   If no ADVERTISER attribute was included in an UPDATE message from a
   route server it is assumed that the route server itself is the
   advertiser of the corresponding route.

   If the NEXT_HOP path attribute of an UPDATE message lists an address
   of the receiving router itself, the route that is carried in such an
   update message must be declared unreachable.

   In addition, it is highly desirable, albeit not required,  to
   slightly modify the "standard" BGP/IDRP operation when acquiring
   routes from RSs:

    when a route is received from an RS and a route with the completely
    identical attributes has been previously acquired from another RS
    in the same cluster,  the previously acquired route should be
    replaced with the newly acquired route.  Such a route replacement
    should not trigger any route advertisement action on behalf of the
    route.

   RSs are designed to operate in such a way that eliminates the need to
   keep multiple copies of the same route by RS clients and minimizes
   the possibility of a route flap when the BGP/IDRP connection to one
   of the redundant route servers is lost.

   It is attempted to subdivide the route dissemination load between
   route servers such that only one RS provides routing updates to a
   given client.  But since, for avoiding an excessive complexity, the
   reconciliation algorithm does not eliminate completely the
   possibility of races, it is still possible that a client may receive
   updates from more than one route server.  Therefore, the client's
   ability to discard duplicate routes may reduce the need for a bigger
   routing database.

4.3 Route Server Operation

   A Route Server maintains BGP-4/IDRP sessions with its clients
   according to the respective BGP-4/IDRP specification with exception
   of protocol modifications outlined in this document.

   UPDATE messages sent by route servers have the same format and
   semantics as it respective BGP-4/IDRP counterparts but also carry the
   ADVERTISER path attribute which specifies the BGP Identifier of the
   border router that submitted the route advertised in the UPDATE
   message. In addition, if the hierarchical model is deployed to
   interconnect Route Server clusters, it is advisable to include the
   "RCID Path" attribute in all routing updates sent between route
   servers as described in 4.3.4.




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   When route servers exchange OPEN messages they include the Route
   Server protocol version (current version is 1) as well as Cluster IDs
   of their respective clusters in an Optional Parameter of the OPEN
   message. The value of Parameter Type for this parameter is 255. The
   length of the parameter data is 3 octets. The format of parameter
   data is shown below:

    +-----------------------+------------------------------------+
    | Version = 1 (1 octet) |      Cluster ID (2 octets)         |
    +-----------------------+------------------------------------+

   Also, route servers that belong to the same cluster send to each
   other LIST messages with lists of clients to which they're providing
   routing information.  In the LIST message an RS specifies the Router
   Identifier of each client to which that RS is providing routing
   updates. Since LIST messages are relatively small there is no need to
   add a processing complexity of generating incremental updates when a
   list changes; instead the complete list is sent when RSs need to be
   informed of the changes.  The format of the LIST message is presented
   in 4.3.1.

4.3.1 LIST Message Format

   The LIST message contains the fixed BGP/IDRP header that is followed
   with the fields shown below.  The type code in the fixed header of
   the LIST message is 255.

     +----------+----------+----------+----------+
     |        Client Identifying Address         | Repeated for each
     +-------------------------------------------+ informed client
   The number of Client Identifying Address" fields is not encoded
   explicitly,  but can be calculated as:

     (<LIST message Length> - <Header Length>) / <Address Length>,

   where <LIST message Length> is the value encoded in the fixed
   BGP/IDRP header, <Header Length> is the length of that header, and
   <Address Length> is 4 for IPv4 and 16 for IPv6.

4.3.2 External Route Acquisition And Advertisement

   A route server acquires external routes from RS clients that are also
   border routers.  A RS also may acquire external routes from other
   RSs.  Route servers relay all acquired routes unaltered to their
   clients.  No route selection is performed for purpose of route re-
   advertisement to RS clients.





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   While route servers receive and store routing data from all their
   client,  Routing Servers in the same cluster coordinate their route
   advertisement in the attempt to ensure that only one RS provides
   routing updates to a given client.  If an RS fails,  other Route
   Servers in the cluster take over the responsibility of providing
   routing updates to the clients that were previously served by the
   failed RS.  A route flap that can result from such switch-over can be
   eliminated by the configuring client's "Hold Time" of their BGP-
   4/IDRP sessions with the route servers to be larger than the switch-
   over time.  The switch-over time is determined by the Hold Time of
   BGP-4/IDRP sessions between the route servers in the cluster and the
   period that is needed for that route servers to reconcile their route
   advertisement responsibilities.  The reconciliation protocol is
   described in 4.3.3.

