Cisco Systems 3750E manual Multicast Routing and Switch Stacks, Understanding Cgmp, 42-9

Chapter 42 Configuring IP Multicast Routing

Multicast Routing and Switch Stacks

DVMRP neighbors build a route table by periodically exchanging source network routing information in route-report messages. The routing information stored in the DVMRP routing table is separate from the unicast routing table and is used to build a source distribution tree and to perform multicast forward using RPF.

DVMRP is a dense-mode protocol and builds a parent-child database using a constrained multicast model to build a forwarding tree rooted at the source of the multicast packets. Multicast packets are initially flooded down this source tree. If redundant paths are on the source tree, packets are not forwarded along those paths. Forwarding occurs until prune messages are received on those parent-child links, which further constrain the broadcast of multicast packets.

Understanding CGMP

This software release provides CGMP-server support on your switch; no client-side functionality is provided. The switch serves as a CGMP server for devices that do not support IGMP snooping but have CGMP-client functionality.

CGMP is a protocol used on Cisco routers and multilayer switches connected to Layer 2 Catalyst switches to perform tasks similar to those performed by IGMP. CGMP permits Layer 2 group membership information to be communicated from the CGMP server to the switch. The switch can then can learn on which interfaces multicast members reside instead of flooding multicast traffic to all switch interfaces. (IGMP snooping is another method to constrain the flooding of multicast packets. For more information, see Chapter 24, “Configuring IGMP Snooping and MVR.”)

CGMP is necessary because the Layer 2 switch cannot distinguish between IP multicast data packets and IGMP report messages, which are both at the MAC-level and are addressed to the same group address.

Multicast Routing and Switch Stacks

For all multicast routing protocols, the entire stack appears as a single router to the network and operates as a single multicast router.

In a Catalyst 3750-E switch stack, the routing master (stack master) performs these functions:

•It is responsible for completing the IP multicast routing functions of the stack. It fully initializes and runs the IP multicast routing protocols.

•It builds and maintains the multicast routing table for the entire stack.

•It is responsible for distributing the multicast routing table to all stack members.

The stack members perform these functions:

•They act as multicast routing standby devices and are ready to take over if there is a stack master failure.

If the stack master fails, all stack members delete their multicast routing tables. The newly elected stack master starts building the routing tables and distributes them to the stack members.

Note If a stack master running the IP services feature set fails and if the newly elected stack master is running the IP base feature set, the switch stack loses its multicast routing capability.

For information about the stack master election process, see Chapter 5, “Managing Switch Stacks.”

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

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