Exam Essentials

Know the difference between IGMP and IGMPv2 and IGMPv3. IGMPv1 and IGMPv2 are very similar. The major difference is that IGMPv2 has a message that the client sends when it doesn't want to receive the multicast stream anymore. The result of this small difference is that they don't work well together, and it makes sense for you to have only one version running. IGMPv3 is better still, and supports extras such as SSM, but because it is very new, you have to make sure that all the hosts in your network support it.

The Catalyst switch can listen for client Leave messages with both CGMP and IGMP Snooping. A switch configured for CGMP can listen for IGMPv2 and v3 Leave messages by being configured with the command set cgmp leave enable. A switch configured for IGMP Snooping can be configured for IGMP Fast-Leave processing with the command set igmp fastleave enable.

Know the difference between CGMP and IGMP Snooping. Although both CGMP and IGMP Snooping allow a switch to get involved in a multicast stream, they are rather different. CGMP is a Cisco proprietary protocol communication based upon communication between a router and any attached switches. Routers receiving IGMP packets from other routers forward specific information to appropriate switches containing information on multicast memberships.

IGMP Snooping enables the switch to learn information from watching IGMP packets go through the switch. IGMP is an Internet standard. Snooping is being considered by the IETF as a standard and is currently in draft. Remember that snooping can't be enabled if CGMP is enabled, so you first need to make sure that CGMP is turned off. Next, enable IGMP Snooping with the ip igmp snooping command.

Know the difference between the multicast routing protocols. There are several options for routing multicast traffic. DVMRP is a distance-vector-based routing protocol, and MOSPF uses OSPF, but neither is the recommended method of doing multicast routing with Cisco equipment. Cisco recommends that PIM be used to route multicast streams because it learns from the preexisting routing protocol. This means that EIGRP can be used to route multicast information.

PIM has two broad modes, sparse and dense. In dense mode PIM, each router is automatically included in the multicast table and has to prune itself off if no clients need the stream. Sparse mode assumes no routers wish to participate. Routers are added as connected clients request access to the multicast streams, and a special router is used as the base for the entire tree. This router, a rendezvous point, needs to be referenced in each multicast router's configuration. Use the command rp pim ip-address ip_address to define the IP address of the rendezvous point.

Know how to troubleshoot your multicast setup. There are many show commands that can be done on the router and switch to show communication but you still need to test the transport. You can use the ping command to reach out and touch a particular multicast IP address. If you want to do a traceroute, use the command mtrace.