HP 3500YL - Manual (Page 51)

Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
IGMP Proxy Forwarding
N o t e For more information about PIM-DM and PIM-SM, see the chapters “PIM-DM
(Dense Mode)
” and “PIM-SM (Sparse Mode)” in this guide.
When a network has a border router connecting a PIM-SM domain to a PIM-DM
domain, the routers that are completely within the PIM-DM domain have no
way to discover multicast flows in the PIM-SM domain. When an IGMP join
occurs on a router entirely within the PIM-DM domain for a flow that origi­
nates within the PIM-SM domain, it is never forwarded to the PIM-SM domain.
The IGMP proxy is a way to propagate IGMP joins across router boundaries.
The proxy triggers the boundary router connected to a PIM-SM domain to
query for multicast flows and forward them to the PIM-DM domain. 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 (refer to figure 2-4).
1. Routing Switch 1 is configured with the IGMP proxy forwarding function
to forward joins towards Border Router 1. Routing Switch 1 is also
configured to forward joins from VLAN 1 toward Border Router 2, as is
VLAN 4 on Routing Switch 3.
2. VLAN 2 on Routing Switch 2 is configured 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.
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