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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 24 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 multicast groups
are present on attached networks. These messages are sent to the all-hosts multicast group (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 upstream 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 is elected according to the multicast
routing protocol that runs on the LAN. The designated router is responsible for sending IGMP
host-query messages to all hosts on the LAN. In sparse mode, the designated router also sends PIM
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 network segment or a host cannot report its group
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 method 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
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the interface to be
configured.
Step 3
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 and
networks. The range is 0 to 65535.
Step 4
end
Return to privileged EXEC mode.
Step 5
show ip igmp interface [interface-id]
Verify your entries.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.