   The BGP-4/IDRP operations of route servers differs from the
   "standard" operation in the following ways:

   -    when receiving routes from another RS, the RS Client mode of
        operation is assumed, i.e., when a route with completely
        identical attributes has been previously acquired from an RS
        belonging to the same cluster as the RS that advertises the new
        route, the previously acquired route should be discarded and
        the newly acquired route should be accepted.  Such a route
        replacement should not trigger any route advertisement action
        on behalf of the route.

   -    all acquired routes are advertised to a client router except
        routes which were acquired from that client (no route echoing);

   -    if the hierarchical model of inter-cluster route exchange is
        used,  all acquired routes are advertised to an RS in another
        RSC except routes that are acquired from that RSC.  In the
        full-mesh model, only routes which are acquired from border
        routers are advertised to route servers in other clusters;

   -    if route servers in the same RS cluster are configured to
        exchange routing information,  only external routes that are
        acquired from border routers are advertised to route servers in
        the local cluster;

   -    the ADVERTISER path attribute is included in every UPDATE
        messages that is generated by RS.  This attribute must
        specify the identifying address of the border router from which
        information provided in UPDATE has been acquired.  All other
        routing attributes should be relayed to RS's peers unaltered.





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   -    when a route advertised by to an RS by a client becomes
        unreachable such a route needs to be declared unreachable to
        all other clients.  In order to withdraw a route,  the route
        server sends an UPDATE for that route to each client (except
        the client that this route was originally acquired) with the
        NEXT_HOP path attribute set to the address of the client to
        which this UPDATE is sent to.  The the ADVERTISER path attribute
        with the identifying address of the border router that
        originally advertised the withdrawn route must be also included
        in such an update message.

   -    if the hierarchical model is deployed to interconnect Route
        Server clusters,  it is advisable to include the RCID_PATH
        attribute in all routing updates sent between route servers as
        described in 4.3.4.  The RCID_PATH attribute is never included
        in UPDATE messages sent to border routers.

4.3.3 Intra-Cluster Coordination

   In order to coordinate route advertisement activities,  route servers
   which are members of the same RS cluster establish and maintain
   BGP/IDRP connections between themselves forming a full-mesh
   connectivity.  Normally, there is no need for more than two-three
   route servers in one cluster.

   Route servers belonging to the same cluster send to each other LIST
   messages with lists of clients to which they're providing routing
   information;  let's call such clients "informed clients".

   Each RS maintains a separate "informed client" list for each RS in
   the local cluster including itself.  All such lists are linked in an
   ascending order that is determined by the number of clients in each
   list; the order among the lists with the same number of clients is
   determined by comparing the identifying addresses of the
   corresponding RSs -- an RS in such a "same number of clients" subset
   is positioned after all RSs with the lower address.

   An RS can be in one of two RS coordination states: 'Initiation' and
   'Active'.

4.3.3.1 Initiation State

   This is the initial state of route server that is entered upon RS
   startup.  When the Initiation state is entered the 'InitiationTimer'
   is started.  The Initiation state transits to the Active state upon
   expiration of the 'InitiationTimer' or as soon as all configured
   BGP/IDRP connections to other route servers in the local RS Cluster
   are established and LIST messages from that route servers are



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   received.

   In the Initiation state an RS:

    o   tries to establish connections with other RSs in the local and
        remote clusters.

    o   accepts BGP/IDRP connections from client routers.

    o   receives and process BGP/IDRP updates but doesn't send any
        routing updates.

    o   stores "informed client" lists received from other RSs in the
        local cluster - a newly received list replaces the existing list
        for the same RS. If a LIST message is received from the route
        server in another RS cluster, it should be silently ignored.

    o   initializes an empty "informed client" list for its own clients.
    o   as soon as a BGP/IDRP connection to an RS in the same RS Cluster
        is established, transmits an empty LIST message to such an RS.

