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Chapter 9

Multicast

Intra-Area MOSPF

OSPF route information is shared via different Link State Advertisement
(LSA) types. LSAs are flooded throughout an area to give all OSPF-enabled
routers a logical image of the network topology. When changes are made to
the topology, new LSAs are flooded to propagate the change.

In addition to the unicast-routing LSA types, in OSPFv2 there is a special

multicast LSA for flooding multicast group information throughout the
area. This additional LSA type required some modification to the OSPF
frame format.

Here is where you need to understand a little about OSPF. Multicast LSA

flooding is done by the Designated Router (DR) when there are multiple
routers connected to a multi-access media, such as Ethernet. On point-to-
point connections, there are no DR and Backup Designated Router (BDR).
Look at the following code from a Cisco router running OSPF over point-to-
point circuits.

Neighbor ID Pri State Dead Time Address Interface

172.16.1.2 1 FULL/ - 00:00:31 172.16.1.2 Serial3/0

192.168.1.2 1 FULL/ - 00:00:39 192.168.1.2 Serial3/1

On a multi-access network, the DR must be multicast enabled; that is,

running MOSPF. If there are any non-MOSPF routers on the same network,
their OSPF priority must be lowered so none of them become the DR. If a
non-MOSPF router were to become the DR, it would not be able to forward
the multicast LSA to the other routers on the segment.

Inside the OSPF area, updates are sent describing which links have active

multicast members on them so that the multicast data can be forwarded to
those interfaces. MOSPF also uses (S, G) notation and calculates the SPT
using the Dijkstra algorithm. You must also understand that an SPT is cre-
ated for each source in the network.

Inter-Area and Inter-AS 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

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