network protocols handbook.pdf

208Protocols GuideLAN - Virtual LAN ProtocolsProtocol NameGMRP: GARP Multicast RegistrationProtocolGARP PDU Format:2 bytesProtocol IDMessageProtocol DescriptionGARP Multicast Registration Protocol (GMRP) is a Generic AttributeRegistration Protocol (GARP) application that provides aconstrained multicast flooding facility similar to IGMP snooping.GMRP and GARP are industry-standard protocols defined bythe IEEE 802.1P.GMRP provides a mechanism that allows bridges and end stationsto dynamically register group membership information withthe MAC bridges attached to the same LAN segment and for thatinformation to be disseminated across all bridges in a BridgedLAN that supports extended filtering services. The operation ofGMRP relies upon the services provided by the GARP.GMRP software components run on both the switch and on thehost. On the host, GMRP is typically used with IGMP: The hostGMRP software spawns Layer 2 GMRP versions of the host’sLayer 3 IGMP control packets. The switch receives both theLayer 2 GMRP and the Layer 3 IGMP traffic from the host. Theswitch uses the received GMRP traffic to constrain multicastsat Layer 2 in the host’s VLAN. In all cases, you can use IGMPsnooping to constrain multicasts at Layer 2 without the need toinstall or configure software on hosts.When a host wants to join an IP multicast group, it sends anIGMP join message, which spawns a GMRP join message.Upon receipt of the GMRP join message, the switch adds theport through which the join message was received to the appropriatemulticast group. The switch propagates the GMRP joinmessage to all other hosts in the VLAN, one of which is typicallythe multicast source. When the source is multicasting to thegroup, the switch forwards the multicast only to the ports fromwhich it has received join messages for the group. The switchsends periodic GMRP queries. If a host wants to remain in amulticast group, it responds to the query. In this case, the switchdoes nothing. If a host does not want to remain in the multicastgroup, it can either send a leave message or not respond to theperiodic queries from the switch. If the switch receives a leavemessage or receives no response from the host for the durationof the leaveall timer, it removes the host from the multicastgroup.GARP message structure1 byteAttributetypeGARP attribute structureAttribute 1 ... Attribute n End mark1 byte 1 byte 1 byteAttribute length Attribute event Attribute value• Protocol ID - Identifies the GARP protocol.• Identifier - Decimal value which aids in matching requestsand replies.• Attribute type - Defines the attribute. Values may be:1 Group attribute; 2 Service Requirement attribute.• Attribute length - Length of the Attribute.• Attribute event - The values of the attribute event canbe:0 Leave_all1 Join_Empty operator2 Join_In operator3 Leave_Empty operator4 Leave_In operator5 Empty operator• Attribute value - This is encoded in accordance withthe specification for the Attribute Type.• End mark - Coded as 0.Related protocolsIEEE 802.1D, 802.1G, 802.1Q, 802.1P, 802.1ac, VTP, GVRP,GARPSponsor SourceGMRP standard is defined by IEEE (http://www.ieee.org)802.1P.Referencehttp://www.alliedtelesyn.co.nz/documentation/at8700/261/pdf/garp.pdfOverview of Generic Attribute Registration ProtocolProtocol StructureGMRP messages have the same structure as GARP with theattribute type specific to GMRP: This can be as follows: 1 GroupAttribute Type; 2 Service Requirement Attribute Type.