KB.15.15

Figure 4 Example of unknown multicast traffic not flooding out ports connected to queriers in

separate VLANs

To display the status of IGMP multicast filtering use the show ip igmp command. If the IGMP

Filter Unknown Multicast setting is different from the IGMP Filter Unknown Multicast status, a reboot

is required to activate the desired setting. This setting will then be reflected in the status.

Example 17 IGMP unknown multicast filter setting being enabled but not yet activated

HP Switch(config)# show igmp filter-unknown-mcast

IGMP Filter Unknown Multicast: Enabled

IGMP Filter Unknown Multicast Status: Disabled

To display information about IGMP multicast filtering by interface, use the show ip igmp

command.

IGMP proxy forwarding

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 originates 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 operates 31