4-21
Multimedia Traffic Control with IP Multicast (IGMP)
Using the Switch as Querier
Figure 4-6. Example of Changing the Forced Fast-Leave Configuration on Port 49
Using the Switch as Querier
Querier Operation
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 Command Prompt 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
Querier
Verifies Forced Fast-Leave enabled.
49 indicates port C1.
1 indicates the default VLAN. (See
the note on page 4-18.)
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