H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 22)

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Configuring IGMP snooping querier
Before you configure IGMP snooping querier, complete the following tasks:
• Enable IGMP snooping in the VLAN
• Determine the IGMP general query interval
• Determine the IGMP last-member query interval
• Determine the maximum response time to IGMP general queries
• Determine the source address of IGMP general queries
• Determine the source address of IGMP group-specific queries
Enabling IGMP snooping querier
In an IP multicast network that runs IGMP, a multicast router or Layer 3 multicast switch sends IGMP
queries, so that all Layer 3 multicast devices can establish and maintain multicast forwarding entries, in
order to forward multicast traffic correctly at the network layer. This router or Layer 3 switch is called the
"IGMP querier."
However, a Layer 2 multicast switch does not support IGMP, and therefore cannot send queries by default.
When you enable IGMP snooping querier on a Layer 2 device in a VLAN where multicast traffic is
switched only at Layer 2 and no multicast routers are present, the Layer 2 device sends IGMP queries, so
that multicast forwarding entries can be established and maintained at the data link layer.
IMPORTANT:
Although an IGMP snooping querier does not participate in IGMP querier elections, it might affect IGMP
querier elections because it sends IGMP general queries with a low source IP address.
To enable IGMP snooping querier:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enter VLAN view. vlan vlan-id N/A
3. Enable IGMP snooping
querier.
igmp-snooping querier Disabled by default
Configuring IGMP queries and responses
You can set the IGMP query interval based on actual condition of the network.
After 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.