Modifying The Igmp Host-Query Message Interval - Cisco Catalyst 3550 series Software Configuration Manual

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 Cisco 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:
Command
Step 1 configure terminal
Step 2
interface interface-id
Step 3
ip igmp query-interval seconds
Step 4
end
Step 5
show ip igmp interface [interface-id]
Step 6 copy running-config startup-config
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:
•
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Catalyst 3550 Multilayer Switch Software Configuration Guide
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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.
Purpose
Enter global configuration mode.
Enter interface configuration mode, and specify the interface to be
configured.
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 1 to 18000.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
Chapter 34 Configuring IP Multicast Routing
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