RFC 3488 (rfc3488) - Page 2 of 17

Cisco Systems Router-port Group Management Protocol (RGMP)

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