RFC 2189 (rfc2189) - Page 3 of 23

Core Based Trees (CBT version 2) Multicast Routing -- Protocol Specification --

RFC 2189              CBTv2 Protocl Specification         September 1997


   The most significant changes to version 2 compared to version 1
   include:

   o new LAN mechanisms, including the incorporation of an HELLO
     protocol.

   o new simplified packet formats, with the definition of a common CBT
     control packet header.

   o each group shared tree has only one active core router.

     This specification revision is a complete re-write of the previous
     revision.

2.  Introduction & Terminology

   In CBT, a "core router" (or just "core") is a router which acts as a
   "meeting point" between a sender and group receivers. The term
   "rendezvous point (RP)" is used equivalently in some contexts [2]. A
   core router need not be configured to know it is a core router.

   A router that is part of a CBT distribution tree is known as an "on-
   tree" router. An on-tree router maintains active state for the group.

   We refer to a broadcast interface as any interface that supports
   multicast transmission.

   An "upstream" interface (or router) is one which is on the path
   towards the group's core router with respect to this interface (or
   router). A "downstream" interface (or router) is one which is on the
   path away from the group's core router with respect to this interface
   (or router).

   Other terminology is introduced in its context throughout the text.

3.  CBT Functional Overview

   The CBT protocol is designed to build and maintain a shared multicast
   distribution tree that spans only those networks and links leading to
   interested receivers.

   To achieve this, a host first expresses its interest in joining a
   group by multicasting an IGMP host membership report [3] across its
   attached link. On receiving this report, a local CBT aware router
   invokes the tree joining process (unless it has already) by
   generating a JOIN_REQUEST message, which is sent to the next hop on
   the path towards the group's core router (how the local router
   discovers which core to join is discussed in section 8). This join



Ballardie                     Experimental