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

Chapter 297
Operation

The local-group database enables the local delivery of multicast datagrams. The datagram’s shortest-path tree enables the delivery of multicast datagrams to distant (i.e., not directly attached) group members. Both of these are used in the calculation of the forwarding cache.

The following are standard assumptions when using MOSPF:

•  All MOSPF routers within the same area calculate the same shortest-path tree for a given multicast datagram. This is accomplished via synchronized link-state databases.

•  Link-state databases can be synchronized using the new group-membership LSA. With each router in an area having the same database, each router should be able to build a source-rooted, shortest-path, multicast tree without having to broadcast the first multicast packet.

•  The shortest-path multicast tree is built “on demand.” This means that the tree is built when a router receives a multicast datagram for the first time for a given multicast group. An MOSPF router does not automatically forward the first multicast datagram like DVMRP does. Since the synchronized link-state database contains group-membership LSAs, this allows a MOSPF router to perform “broadcast the first datagram” in its memory and the tree is built on demand. The routers already know where the active group memberships are and can build the tree without forwarding the first datagram and then waiting for Prune messages.

When the first multicast datagram arrives (at any router), the source subnet (IP address of the source) is located in the link-state database. This is sometimes called the MOSPF link-state database, but the single database (link-state database) contains entries for both unicast and multicast. A source-rooted multicast tree is calculated using the router LSAs and the network LSAs using the Dykstra algorithm (same as unicast OSPF). Once the tree is built, it is pruned (to eliminate links that do not contain at least one member of a group) using the group-membership LSAs. The final result of the Dykstra algorithm is the pruned shortest-path tree that is rooted at the source (remember, OSPF calculates its shortest-path tree using itself as the root). This shortest-path tree is used to understand which ports should be used for the forwarding of multicast datagrams that are distant (i.e., no local-group membership, but there are members of the group downstream from this router), and which ports should receive which multicast datagram (source).

Now we have two sources of information: the source-routed shortest-path tree and the local-group database. Both of these are used to determine the forwarding cache, which is the only place the router will look to determine forwarding of a multicast datagram.