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New Member

OSPF Route Selection based on LSA Types ?

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

2 ACCEPTED SOLUTIONS

Accepted Solutions
Gold

Re: OSPF Route Selection based on LSA Types ?

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

Cisco Employee

Re: OSPF Route Selection based on LSA Types ?

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
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
6 REPLIES
New Member

Re: OSPF Route Selection based on LSA Types ?

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.

New Member

Re: OSPF Route Selection based on LSA Types ?

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.

"

Gold

Re: OSPF Route Selection based on LSA Types ?

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

New Member

Re: OSPF Route Selection based on LSA Types ?

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

Cisco Employee

Re: OSPF Route Selection based on LSA Types ?

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
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Cisco Employee

Re: OSPF Route Selection based on LSA Types ?

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

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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