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