4.3.3.2 Active State

This state is entered upon expiration of the 'InitiationTimer' or as
soon as all configured BGP/IDRP connections to other route servers in
the local RS Cluster are established and LIST messages from that route
servers are received.

In the Active state an RS:

    o   continues attempts to establish connections with other route
        servers in the local and remote clusters;

    o   accepts new BGP/IDRP connections;

    o   transmits a LIST message to an RS in the local cluster as soon
        as an BGP/IDRP session with the RS is established and then
        whenever the local "informed client" list changes;

    o   receives and process BGP/IDRP updates;

    o   receives and processes "informed client" lists as described
        below:

        a) If a LIST message is received from the route server in
           another RS cluster, it should be silently ignored.





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        b) If a LIST message is received from a route server that
           belongs to the same RS Cluster,  the differences between
           the old and the new list are determined and the old "informed
           client" list for that RS is replaced by the list from the new
           message.  For each client that was in the old list but not in
           the new list it is checked whether the server has
           an established BGP/IDRP connection to that client and
           the client is not in any of the other "informed client"
           lists.  If both conditions are met,  the processing described
           for a new client takes place (see 4.3.3.3).

    o   for each new BGP/IDRP client (including connections established
        in Initiation state),  decides if that client should become an
        "informed client", i.e. whether routing updates are to be sent
        to the client or that client has been already taken care by
        another RS in the local cluster.  The decision process is
        described in the next section.

4.3.3.3 New Client Processing

   Whenever an RS acquires a new BGP/IDRP peer it scans through all
   "informed client" lists in order to determine if this peer has
   already been receiving routing updates from another RS in the local
   RS cluster.  If the identifying address of the peer is found in one
   of the list,  no routing updates are sent to that peer.

   If the peer's Router Id is not found,  the route server initiates a
   'DelayTimer' timer for that peer and the decision is postponed until
   that timer expires.  The delay value is calculated as followed:

      the RS determines the relative position of its own "informed
      client" list in the linked list of all "informed client" lists.
      If such position is expressed with a number, say N,  in the 1 to
      "maximum number of lists" range, then the delay value is set to
      (N-1)*<DelayGranularity>.

   Upon expiration of the DelayTimer,  the "informed client" lists are
   scanned once again to see if the corresponding peer has already been
   receiving routing updates from another RS in the local RS cluster.
   If the Router Id of the peer is found in one of the lists as a result
   of receiving a new LIST message, no routing updates are sent to that
   peer.  Otherwise,  the peer's Router ID is entered in the "informed
   client" list that belongs to the RS,  the transmission of the updated
   LIST message is immediately scheduled, and routing updates are sent
   to the client.

   The rational for the delay is to minimize races in the decision as
   which RS among route servers in the same RSC is going to provide



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   routing information to a given client.  The RS with least number of
   "informed clients" would have a shortest delay and is the most
   probable to win the race.  This helps to equalize the number of
   "informed clients" between RSc in a cluster.

   After an BGP/IDRP peer is placed in the "informed client" list, it is
   only removed from the list when the BGP/IDRP connection to this peer
   is lost.  While an RS client is in the list it is accurately updated
   with all routing changes.

4.3.3.5 Inter-RS Connection Failure

   If a route server loses a routing session with a route server in the
   same cluster,  it must consider taking the responsibilities of route
   advertisement to the clients that are in the "informed client" list
   of the remote route server of the failed session.

   For each such client it is checked whether the server has an
   established BGP/IDRP connection to that client and the client is not
   in any of the "informed client" lists of active RS.  If both
   conditions are true,  the processing described for a new client takes
   place (see 4.3.3.3).

   After advertisement responsibilities are reconciled the "informed
   client" list associated with the failed session should be discarded.

4.3.4 RCID_PATH Attribute

   The RCID_PATH is an optional non-transitive attribute that is
   composed of a sequence of RS Cluster Identifiers (RCID) that
   identifies the RS Cluster through which routing information carried
   in the UPDATE message has passed.  Type Code of the RCID_PATH
   attribute is 254.  The attribute value field contains one or more RS
   Cluster Identifiers, each encoded as a 2-octets long field.

   When a route server propagates a route which has been learned from
   nother Route Server's UPDATE message, the following is performed with
   respect to the the RCID_PATH attribute:

  -     if the destination of the route is not a route server, the
        RCID_PATH Attribute is excluded from the UPDATE message sent to
        that client.

