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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter24 Configuring IP Multicast Routing
Configuring Optional Multicast Routing Features
Enabling CGMP Server Support
The multilayer switch serves as a CGMP server for devices that do not support IGMP snooping but have
CGMP client functionality. CGMP is a protocol used on Cisco routers and multilayer switches connected
to Layer 2 Catalyst switches to perform tasks similar to those performed by IGMP. CGMP is necessary
because the Layer 2 switch cannot distinguish between IP multicast data packets and IGMP repo rt
messages, which are both at the MAC-level and are addressed to the same group address.
Beginning in privileged EXEC mode, follow these steps to enable the CGMP server on the multilayer
switch interface:
To disable CGMP on the interface, use the no ip cgmp interface configuration command.
When multiple Cisco CGMP-capable devices are connected to a switched network an d the ip c gmp
proxy command is needed, we recommend that all devices be conf igur ed wi th the sam e CG MP opt ion
and have precedence for becoming the IGMP querier over non-Cisco routers.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Enter interface configuration mode, and specify the interface that is
connected to the Layer 2 Catalyst switch.
Step3 ip cgmp [proxy] Enable CGMP on the interface.
By default, CGMP is disabled on all interfaces.
Enabling CGMP triggers a CGMP join message. Enable CGMP only on
Layer 3 interfaces connected to Layer 2 Catalyst switches.
(Optional) When you enter the proxy keyword, the CGMP proxy function
is enabled. The proxy router advertises the existence of
non-CGMP-capable routers by sending a CGMP join messag e with the
non-CGMP-capable router MAC address and a group address
of 0000.0000.0000.
Note To perform CGMP proxy, the multilayer switch must be the IGMP
querier. If you configure the ip cgmp proxy command, you must
manipulate the IP addresses so that the switch is the IGMP
querier, which might be the highest or lowest IP address,
depending on which version of IGMP is running on the network.
An IGMP Version 2 querier is selected based on the lowest IP
address on the interface. An IGMP Version 1 querier is selected
based on the multicast routing protocol used on the interface.
Step4 end Return to privileged EXEC mode.
Step5 show running-config Verify your entries.
Step6 copy running-config startup-config (Optional) Save your entries in the configuration file.
Step7 Verify the Layer 2 Catalyst switch CGMP-client configuration. For more
information, refer to the documentation that shipped with the product.