RE: OSPF LSA type 3 filtering

The router isn't trying to solve SPF for R1 -> DR though, it's trying to solve SPF for R1 -> R2 and R1 -> R3. This is the topology information that LSA 2 maintains. The DR/BDR information in LSA 2 isn't used just to minimize flooding of n*(n-1)/2, it's also used to cut down the discovery of candidates in the tree on a broadcast/non-broadcast segment from n*(n-1)/2 to 2. In other words if you have 100 routers on a LAN and in one case you run the link as network type point-to-multipoint, and in the other case you run it as network broadcast, network broadcast will have a shorter SPF completion time because there are less operations of discovering candidates. The time might be negligible nowadays but it's still measurable.

Think of it this way, with 100 routers on the LAN running network type broadcast, R1 says "I have an adjacency to DR with cost X via LSA 1. DR has adjacency with R2, R3, R4... R100 via LSA 2. Therefore I have adjacency with R2, R3, R4... R100 with cost X".

In network type point-to-multipoint this would read as "I have an adjacency to R2 with cost X via LSA 1. I have an adjacency to R3 with cost X via LSA 1. I have an adjacency to R4 with cost X via LSA 1... I have an adjacency to R100 with cost X via LSA 1."

The resulting Shortest Path Trees will be the same, but programmatically the amount of operations needed to reach the end result are not the same between the two. When LSA 1 points at a transit link (broadcast/non-broadcast) the recursive lookup to LSA 2 immediately returns *all* candidates on that portion of the tree, whereas with point-to-multipoint there is a separate LSA 1 that represents each candidate. Therefore with broadcast/non-broadcast you need two lookups (one for LSA 1 and the second recursive lookup to LSA 2) vs. n*(n-1)/2 LSA 1 lookups for point-to-multipoint.

Of course if your Ethernet links are physically point-to-point then you would want to run them as OSPF network point-to-point, which means one LSA 1 lookup instead of one LSA 1 lookup and a second recursive LSA 2 lookup.

Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com
 
Internetwork Expert, Inc.
http://www.INE.com

-----Original Message-----
From: Carlos G Mendioroz [mailto:tron_at_huapi.ba.ar]
Sent: Friday, January 04, 2013 4:03 AM
To: Marko Milivojevic
Cc: Brian McGahan; Narbik Kocharians; Cisco certification
Subject: 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