Cisco WS-C4003 Software Guide - Page 233

Chapter 15 Configuring Multicast Services Understanding How Multicasting Works Note If there are other hosts in the same multicast group and they do respond to the multicast group query, the router does not request the switch to remove the group from its forwarding tables. The router does not remove a multicast group from the forwarding tables of the switch until all the hosts in the group ask to leave the group. CGMP fast-leave-processing allows the switch to detect IGMP version 2 leave messages sent to the all-routers multicast address by hosts on any of the supervisor engine module ports. When the supervisor engine module receives a leave message, it starts a query-response timer. If this timer expires before a CGMP join message is received, the port is pruned from the multicast tree for the multicast group specified in the original leave message. Fast-leave processing ensures optimal bandwidth management for all hosts on a switched network, even when multiple multicast groups are in use simultaneously. Understanding GMRP Operation GARP Multicast Registration Protocol (GMRP) is a Generic Attribute Registration Protocol (GARP) application that provides a constrained multicast flooding facility similar to IGMP snooping and CGMP. GMRP and GARP are industry-standard protocols defined by the IEEE. For detailed protocol operational information, refer to IEEE 802.1p. GMRP can register and deregister multicast group addresses at the MAC layer throughout the Layer 2-connected network. GMRP is Layer 3-protocol independent, which allows it to support the multicast traffic of any Layer 3 protocol (such as IP, IPX, and so forth). GMRP software components run on both the switch and on the host (Cisco is not a source for GMRP host software). On the host, GMRP is typically used with IGMP: the host GMRP software generates Layer 2 GMRP versions of the host's Layer 3 IGMP control packets. The switch receives both the Layer 2 GMRP and the Layer 3 IGMP traffic from the host. The switch uses the received GMRP traffic to constrain multicasts at Layer 2 in the host's VLAN. Note In all cases, you can use CGMP or IGMP snooping to constrain multicasts at Layer 2 without the need to install or configure software on hosts. When a host wants to join an IP multicast group, it sends an IGMP join message, which creates a corresponding GMRP join message. When the switch receives the GMRP join message, it adds the port through which the join message was received to the appropriate multicast group. The switch propagates the GMRP join message to all other hosts in the VLAN, one of which is typically the multicast source. When the source is multicasting to the group, the switch forwards the multicast only to the ports 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, it responds to the query. In this case, the switch does nothing. If a host does not want to remain in the multicast group, it can either send a leave message or not respond to the periodic queries from the switch. If the switch receives a leave message or receives no response from the host for the duration of the leaveall timer, the switch removes the host from the multicast group. Note To use GMRP in a routed environment, enable the GMRP forward-all option on all ports where routers are attached. 78-12647-02 Software Configuration Guide-Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4 15-3