Subject: Re: [OSPF] ASBR election

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Fri, 19 May 2006 08:03:42 -0700 (PDT)

Pradeep,

>As Pat noted,
>RFC 3101 adds the highest router ID as a tie breaker which force
>picking the two ASBRs.

And this happens only when comparing functionally equivalent LSAs with 
the same non-zero forwarding address.

OSPF has always built a routing table of multiple-equal cost paths. When 
dealing with external LSAs the external path preference rules prune them 
to a single area, including the default prefix 0.0.0.0/0. When two equal 
cost external default LSAs have a zero forwarding address (or different 
non-zero forwarding addresses) the OSPF routing table installation 
algorithm must assume that these routes are separate equal cost paths 
and it may install them. OSPF makes no recommendations about how 
implementations balance load over multiple equal cost paths. Most 
implementations allow the configuration of administrative metrics to 
define a preference pecking order of multiple paths, ala cisco's 
distance metrics (which I have seen used for the default route); but 
lacking any specifically configured administrative metrics or traffic 
engineering, it has been my experience that implementations simply 
balance the load over the multiple paths.

Pat

_______________________________________________
OSPF mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/ospf

---

From: Acee Lindem <[email-masked]>
Date: Fri, 19 May 2006 11:16:53 -0400

>balance the load over the multiple paths.
Thanks Pat - I guess I missed the part about the LSA being functionally 
the same. In this case, it
really shouldn't matter which LSA you use since they are functionally 
the same (although I
could envision an implementation doing something stupid like flapping 
the route when the
functionally equivalent LSA is purged ;^).

Acee