Using The Switch As Querier - HP 2920 Series Multicast And Routing Manual

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:

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 Proxy loop scenario

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.

Be careful to avoid configuring a IGMP forward loop, because this would leave the

Figure

3.

Using the switch as querier

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