Re: OSPF LSA type 3 filtering

Marko,
say we have an area with 3 routers, R1, R2, R3, connected by a LAN.
Then OSPF would choose one as DR. Say that lan is X.

Would you agree that the database representation would be:

Router links:
R1: R1 -> DR (transit)
R2: R2 -> DR (transit)
R3: R3 -> DR (transit)

Net link:
DR: X (R1,R2,R3)

You can draw the topology just by looking at the router links.
What is missing ?

-Carlos

Marko Milivojevic @ 04/01/2013 01:11 -0300 dixit:
> 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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-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net