OSPF: Type 1 and 2 LSAs

Each router creates and floods a Type 1 LSa for itself, Type 1 LSAs describe the router and its interfaces, and a list of the neighboring routers on each interface in that specific area.  the LSA itself is identified by the LSID equal to the RID of the router.

Type 2 LSAs represent a transit subnet for the DR to be elected.  The LSID is the DRs interface IP.  Type 2 LSAs are not created for subnets with no DR elected.

With these two LSA types being flooded to neighbors/DRs, SPF is able to create a topological graph of the network and calculate the best routes.

For subnets with no Dr, the type 1 LSA have enough information for SPF to create the math model of the topology.

For transit networks with DRs, OSPF uses Type 2 LSAs to model the subnet as a node in a SPF math model, since OSPF treats type 2 LSAs as a node in the graph this LSA is referred to sometimes as the pseudonode.  The type 2 LSAs include references t othe RIDs of all the routers that are currently neighbors of the DR.



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