Re: OSPF LSA type 3 filtering

Writing on a phone. Pardon the brevity

>
> I would argue that you can make the topology of an area only with type 1 LSAs, and that type 2 LSAs are just for "condensing" the multiaccess
> link reachability information in one place.

Not quite. You would know which routers exist in the area, but not how they are interconnected.

To calculate the SPF tree, routers need two pieces of information for all non-leaf links: the link state, and relationship with other routers.

OSPF recognizes three link types in Type 1: stub, transit, and point to point.

For point to point links, link state is carried in two link state entries. Link itself is described as a "stub link", and the relationship with other router is described as a point-to-point link. These are both in Type 1 LSA.

However, for transit link the actual link is described as a link entry in Type 1 LSA, with a reference to a Type 2 LSA (in a form of a DR address). The Type 2 carries the topological information about the relationships between touters in the segment. Both are crucial for the topological calculation.

Note - this was all about the topological information and not the reachability.

-Marko

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Received on Thu Jan 03 2013 - 20:11:15 ART