Cisco Systems 78-11741-02 Enabling Cgmp, Configuring Stub IP Multicast Routing, Enables Cgmp

Configuring IP Multicast Routing

Enabling CGMP

To allocate a circular buffer to store IP multicast packet headers that the router receives, use the following command in global configuration mode:

Command	Purpose
Router(config)# ip multicast cache-headers	Allocates a buffer to store IP multicast packet headers.

Note The ip multicast cache-headersglobal 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:

Command	Purpose
Router(config-if)# ip cgmp [proxy]	Enables CGMP.

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.

Cisco IOS IP Configuration Guide

IPC-440