Understanding How Gmrp Works - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Understanding How Multicasting Works
When the router receives the IGMP leave, it sends several IGMP group-specific queries. If no join
messages are received in response to the queries, and there are no downstream routers connected through
that interface, the router removes the interface from the OIL for that IP multicast group entry in the
multicast routing table.
IGMP Fast-Leave Processing
IGMP snooping fast-leave processing allows the switch processor to remove an interface from the port
list of a forwarding-table entry without first sending out a MAC-based general query on the port. When an
IGMP leave is received on a port, the port is immediately removed from the multicast forwarding entry (or
the entire entry is removed).
Do not use the fast-leave processing feature if more than one host is connected to each port. If
Note
fast-leave is enabled when more than one host is connected to a port, some hosts might be dropped
inadvertently. Fast leave is supported with IGMP version 2 hosts only.

Understanding How GMRP Works

GMRP is a Generic Attribute Registration Protocol (GARP) application that provides a constrained
multicast flooding facility similar to IGMP snooping. GMRP and GARP are industry-standard protocols
defined by the IEEE. For detailed protocol operational information, refer to 802.1p.
GMRP software components run on both the switch and on the host. (Cisco is not a source for GMRP
host software.) On the host, in an IP multicast environment, you must use IGMP with GMRP; the host
GMRP software spawns 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
forwards the Layer 3 IGMP control packets to the router and uses the received GMRP traffic to constrain
multicasts at Layer 2 in the host's VLAN.
When a host wants to join an IP multicast group, it sends an IGMP join, which spawns a GMRP join.
When the switch receives the GMRP join, it adds the port through which the join was received to the
appropriate multicast group. The switch propagates the GMRP join 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 and 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 forwardall option on all ports where
Note
routers are attached. (See the
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
40-4
"Enabling GMRP Forward-All Option" section on page
Chapter 40 Configuring Multicast Services
40-15.)
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