Intra-Area MOSPF

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 multiple routers are 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-topoint
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
any non-MOSPF routers are on the same network, their OSPF priority must be lowered so that
none of them becomes 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 by using Dijkstra’s algorithm. You must also understand
that an SPT is created for each source in the network.