Intra-Area and Inter-Area MOSPF

Intra-Area and Inter-Area MOSPF
When discussing the difference between intra-area and inter-area MOSPF, you must remember
that all areas connect through Area 0, the backbone. In large networks, having full multicast
tables in addition to all the unicast tables flow across Area 0 would cause a great deal of overhead
and possibly latency.
Unicast OSPF uses a Summary LSA to inform the routers in Area 0 about the networks and
topology in an adjacent area. This task is performed by the area’s area border router (ABR). The
ABR summarizes all the information about the area and then passes it on to the backbone
(Area 0) routers in a summary LSA. The same is done for the multicast topology. The ABR summarizes
which multicast groups are active and which groups have sources within the area. This
information is then sent to the backbone routers.
In addition to summarizing multicast group information, the ABR is responsible for the
actual forwarding of multicast group traffic into and out of the area. Each area has an ABR that
performs these two functions within an OSPF network.
OSPF implements autonomous system border routers to be the bridges between different
autonomous systems. These routers perform much the same as an ABR, but they must be able
to communicate with non-OSPF-speaking devices. Multicast group information and data is forwarded
and received by the multicast autonomous system border router (MASBR). Because
MOSPF runs natively within OSPF, there must be a method or protocol by which the multicast
information can be taken from MOSPF and communicated to the external AS. Historically,
DVRMP has provided this bridge.