• You must remove all VLAN associations with the domain name before that domain name can
be removed.
• The appropriate border routers must be used for each VLAN, or PIM-DM will not forward the
traffic. This could occur when multiple border routers exist. It may be necessary to configure
multiple overlapping domains if the multicast source address can generate the same multicast
address and have different best paths to the PIM-DM domain.
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; see Figure 3.
Figure 3 Proxy loop scenario
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.
Using the switch as querier 23
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