Saturday, July 4, 2009

OSPFv3 LSA types and their OSPFv2 counterparts

By Jeff Doyle

Although Router and Network LSAs have the same names, there is a significant difference in how the OSPFv3 and OSPFv2 Router and Network LSAs function. Specifically, OSPFv3 Router and Network LSAs do not advertise prefixes. This is an important improvement in the scaling of the protocol. These LSAs are, as you know from Chapter 8, "OSPFv2," primarily to represent the router as a node on the SPF tree. So when a Router or Network LSA is flooded, there is an assumption that a topological change has taken place and all routers in the area, on receipt of the LSA, rerun SPF. But because OSPFv2 routers also use these LSAs to advertise their connected subnets, if a subnet changes, the LSA must also be flooded to advertise the change. Even though something like an address change does not affect the SPF topology, the reception of a Router or Network LSA triggers an SPF run anyway. This can be particularly problematic for edge or access routers that have many stub links that change regularly.

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