OSPF Route Selection based on LSA Types ?

[s.phillipson]

Hi,

Does OSPF determine routes based on LSA Type? For example, if a Router has received two routing updates referring to the same IP subnet from (1) an Intra-Area route, received from an OSPF peer in Area 0 and (2) an External route received from an ASBR redistributing routes into Area 0

how will the Router make its selection?

I am specifically concerned in the situation where you have an OSPF area interconnecting into another AS at two points. Could this scenario result in sub-optimal routing due to a Router sending intra-area data via the other AS (ie. a routing loop being formed resulting in sub-optimal routing).

Regards,

Steve

[gleithner]

no, LSA type isn't figured in. Assumig prefix length is the same, only cost matters. alter the metric at the redistribution point to setup which one you want to be preferred.

[gleithner]

guess I'm wrong, That's what i get for not double-checking something.

the RFC says "There are four possible types of paths used to route traffic to

the destination, listed here in decreasing order of preference:

intra-area, inter-area, type 1 external or type 2 external.

"

[ruwhite · Accepted solution]

Intr-area routes are always preferred over inter-area routes, and internals are always preferred over externals. So, the preference order is:

Type 1/2

Type 3

Type 7

Type 5

:-)

Russ.W

[s.phillipson]

Russ,

Are there any whitepapers which explain this in a little more detail?

Interesting that there seems to be conflicting points of view on this matter. Your view confirms what I thought, but I'd appreciate it if you knew of a reference resource which states it just to stifle any further debate on this matter (I did a search on Cisco's site prior to making this post, but couldn't find anything specific)

Regards,

Steve

[Harold Ritter · Accepted solution]

Steve,

The best way to put an end to any discussion is to refer back to the RFC (2328). Section 11 under Path-type.

Hope this helps,

[Harold Ritter]

To add to that list an external type 1 is always preferred over a type 2.