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Enabling IGMP snooping querier
In an IP multicast network running IGMP, a multicast router or Layer 3 multicast switch is responsible
for sending IGMP general queries, so that all Layer 3 multicast devices can establish and maintain
multicast forwarding entries to forward multicast traffic correctly at the network layer. This router or
Layer 3 switch is called "IGMP querier".
However, a Layer 2 multicast switch does not support IGMP, and cannot send general queries by
default. By enabling IGMP snooping on a Layer 2 switch in a VLAN where multicast traffic needs to
be Layer-2 switched only and no multicast routers are present, the Layer 2 switch will act as the
IGMP snooping querier to send IGMP queries, allowing multicast forwarding entries to be
established and maintained at the data link layer.
Follow these steps to enable IGMP snooping querier:
To do...
Use the command...
Remarks
Enter system view
system-view
—
Enter VLAN view
vlan
vlan-id
—
Enable IGMP snooping querier
igmp-snooping
querier
Required
Disabled by default
CAUTION:
It is meaningless to configure an IGMP snooping querier in a multicast network running IGMP.
Although an IGMP snooping querier does not take part in IGMP querier elections, it might affect
IGMP querier elections because it sends IGMP general queries with a low source IP address.
Configuring IGMP queries and responses
You can tune the IGMP general query interval based on actual condition of the network.
Upon receiving an IGMP query—general query or group-specific query, a host starts a timer for each
multicast group it has joined. This timer is initialized to a random value in the range of 0 to the
maximum response time—the host obtains the value of the maximum response time from the Max
Response Time field in the IGMP query it received. When the timer value comes down to 0, the host
sends an IGMP report to the corresponding multicast group.
An appropriate setting of the maximum response time for IGMP queries allows hosts to respond to
queries quickly and avoids bursts of IGMP traffic on the network caused by reports simultaneously
sent by a large number of hosts when the corresponding timers expire simultaneously.
•
For IGMP general queries, configure the maximum response time to fill their Max Response
time field.
•
For IGMP group-specific queries, configure the IGMP last-member query interval to fill their
Max Response time field. Namely, for IGMP group-specific queries, the maximum response
time equals to the IGMP last-member query interval.
Configuring IGMP queries and responses globally
Follow these steps to configure IGMP queries and responses globally:
To do...
Use the command...
Remarks
Enter system view
system-view
—
Enter IGMP snooping view
igmp-snooping
—
Configure the maximum response
time to IGMP general queries
max-response-time
interval
Optional
10 seconds by default