Multimedia Traffic Control with IP Multicast (IGMP)
Using the Switch as Querier
Using the Switch as Querier
The function of the IGMP Querier is to poll other IGMP-enabled devices in an
IGMP-enabled VLAN to elicit group membership information. The switch
performs this function if there is no other device in the VLAN, such as a
multicast router, to act as Querier. Although the switch automatically ceases
Querier operation in an IGMP-enabled VLAN if it detects another Querier on
the VLAN, you can also use the switch’s CLI to disable the Querier capability
for that VLAN.
Note A Querier is required for proper IGMP operation. For this reason, if you disable
the Querier function on a switch, ensure that there is an IGMP Querier (and,
preferably, a backup Querier) available on the same VLAN.
If the switch becomes the Querier for a particular VLAN (for example, the
DEFAULT_VLAN), then subsequently detects queries transmitted from
another device on the same VLAN, the switch ceases to operate as the Querier
for that VLAN. If this occurs, the switch Event Log lists a pair of messages
similar to these:
I 01/15/01 09:01:13 igmp: DEFAULT_VLAN: Other Querier detected
I 01/15/01 09:01:13 igmp: DEFAULT_VLAN: This switch is no longer Queri
e
In the above scenario, if the other device ceases to operate as a Querier on the
default VLAN, then the switch detects this change and can become the Querier
as long as it is not pre-empted by some other IGMP Querier on the VLAN. In
this case, the switch Event Log lists messages similar to the following to
indicate that the switch has become the Querier on the VLAN:
I 01/15/01 09:21:55 igmp: DEFAULT_VLAN: Querier Election in process
I 01/15/01 09:22:00 igmp: DEFAULT_VLAN: This switch has been elected
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