Subject: [OSPF] is N1 Route preffered over E1

From: Pranjal Chakravarty <[email-masked]>
Date: Thu, 9 Oct 2008 13:36:39 +0530 (IST)

Suppose a Router recive the info abt a external Route by a E1 Route and another by a N1 Route .Will ECMP will happen and two routes will be installed on the router or a single route will be preffered .We know that E1 route is always preffered over E2 Route .My question is does N1 route is preffered over E1 Route or vice versa?

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From: Acee Lindem <[email-masked]>
Date: Thu, 9 Oct 2008 09:38:50 -0400

On Oct 9, 2008, at 4:06 AM, Pranjal Chakravarty wrote:

> Suppose a Router recive the info abt a external Route by a E1 Route  
> and another by a N1 Route .Will ECMP will happen and two routes  
> will be installed on the router or a single route will be  
> preffered .We know that E1 route is always preffered over E2  
> Route .My question is does N1 route is preffered over E1 Route or  
> vice versa?

I assume by N1 you're referring to IOS's "show ip route" type for an  
NSSA type 1 route. RFC 3101 will prefer the E1 or N1 route based on  
the following rules from section 2.5:

           (d) If the new AS external path is still indistinguishable
               from the current paths in N's routing table entry, select
               the preferred path based on a least cost comparison.   
Type
               1 external paths are compared by looking at the sum of  
the
               distance to the ASBR/forwarding addresses and the
               advertised type 1 metric (X+Y).  Type 2 external paths
               advertising equal type 2 metrics are compared by looking
               at the distance to the ASBR/forwarding addresses.
               ~[OSPF]

           (e) If the current LSA is functionally the same as an
               installed LSA (i.e., same destination, cost and non-zero
               forwarding address) then apply the following  
priorities in
               deciding which LSA is preferred:

                  1. A Type-7 LSA with the P-bit set.

                  2. A Type-5 LSA.

                  3. The LSA with the higher router ID.

However, if the E1 route and the N1 route are, in fact, equal-cost  
then usually this is a moot point since they both use the same  
forwarding address.
Hope this helps,
Acee

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