KB.15.15

Figure 6 Proxy loop scenario

About 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 multicastrouter, 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 01/15/12 09:01:13 igmp: DEFAULT_VLAN: Other Querier detected

I 01/15/12 09:01:13 igmp: DEFAULT_VLAN: This switch is no longer Querier

In the above scenario, if the other device ceases to operate as a Querier on the default VLAN, 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 01/15/12 09:21:55 igmp: DEFAULT_VLAN: Querier Election in process

I 01/15/12 09:22:00 igmp: DEFAULT_VLAN: This switch has been elected as Querie

34 Multimedia Traffic Control with IP Multicast (IGMP)