Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 294
Local-Group Database and the Group-Membership LSA

MOSPF provides for the ability to forward multicast datagrams from one IP subnet to another. A new OSPF link-state advertisement (group-membership LSA) has been added to accommodate this by allowing a source-rooted, pruned, shortest-path tree to be built. This new LSA augments the link-state database; therefore, the MOSPF database is the link-state database of OSPF but with entries that pertain to multicast networks.

The new LSA places the location of multicast destinations in the database. Using this information, MOSPF can build a shortest-path tree for a source group. MOSPF is an extension of OSPF and MOSPF routers will interoperate in nonmulticast routers when forwarding unicast datagrams.

An MOSPF router bases its forwarding decision on the contents of a data cache known as the forwarding cache. Each entry in the cache represents a source/destination combination (and possibly ToS). This forwarding cache is built from two components: a local group database (built by IGMP) datagram’s shortest-path tree.

The local-group database keeps track of the local membership for the routers directly attached to networks. These entries are paired in the form of (group, attached network). The attached network is the IP address of the network, and the group is the IP multicast address of the multicast group. All we have to have is one host on that network indicating membership and the router will place an entry in the local-group database. Similar to the other multicast routing protocols, this database allows the router to determine which port(s) it should forward a received multicast datagram. The IGMP protocol assists in building this database.

To allow for multicast datagrams to be forwarded to all members of the group in an area, the local-group database is flooded throughout the area (including being received by ABRs) using the group-membership LSA. There is a separate group-membership LSA for each multicast group in the router’s group database. The router’s group-membership LSA for a specified group lists those local router ports (i.e., the router itself and/or any directly connected transit networks) that should not be pruned from the group’s datagram shared trees.