Cisco Systems RJ-45-to-AUX manual Internet Group Management Protocol Igmp, IGMPv1, 145

receive the multicast traffic sends a PIM join message toward the root node (rendezvous point). As this join message travels up the tree, the multicast routers along the way forward the requested traffic back down the tree to the destination.

To put it in a simple form, PIM−DM will send the multicast traffic everywhere until it is told not to. PIM−SM will not send any multicast traffic until it is asked to.

Note Cisco routers use PIM−SM. They can receive and forward messages to DVMRP neighbors, but they do not actually implement DVMRP in their networks.

Multicast Open Shortest Path First

Multicast Open Shortest Path First (MOSPF) is an extension of the Open Shortest Path First (OSPFv2) unicast routing protocol. OSPF must be running on the network for MOSPF to work. OSPF is a link−state routing protocol that allows a network to be split into multiple areas. The OSPF link−state database provides the complete map of an area at each router. MOSPF’s extension is a new link−state advertisement (LSA) to distribute multicast group membership throughout the OSPF area. The group membership LSA includes the multicast group address, the router ID, and the interfaces on the router that have members in the multicast group. This information reaches all the MOSPF routers in the area, so each router will have a complete database of all the multicast group members. Each router will then construct a source tree from the link−state information.

MOSPF is a very bandwidth−efficient multicast routing protocol, but it does have a downside: It is very CPU intensive. If the network topology is not stable, the router will have to recalculate the routing tabling. Multicast increases the problem, because each membership change will cause a new computation. In a many−to−many multicast environment, by utilizing the source tree model, a new SPT is created for each source. Because of this, MOSPF is most suitable in a stable environment where the location of sources, number of sources, number of groups, and group membership are under tight control.

Internet Group Management Protocol (IGMP)

Internet Group Management Protocol (IGMP) is used by hosts to request multicast traffic. An individual host sends out an IGMP member report to inform the multicast router that it wants to receive data transmissions. The router maintains a list of multicast group memberships learned from IGMP. The multicast group membership list is built on a per−interface basis and is considered active if one host maintains its membership.

There are currently two versions of IGMP: version 1 (IGMPv1), which is defined in RFC 1112; and version 2 (IGMPv2), which is defined in RFC 2236. Let’s look at the two versions of IGMP and how they differ.

IGMPv1

You may wonder why we’re explaining version 1, when version 2 is the standard. Well, there are PCs that still use IGMPv1. Windows 95 (the OS that never goes away) supports IP multicast utilizing IGMPv1, unless you download an upgrade version of Microsoft’s Winsock dynamic link library (DLL). (Windows 98 and Windows 2000 have native support for IGMPv2.) The same situation applies to the different versions of Unix. Unless all of the computers on your network are running the latest version of an OS or you’ve installed a patch, you’re going to have computers using IGMPv1.

IGMP uses a query and response format. The router sends a host membership query message to the all−hosts multicast address, 244.0.0.1. (If multiple routers are on the network, one will be designated to send the message.) When a host receives the query message, it responds with a membership report to the multicast group of which the host is a member. If more than one host on the subnet belongs to the same multicast group, it will see the membership report and not respond to the membership query. As the router receives the

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