KB.15.15

• The domains may have overlapping multicast ranges.

• The IP address of the border router may be the same or different in each configured domain.

• Duplicate IGMP joins are automatically prevented, or leaves that would remove a flow currently

joined by multiple hosts.

• Range overlap allows for redundant connectivity and the ability for multicasts to arrive from

different border routers based on the shortest path back to the source of the traffic.

• The configured domain names must be associated with one or more VLANs for which the

proxy joins are to be done.

• All routers in the path between the edge router receiving the initial IGMP packets and the

border router have to be configured to forward IGMP using IGMP proxy.

• All upstream and downstream interfaces using IGMP proxy forwarding require IGMP and PIM

to be enabled.

• 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 6 (page 34).

How IGMP operates 33