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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter24 Configuring IP Multicast Routing
Configuring Optional IGMP Features
Modifying the IGMP Host-Query Message Interval
The multilayer switch periodically sends IGMP host-query messages to discover which multica st groups
are present on attached networks. These messages are sent to the a ll-ho sts m ultic as t gr oup (224. 0. 0.1)
with a time-to-live (TTL) of 1. The switch sends host-query messages to refresh its knowledge of
memberships present on the network. If, after some number of queries, the IOS software discovers that
no local hosts are members of a multicast group, the software stops forwarding multicast packets to the
local network from remote origins for that group and sends a prune message upstrea m toward the source.
The switch elects a PIM designated router (DR) for the LAN (subnet). The DR is the router or multilayer
switch with the highest IP address for IGMPv2; for IGMPv1 , the DR i s elected according t o the multi cast
routing protocol that runs on the LAN. The designated router is r esponsi b le for se ndin g I GMP
host-query messages to all hosts on the LAN. In sparse mode, the desig nat ed rou ter also s end s PI M
register and PIM join messages toward the RP router.
Beginning in privileged EXEC mode, follow these steps to modify the host-query interval:
To return to the default frequency, use the no ip igmp query-interval interface configuration command.
Configuring the Multilayer Switch as a Statically Connected Member
Sometimes there is either no group member on a networ k segm en t or a h ost c anno t repo rt i ts gro up
membership by using IGMP. However, you might want multicast traffic to go to that network segment.
These are ways to pull multicast traffic down to a network segment:
•Use the ip igmp join-group interface configuration command. With this method, the multilayer
switch accepts the multicast packets in addition to forwarding them. Accepting the multicast packets
prevents the switch from fast switching.
•Use the ip igmp static-group interface configuration command. With this method, the multilayer
switch does not accept the packets itself, but only forwards them. This m ethod allows fast switching.
The outgoing interface appears in the IGMP cache, but the switch itself is not a member, as
evidenced by lack of an L (local) flag in the multicast route entry.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Enter interface configuration mode, and specify the interface to be
configured.
Step3 ip igmp query-interval seconds Configure the frequency at which the designated router sends IGMP
host-query messages.
By default, the designated router sends IGMP host-query messages
every 60 seconds to keep the IGMP overhead very low on hosts an d
networks. The range is 0 to 65535.
Step4 end Return to privileged EXEC mode.
Step5 show ip igmp interface [interface-id] Verify your entries.
Step6 copy running-config startup-config (Optional) Save your entries in the configuration file.