Multicast Open Shortest Path First
MOSPF is defined in RFC 1584, “Multicast Extensions to OSPF,” which is an extension to the
OSPFv2 unicast routing protocol. The basic operation of MOSPF is described here:
■ MOSPF uses the group membership LSA, Type 6, which it floods throughout the originating
router’s area. As with unicast OSPF, all MOSPF routers in an area must have identical linkstate
databases so that every MOSPF router in an area can calculate the same SPT.
■ The SPT is calculated “on-demand,” when the first multicast packet for the group arrives.
■ Through the SPF calculation, all the routers know where the attached group members are,
based on the group membership LSAs.
■ After the SPF calculation is completed, entries are made into each router’s multicast
forwarding table.
■ Just like unicast OSPF, the SPT is loop free, and every router knows the upstream interface
and downstream interfaces. As a result, an RPF check is not required.
■ Obviously, MOSPF can only work with the OSPF unicast routing protocol. MOSPF is suitable
for small networks. As more hosts begin to source multicast traffic, routers have to perform
a higher number of Dijkstra algorithm computations, which demands an increasing level of
router CPU resources. Cisco IOS does not support MOSPF.