Intra-LIS IP multicast among routers over ATM using Sparse Mode PIM
RFC 2337

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Network Working Group                                       D. Farinacci
Request for Comments: 2337                                 Cisco Systems
Category: Experimental                                          D. Meyer
                                                           Cisco Systems
                                                              Y. Rekhter
                                                           Cisco Systems
                                                              April 1998

  Intra-LIS IP multicast among routers over ATM using Sparse Mode PIM

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

2. Abstract

   This document describes how intra-LIS IP multicast can be efficiently
   supported among routers over ATM without using the Multicast Address
   Resolution Server (MARS). The method described here is specific to
   Sparse Mode PIM [PIM-SM], and relies on the explicit join mechanism
   inherent in PIM-SM to notify routers when they should create group
   specific point-to-multipoint VCs.

3. Overall model

   This document focuses on forwarding of multicast traffic among PIM-SM
   routers connected to an ATM network. Routers on an ATM network are
   partitioned into Logical IP Subnets, or LISs.  This document deals
   with handling multicast within a single LIS. Handling inter-LIS
   multicast traffic, including handling shortcuts, is outside the scope
   of this document.  In addition, this document does not address
   forwarding of multicast traffic to or from hosts connected to an ATM
   network.

Farinacci, et. al.            Experimental                      [Page 1]
RFC 2337            IP multicast over ATM using PIM           April 1998

4. Router behavior

   This document requires that each router within a LIS knows IP and ATM
   addresses of all other routers within the LIS. The mapping between IP
   and ATM addresses may be provided by an ARP server [RFC2225], or by
   any other means (e.g., static configuration).

   Each PIM router within a LIS is required to maintain a single
   (shared) point-to-multipoint distribution VC rooted at the router
   with all other PIM routers in the LIS as the leaf nodes. The VC is
   expected to be used for forwarding of multicast traffic (both data
   and control) among routers within the LIS. For example, this VC would
   be used for distributing PIM [PIM-SM] control messages (Join/Prune
   messages).

   In addition, if a PIM router receives a IGMP report from an non-PIM
   neighbor, then the router may add the reporter to the existing shared
   distribution VC or to the group specific distribution VC (if it
   exists). The PIM router may also create a specific VC for this IGMP
   proxy.

4.1. Establishing Dedicated, Per Group Point-to-Multipoint VCs

   Routers may also maintain group specific, dedicated point-to-
   multipoint VCs. In particular, an upstream router for a group may
   choose to become the root of a group specific point-to-multipoint VC
   whose leaves are the downstream routers that have directly connected
   or downstream receivers for the group. While the criteria for
   establishing a group specific point-to-multipoint VC are local to a
   router, issues such as the volume of traffic associated with the
   group and the fanout factor within the LIS should be considered.
   Finally, note that a router must minimally support a single shared
   point-to-multipoint VC for distribution of control messages and data
   (to all group addresses).

   A router can choose to establish a dedicated point-to-multipoint VC
   (or add another leaf to an already established dedicated point-to-
   multipoint VC) when it receives a PIM Join or IGMP report messages
   from another device in the same LIS. When a router that is the root
   of a point-to-multipoint VC receives PIM Prune message or IGMP leave,
   it removes the originator of the message from its dedicated point-
   to-multipoint VC.

Farinacci, et. al.            Experimental                      [Page 2]
RFC 2337            IP multicast over ATM using PIM           April 1998

4.2. Switching to a Source-Rooted Tree

   If at least one of the routers within a LIS decides to switch to a
   source-rooted tree (by sending (S,G) PIM Joins), then all other
   routers within the LIS that have downstream members for G should
   switch to that source-rooted tree as well. Since a router that
   switches to a source-rooted tree sends PIM Join messages for (S,G)
   over its shared point-to-multipoint VC, the other routers within the
   LIS are able to detect this. Once a router that has downstream
   members for G detects this, the router should send (S,G) PIM Join