25. Multicast Filtering
ROX™ v2.2 User Guide
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RuggedBackbone™ RX1500
25.1.3. Combined Router and Switch IGMP Operation
This section describes the additional challenges of multiple routers, VLAN support and switching.
Producer P1 resides on VLAN 2 while P2 resides on VLAN 3. Consumer C1 resides on both VLANs
whereas C2 and C3 reside on VLANs 3 and 2, respectively. Router 2 resides on VLAN 2, presumably
to forward multicast traffic to a remote network or act as a source of multicast traffic itself.
Figure 25.2. IGMP Operation Example 2
In this example, we will assume that all the devices agree that router 1 is the querier for VLAN 2 and
router 2 is simply a non-querier. In this case, the switch will periodically receive queries from router 1
and, thus, maintain the information concerning which of its ports links to the multicast router. However,
the switch port that links to router 2 must be manually configured as a “router port”. Otherwise, the
switch will send neither multicast streams nor joins/leaves to router 2.
Note that VLAN 3 does not have an external multicast router. The switch should be configured to operate
in its “routerless” mode and issue general membership queries as if it is the router.
Processing Joins
If host C1 desires to subscribe to the multicast streams for both P1 and P2, it will generate two joins.
The join from C1 on VLAN 2 will cause the switch to immediately initiate its own join to multicast router
1 (and to issue its own join as a response to queries).
The join from C1 for VLAN 3 will cause the switch to immediately begin forwarding multicast traffic from
P2 to C2.
Processing Leaves
When host C1 decides to leave a multicast group, it will issue a leave request to the switch. The switch
will poll the port to determine if C1 is the last member of the group on that port. If C1 is the last (or only)
member, the group will immediately be pruned from the port.
Should host C1 leave the multicast group without issuing a leave group message and then fail to respond
to a general membership query, the switch will stop forwarding traffic after two queries.
When the last port in a multicast group leaves the group (or is aged-out), the switch will issue an IGMP
leave report to the router.
25.2. GMRP (GARP Multicast Registration Protocol)
The GARP Multicast Registration Protocol (GMRP) is an application of the Generic Attribute Registration
Protocol (GARP) that provides a mechanism at Layer 2 for managing multicast group membership
in a bridged Layer 2 network. It allows Ethernet switches and end stations to register and unregister