  -     if the destination of the route is another route server that is
        located in the advertising server's own RS cluster,  the
        RCID_PATH attribute is sent unmodified.





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  -     if the destination of the route is a route server in a different
        RS cluster,  the advertising route server shall verify that the
        RCID of the destination speaker's cluster is not present in
        the RCID_PATH attribute associated with route.  If it does,
        the route shall not be advertised and an event indicating
        that a route loop was detected should be logged, otherwise
        the advertising router shall prepend its own RCID to the RCID
        sequence in the RCID_PATH attribute (put it in the leftmost
        position).

   When a route server propagates a route which has been learned from a
   border router to another route server then:

  -     if the destination of the route is a route server that is
        located in the advertising router's own RS cluster,  an empty
        RCID_PATH attribute shall be included in the UPDATE message
        (an empty RCID_PATH attribute is one whose length field contains
        the value zero).

  -     if the destination of the route is a route server in a different
        RS cluster,  the advertising route server shall include its own
        RCID in the RCID_PATH attribute.  In this case, the RCID of
        advertising route server will be the only entry in the RCID_PATH
        attribute.

4.3.5 NOTIFICATION Error Codes

   In addition to the error codes defined in the BGP-4/IDRP
   specification, the following error can be indicated in a NOTIFICATION
   message that is sent by a route server:

     255  LIST Message Error

   The following error subcodes can be associated with the LIST Message
   Error:

     1  - Bad Address.  This subcode indicates that a Client Identifying
          Address in the received LIST message does not represent
          a valid network layer address of a router interface.

   The following additional UPDATE error subcodes are also defined:

     255 - Invalid ADVERTISER Attribute.  This subcode indicates that
           a value of the ADVERTISER Attribute does not represent
           a valid network layer address of a router interface.






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RFC 1863                A BGP/IDRP Route Server             October 1995


4.3.7 Timers

   The InitiationTimer value of 5 minutes is suggested.

   In order to avoid route flaps during an RS switch-over, a value of
   DelayGranularity should be such so the maximum possible value of the
   DelayTimer (see 4.3.3.3) combined with the Hold Time of inter-RS
   connections would be shorter than two-third of the smallest Hold Time
   interval of all BGP/IDRP connections between the route servers and
   their clients (including RSs in other clusters).  So in a cluster
   with three RSs and the respective Hold Times of 30 and 90 seconds the
   DelayGranularity of 15 seconds would be a recommended value.

   For the same reason it is recommended that the Hold Time of BGP/IDRP
   connections between route servers in the same cluster is set to one-
   third of the smallest Hold Time of all BGP/IDRP connections between
   the route servers and their clients (including RSs in other
   clusters).  So, if the smallest Hold Time of BGP/IDRP sessions with
   clients is 90 seconds,  the recommended  value of the Hold Time of
   BGP/IDRP connections between route servers in that cluster would be
   30 seconds.

5. Route Server Discovery

   This document does not propose any mechanism for the dynamic RS
   discovery by RS clients or/and by other route servers.  It is assumed
   that at minimum a manual configuration will be provided in
   participating routers to achieve the needed connectivity.

7. Security Considerations

   Security issues are not discussed in this document.

8. Acknowledgment

   Some design concepts presented in this paper benefited from
   discussions with Tony Li (cisco Systems).

   Author likes to thank John Krawczyk (Bay Networks) and Susan Harris
   (Merit) for their review and valuable comments.

   Also, author would like to thank Yakov Rekhter (IBM) for the review
   of the earlier version of this document and constructive comments.

   Special thanks to Ray Chang (Bay Networks) whose experience in
   implementing the concepts presented in this document helped to refine
   the route server design.




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RFC 1863                A BGP/IDRP Route Server             October 1995


9. References

   [BGP4] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
          (BGP-4)", RFC 1771, T.J. Watson Research Center, IBM Corp.,
          cisco Systems, March 1995.

   [IDRP] Rekhter, Y., and P. Traina, "IDRP for IPv6", Work In Progress.

10. Author's Address

   Dimitry Haskin
   Bay Networks, Inc.
   2 Federal Street
   Billerica, MA 01821

   EMail: dhaskin@baynetworks.com



































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