RGMP: Cisco Router Port Group Management Protocol

The Cisco Router Port Group Management Protocol (RGMP) is defined to address the limitations of Internet Group Management Protocol (IGMP) in its Snooping mechanism.  RGMP is used between multicast routers and switches to restrict multicast packet forwarding in switches to those routers where the packets may be needed. RGMP is designed for backbone switched networks where multiple, high speed routers are interconnected.

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.

The RGMP protocol restricts multicast traffic to router ports. To effectively restrict traffic, it must be supported by both the switches and the routers in the network.

Backbone switches use RGMP to learn which groups are desired at each of their ports. Multicast routers use RGMP to pass such information to the switches. Only routers send RGMP messages. They ignore received RGMP messages. When a router no longer needs to receive traffic for a particular group, it sends an RGMP Leave message for the group. A switch enabled for RGMP on a network consumes RGMP messages received from ports of the network and processes them. If enabled for RGMP, the switch must NOT forward/flood received RGMP messages out to other ports of the network.

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.

• Type - There are four types of RGMP messages of concern to the router-switch interaction. The type codes are defined to be the highest values in an octet to avoid the re-use of already assigned IGMP type codes: 0xFF = Hello; 0xFE = Bye; 0xFD = Join a group; 0xFC = Leave a group.
• Reserved - The reserved field in the message MUST be transmitted as zeros and ignored on receipt.
• Checksum - Checksum covers the RGMP message (the entire IPv4 payload). The algorithm and handling of checksum are the same as those for IGMP messages.
• Group Address - In an RGMP Hello or Bye message, the group address field is set to zero. In an RGMP Join or Leave message, the group address field holds the IPv4 multicast group address of the group being joined or left.

RGMP is a Cisco protocol.