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Configuring IP Multicast Routing
Enabling CGMP
IPC-440
Cisco IOS IP Configuration Guide
To allocate a circular buffer to store IP multicast packet headers that the router receives, use the
following command in global configuration mode:
Note The ip multicast cache-headers global configuration command allocates a circular buffer of
approximately 32 KB.
Use the show ip mpacket EXEC command to display the buffer.
Enabling CGMP
CGMP is a protocol used on routers connected to Catalyst switches to perform tasks similar to those
performed by IGMP. CGMP is necessary because the Catalyst switch cannot distinguish between IP
multicast data packets and IGMP report messages, which are both at the MAC level and are addressed
to the same group address.
Enabling CGMP triggers a CGMP join message. CGMP should be enabled only on 802 or ATM media,
or LAN emulation (LANE) over ATM. CGMP should be enabled only on routers connected to Catalyst
switches.
To enable CGMP for IP multicast on a LAN, use the following command in interface configuration
mode:
When the proxy keyword is specified, the CGMP proxy function is enabled. That is, any router that is
not CGMP-capable will be advertised by the proxy router. The proxy router advertises the existence of
other non-CGMP-capable routers by sending a CGMP join message with the MAC address of the
non-CGMP-capable router and group address of 0000.0000.0000.
Configuring Stub IP Multicast Routing
When you use PIM in a large network, there are often stub regions over which the administrator has
limited control. To reduce the configuration and administration burden, you can configure a subset of
PIM functionality that provides the stub region with connectivity, but does not allow it to participate in
or potentially complicate any routing decisions.
Stub IP multicast routing allows simple multicast connectivity and configuration at stub networks. It
eliminates periodic flood-and-prune behavior across slow-speed links (ISDN and below) using dense
mode. It eliminates that behavior by using forwarded IGMP reports as a type of join message and using
selective PIM message filtering.
Command Purpose
Router(config)# ip multicast cache-headers
Allocates a buffer to store IP multicast packet headers.
Command Purpose
Router(config-if)# ip cgmp [proxy]
Enables CGMP.