RFC 3488 (rfc3488) - Page 2 of 17
Cisco Systems Router-port Group Management Protocol (RGMP)
Alternative Format: Original Text Document
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 224.0.0.25. This
address has been assigned by IANA to carry messages from routers to
switches [3].
RGMP is designed to work in conjunction with multicast routing
protocols where explicit join/prune to the distribution tree is
performed. PIM-SM [4] 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.
Wu & Eckert Informational