network protocols handbook.pdf

275Protocols GuideCisco ProtocolsProtocol NameRGMP: Cisco Router PortGroup Management ProtocolProtocol DescriptionThe Cisco Router Port Group Management Protocol (RGMP) isdefined to address the limitations of Internet Group ManagementProtocol (IGMP) in its Snooping mechanism. RGMP is used betweenmulticast routers and switches to restrict multicast packetforwarding in switches to those routers where the packets maybe needed. RGMP is designed for backbone switched networkswhere multiple, high speed routers are interconnected.The main limitation of IGMP Snooping is that it can only restrictmulticast traffic onto switch ports where receiving hosts are connecteddirectly or indirectly via other switches. IGMP Snoopingcan not restrict multicast traffic to ports where at least one multicastrouter is connected. It must instead flood multicast trafficto these ports. Snooping on IGMP messages alone is an intrinsiclimitation. Through it, a switch can only learn which multicastflows are being requested by hosts. A switch cannot learnthrough IGMP which traffic flows need to be received by routerports to be routed because routers do not report these flows viaIGMP.The RGMP protocol restricts multicast traffic to router ports.To effectively restrict traffic, it must be supported by both theswitches and the routers in the network. Backbone switchesuse RGMP to learn which groups are desired at each of theirports. Multicast routers use RGMP to pass such information tothe switches. Only routers send RGMP messages. They ignorereceived RGMP messages. When a router no longer needs toreceive traffic for a particular group, it sends an RGMP Leavemessage for the group. A switch enabled for RGMP on a networkconsumes RGMP messages received from ports of thenetwork and processes them. If enabled for RGMP, the switchmust NOT forward/flood received RGMP messages out to otherports of the network.RGMP is designed to work in conjunction with multicast routingprotocols where explicit join/prune to the distribution tree is performed.PIM-SM is an example of such a protocol. The RGMPprotocol specifies operations only for IP version 4 multicast routing.IP version 6 is not considered.Protocol StructureRGMP message format is the same as the IGMPv2:• Type - There are four types of RGMP messages ofconcern to the router-switch interaction. The typecodes are defined to be the highest values in anoctet to avoid the re-use of already assigned IGMPtype codes: 0xFF = Hello; 0xFE = Bye; 0xFD = Join agroup; 0xFC = Leave a group.• Reserved - The reserved field in the message MUSTbe transmitted as zeros and ignored on receipt.• Checksum - Checksum covers the RGMP message(the entire IPv4 payload). The algorithm and handlingof 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 RGMPJoin or Leave message, the group address fieldholds the IPv4 multicast group address of the groupbeing joined or left.Related protocolsIPv4, IGMP, PIM-SM, CGMPSponsor SourceRGMP is a Cisco protocol.Referencehttp://www.javvin.com/protocol/rfc3488.pdfCisco Systems Router Port Group Management Protocol8 16 32bitType Reserved ChecksumGroup Address