Cisco Systems Router-port Group Management Protocol (RGMP)
RFC - Informational
(March 2003; No errata)
||RFC Editor Note
RFC 3488 (Informational)
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Network Working Group I. Wu
Request for Comments: 3488 T. Eckert
Category: Informational Cisco Systems
Router-port Group Management Protocol (RGMP)
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document describes the Router-port Group Management Protocol
(RGMP). This protocol was developed by Cisco Systems and is used
between multicast routers and switches to restrict multicast packet
forwarding in switches to those routers where the packets may be
RGMP is designed for backbone switched networks where multiple, high
speed routers are interconnected.
1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 .
IGMP Snooping is a popular, but not well documented mechanism to
restrict multicast traffic, in switched networks, to those ports that
want to receive the multicast traffic. It dynamically establishes
and terminates multicast group specific forwarding in switches that
support this feature.
Wu & Eckert Informational [Page 1]
RFC 3488 Cisco Systems RGMP February 2003
The main limitation of IGMP Snooping is that it can only restrict
multicast traffic onto switch ports where receiving hosts are
connected directly or indirectly via other switches. IGMP Snooping
can not restrict multicast traffic to ports where at least one
multicast router is connected. It must instead flood multicast
traffic to these ports. Snooping on IGMP messages alone is an
intrinsic limitation. Through it, a switch can only learn which
multicast flows are being requested by hosts. A switch can not learn
through IGMP which traffic flows need to be received by router ports
to be routed because routers do not report these flows via IGMP.
In situations where multiple multicast routers are connected to a
switched backbone, IGMP Snooping will not reduce multicast traffic
load. Nor will it assist in increasing internal bandwidth through
the use of switches in the network.
In switched backbone networks or exchange points, where predominantly
routers are connected with each other, a large amount of multicast
traffic may lead to unexpected congestion. It also leads to more
resource consumption in the routers because they must discard the
unwanted multicast traffic.
The RGMP protocol described in this document restricts multicast
traffic to router ports. To effectively restrict traffic, it must be
supported by both the switches and the routers in the network.
The RGMP message format resembles the IGMPv2 message format so that
existing switches, capable of IGMP Snooping, can easily be enhanced
with this feature. Its messages are encapsulated in IPv4 datagrams,
with a protocol number of 2, the same as that of IGMP. All RGMP
messages are sent with TTL 1, to destination address 188.8.131.52. This
address has been assigned by IANA to carry messages from routers to
RGMP is designed to work in conjunction with multicast routing
protocols where explicit join/prune to the distribution tree is
performed. PIM-SM  is an example of such a protocol.
The RGMP protocol specifies operations only for IP version 4
multicast routing. IP version 6 is not considered.
To keep RGMP simple, efficient and easy to implement, it is designed
for switches to expect RGMP messages from only one source per port.
For this reason, RGMP only supports a single RGMP enabled router to
be connected directly to a port of an RGMP enabled switch. Such a
topology should be customary when connecting routers to backbone
switches and thus not pose a limitation on the deployment of RGMP.
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RFC 3488 Cisco Systems RGMP February 2003
All RGMP messages have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Type | Reserved | Checksum |
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