RFC 1654 (rfc1654) - Page 4 of 56

A Border Gateway Protocol 4 (BGP-4)

RFC 1654                         BGP-4                         July 1994


   closed.

   The hosts executing the Border Gateway Protocol need not be routers.
   A non-routing host could exchange routing information with routers
   via EGP or even an interior routing protocol.  That non-routing host
   could then use BGP to exchange routing information with a border
   router in another Autonomous System.  The implications and
   applications of this architecture are for further study.

   If a particular AS has multiple BGP speakers and is providing transit
   service for other ASs, then care must be taken to ensure a consistent
   view of routing within the AS.  A consistent view of the interior
   routes of the AS is provided by the interior routing protocol.  A
   consistent view of the routes exterior to the AS can be provided by
   having all BGP speakers within the AS maintain direct BGP connections
   with each other.  Using a common set of policies, the BGP speakers
   arrive at an agreement as to which border routers will serve as
   exit/entry points for particular networks outside the AS.  This
   information is communicated to the AS's internal routers, possibly
   via the interior routing protocol.  Care must be taken to ensure that
   the interior routers have all been updated with transit information
   before the BGP speakers announce to other ASs that transit service is
   being provided.

   Connections between BGP speakers of different ASs are referred to as
   "external" links.  BGP connections between BGP speakers within the
   same AS are referred to as "internal" links.  Similarly, a peer in a
   different AS is referred to as an external peer, while a peer in the
   same AS may be described as an internal peer.

3.1 Routes: Advertisement and Storage

   For purposes of this protocol a route is defined as a unit of
   information that pairs a destination with the attributes of a path to
   that destination:

      - Routes are advertised between a pair of BGP speakers in UPDATE
      messages: the destination is the systems whose IP addresses are
      reported in the Network Layer Reachability Information (NLRI)
      field, and the the path is the information reported in the path
      attributes fields of the same UPDATE message.

      - Routes are stored in the Routing Information Bases (RIBs):
      namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes
      that will be advertised to other BGP speakers must be present in
      the Adj-RIB-Out; routes that will be used by the local BGP speaker
      must be present in the Loc-RIB, and the next hop for each of these
      routes must be present in the local BGP speaker's forwarding



Rekhter & Li