Re: purpose of having p2p ospf link information in router-LSA

Hi, 

The difference is on the LInk ID and Link Data.

For P2P: LInk ID= Neighbor Router ID ; Link Data=Local router interface IP

For Stub: Link ID=Network Address; Link Data= Subnet mask

On a P2P interface the adjacency is formed over an unnumbered connection, so Type 1 [P2P] router LSA does not provide any info on the P2P link IP addressing.

From RFC 2328:

 Interfaces to point-to-point networks need
        not be assigned IP addresses.  When interface addresses are
        assigned, they are modelled as stub links, with each router
        advertising a stub connection to the other router's interface
        address. Optionally, an IP subnet can be assigned to the point-
        to-point network. In this case, both routers advertise a stub
        link to the IP subnet, instead of advertising each others' IP
        interface addresses.

Hope this helps.

Cheers,

Ashvin