Cisco Systems 78-11741-02 manual Changing the Igmp Version, Routers That Run Igmp Version, IPC-410

Configuring IP Multicast Routing

IGMP Features Configuration Task List

Changing the IGMP Version

By default, the router uses IGMP Version 2 (IGMPv2), which allows such features as the IGMP query timeout and the maximum query response time.

All routers on the subnet must support the same version. The router does not automatically detect Version 1 routers and switch to Version 1 as did earlier releases of the Cisco IOS software. However, a mix of IGMP Version 1 and Version 2 hosts on the subnet is acceptable. IGMP Version 2 routers will always work correctly in the presence of IGMP Version 1 hosts.

To control which version of IGMP the router uses, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip igmp version {3 2 1}	Selects the IGMP version that the router uses.

Modifying the IGMP Host-Query Message and Query Timeout Intervals

Multicast routers send IGMP host-query messages to discover which multicast groups are present on attached networks. These messages are sent to the all-systems group address of 224.0.0.1 with a time-to-live (TTL) of 1.

Multicast routers send host-query messages periodically to refresh their knowledge of memberships present on their networks. If, after some number of queries, the Cisco IOS software discovers that no local hosts are members of a multicast group, the software stops forwarding onto the local network multicast packets from remote origins for that group and sends a prune message upstream toward the source.

Routers That Run IGMP Version 1

If there are multiple routers on a LAN, a designated router (DR) must be elected to avoid duplicating multicast traffic for connected hosts. PIM routers follow an election process to select a DR. The PIM router with the highest IP address becomes the DR.

The DR is responsible for the following tasks:

•Sending PIM register and PIM join and prune messages toward the RP to inform it about host group membership.

•Sending IGMP host-query messages.

By default, the DR sends host-query messages every 60 seconds in order to keep the IGMP overhead on hosts and networks very low.

To modify this interval, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)#ip igmp query-interval seconds	Configures the frequency at which the designated router sends
	IGMP host-query messages.

Cisco IOS IP Configuration Guide

IPC-410