IGMP needs to be configured on all VLAN interfaces on which the proxy is to be forwarded or received, and PIM-
DM must be running for the traffic to be forwarded.
You can configure an IGMP proxy on a selected VLAN that will forward IP joins (reports) and IGMP leaves to the
upstream border router between the two multicast domains. You must specify the VLANs on which the proxy is
enabled as well as the address of the border router to which the joins are forwarded.
How IGMP proxy forwarding works
The following steps illustrate how to flood a flow from the PIM-SM domain into the PIM-DM domain when an
IGMP join for that flow occurs in the PIM-DM domain. See figure
on page 37.
Procedure
1.
Configure Routing Switch 1 with the IGMP proxy forwarding function to forward joins toward Border Router 1;
in addition, configure Routing Switch 1 to forward joins from VLAN 1 toward Border Router 2, as is VLAN 4 on
Routing Switch 3.
2.
Configure VLAN 2 on Routing Switch 2 to forward joins toward Border Router 1.
3.
When the host connected in VLAN 1 issues an IGMP join for multicast address 235.1.1.1, the join is proxied by
Routing Switch 1 onto VLAN 2 and onto VLAN 4. The routing information table in Routing Switch 1 indicates
that the packet to Border Router 1 and Border Router 2 is on VLAN 2 and VLAN 4, respectively.
Figure 5: IGMP proxy example
4.
Routing Switch 2 then proxies the IGMP join into VLAN 3, which is connected to Border Router 1.
5.
Border Router 1 uses PIM-SM to find and connect to the multicast traffic for the requested traffic. The traffic is
flooded into the PIM-DM network where it is routed to the original joining host.
6.
Additionally, the join was proxied from Routing Switch 3 to Border Router 2. At first, both border routers will
flood the traffic into the PIM-DM domain. However, PIM-DM only forwards multicasts based on the shortest
Chapter 2 Multimedia traffic control with IP multicast (IGMP)
37
