Understanding Gmrp Operation - Cisco WS-C4003 - Catalyst 4000 Chassis Switch Software Configuration Manual

Chapter 15 Configuring Multicast Services
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.
To use GMRP in a routed environment, enable the GMRP forward-all option on all ports where routers
Note
are attached.
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Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
Understanding How Multicasting Works
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