Subject: Re: [OSPF] is N1 Route preffered over E1

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Thu, 09 Oct 2008 11:42:50 -0700 (PDT)

Acee,

Regarding your answer to 

> 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?

don't forget the pruning defined in OSPF RFC 2328 16.4.1. There

 o Intra-area paths using non-backbone areas are always the
   most preferred.

For the purpose of the AS External route calculation defined in RFC 
3101 an E1 Type 5 LSA has no inherent preference over an N1 Type 7 
LSA. Hence an N1 Type 7 LSA path over its non-backbone NSSA will 
prune an E1 Type 5 LSA path over the transit topology regardless of 
the metric. Of course this only applies when RFC1583Compatibility is 
set to "disabled". This pruning happens in Step 3 (for 0.0.0.0 fwd 
addrs only) and again in Step 6(c), which is before the metric 
comparison in Step 6(d).

Now regarding equal-cost E1 and N1 routes with the same non-zero 
forwarding address, a proper Type 5/7 LSA calculation will have long 
since pruned the E1 Type 5 LSA path based on the following text in 
Step 3.

   If the forwarding address is non-zero look up the forwarding
   address in the routing table.  For a Type-5 LSA the matching
   routing table entry must specify an intra-area or inter-area
   path through a Type-5 capable area.  For a Type-7 LSA the
   matching routing table entry must specify an intra-area path
   through the LSA's originating NSSA.  If no such path exists
   then do nothing with this LSA and consider the next in the
   list.

Note that the only time an E1 Type 5 LSA and an N1 Type 7 LSA should 
have the same non-zero forwarding address is during Type 7 LSA 
translation. This paragraph should cause the E1 Type 5 LSA to be 
skipped. When originating other Type 5 LSAs the forwarding address 
should never be picked from a stub area or an NSSA except during 
translation. To do so can cause routing loops and dropped packets. Of 
course Type 7 LSAs always have forwarding addresses from the NSAA.  

Note it is hard to envision where Step 6(e) plays much of a role 
during a steady state NSSA topology. It might play a more significant 
role when the NSSA topology loosely splits.

Pat

_______________________________________________
OSPF mailing list
[email-masked]
https://www.ietf.org/mailman/listinfo/ospf