Understanding Gmrp Operation - Cisco WS-C2948G-GE-TX Configuration Manual

Chapter 15 Configuring Multicast Services
CGMP leave-processing allows the switch to detect IGMP version 2 leave messages that were 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 that
is specified in the original leave message. CGMP leave processing optimizes bandwidth management for
all hosts on a switched network, even when multiple multicast groups are in use simultaneously.
When CGMP fast-leave processing is enabled, the switch does not start a query response timer. The
switch immediately prunes the port from the multicast tree for the multicast group by deleting the
multicast MAC address from the port that received an IGMP leave message.

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 that are 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.
In all cases, you can use CGMP or IGMP snooping to constrain multicasts at Layer 2 without installing
Note
or configuring 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.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
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Understanding How Multicasting Works
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