CAUTION:
Be careful to avoid configuring a IGMP forward loop, because this would leave the
VLANs in a joined state forever once an initial join is sent from a host. For example, a join is issued
from the host in VLAN 2 and Routing Switch 2 will proxy the join onto VLAN 1. Routing Switch 3 will
then proxy the join back onto VLAN 2 and increment its internal count of the number of joins on
VLAN 2. Even after the host on VLAN 2 issues a leave, the proxy join will continue to remain and
refresh itself each time a query occurs on VLAN 2. This type of loop could be created with multiple
routers if an IGMP proxy is allowed to get back to the VLAN of the router that initially received the
IGMP join from a host as shown in the following figure.
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
Chapter 2 Multimedia traffic control with IP multicast (IGMP)
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