MN700004 Rev 01 18615. GARP Multicast RegistrationProtocol (GMRP)IntroductionGARP Multicast Registration Protocol (GMRP) is a Generic Attribute Registration Protocol(GARP) application that provides a constrained multicast flooding facility similar to IGMPsnooping. GMRP and GARP are industry-standard protocols defined by the IEEE 802.1p.Feature OverviewGMRP can register and deregister multicast group addresses at the MAC layer throughout theLayer 2 connected network. GMRP is Layer 3 protocol independent, which allows it tosupport the multicast traffic of any Layer 3 protocol (such as IP, IPX etc.).GMRP software components run both on the switch and on the host (The switch is not asource for GMRP host software). On the host, GMRP is typically used with IGMP: the hostGMRP software generates Layer 2 GMRP versions of the host’s Layer 3 IGMP controlpackets. The switch receives both the Layer 2 GMRP and the Layer 3 IGMP traffic from thehost. The switch uses the received GMRP traffic to constrain multicasts at Layer 2 in thehost’s VLAN.NOTE In all cases, you can use IGMP snooping to constrain multicasts at Layer 2 without the needto install or configure software on hosts.When a host wants to join an IP multicast group, it sends an IGMP join message, whichcreates a corresponding GMRP join message. When the switch receives the GMRP joinmessage, it adds the port through which the join message was received to the appropriatemulticast group. The switch propagates the GMRP join message to all other hosts in theVLAN, one of which is typically the multicast source.When the source is multicasting to the group, the switch forwards the multicast only to theports from which it received join messages for the group.The switch sends periodic GMRP queries. If a host wants to remain in a multicast group, itresponds to the query. In this case, the switch does nothing. If a host does not want to remainin the multicast group, it can either send a leave message or not respond to the periodicqueries from the switch. If the switch receives a leave message or receives no response fromthe host for the duration of the leave-all timer, the switch removes the host from the multicastgroup.
