Displays the multicast groups that are directly connected to the router and that were learned via IGMP

If the multicast traffic is not flowing to receivers, the IGMP group membership has to be checked on the leaf routers. The show ip igmp interface command shows information about the selected interface, and show ip igmp groups lists the local groups known to the router.

Enabling PIM on an interface also enables IGMP operation on that interface. An interface can be configured to be in dense mode, sparse mode, or sparse-dense mode. The mode determines how the router populates its multicast routing table and how the router forwards multicast packets that it receives from its directly connected LANs. You must enable PIM in one of these modes for an interface to perform IP multicast routing.

Configure a Router to Be a Member of a Group or a Statically Connected Member

Sometimes either there is no group member on a network segment or a host cannot report its group membership using IGMP. However, you may want multicast traffic to go to that network segment. These commands are often used in lab environments where no multicast servers and receivers are configured. The following are two ways to pull multicast traffic down to a network segment:

■ ip igmp join-group: With this command, the router accepts the multicast packets in addition to forwarding them. Accepting the multicast packets prevents the router from fast switching.

Cisco Systems routers can be configured to be members of a multicast group, which is useful for determining multicast reachability in a network. If a device is configured to be a group member and supports the protocol that is being transmitted to the group, it can respond (for example, to the ping command). The device responds to Internet Control Message Protocol (ICMP) echo request packets addressed to a group of which it is a member.

7-58 Building Scalable Cisco Internetworks (BSCI) v3.0 © 2006 Cisco Systems, Inc.

Another example is the multicast traceroute tools provided in the Cisco IOS software. To have the router join a multicast group and enable IGMP, use the command shown in the table in interface configuration mode. This command is often used in lab environments where no multicast servers and receivers are configured.

Command

Purpose

ip igmp join-group group-address

Joins a multicast group

■ ip igmp static-group: With this command, the router does not accept the packets itself but only forwards them. Hence, this method allows fast switching. The outgoing interface appears in the IGMP cache, but the router itself is not a member, as evidenced by lack of an "L" (local) flag in the multicast route entry.

To configure the router itself to be a statically connected member of a group (and allow fast switching), use the command shown in the table in interface configuration mode.

Command

Purpose

ip igmp static-group group-address

Configures the router as a statically connected member of a group

© 2006 Cisco Systems, Inc. Implementing Multicast 7-59

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