Hi All, I just got curious to know (since I have a related problem to solve in our router), how the router should behave in the case of multiple ASBRs scenario, in both the cases when rfc1583compatible is enabled/disabled.
When there are multiple ASBRs in the autonomous system, each originating a type-5 AS external LSA for a single external destination say N1, how does the rules in section 16.4 should be applied by a router to compute the paths to N1. How does the router prune from among the number of ECMP paths available to each of those ASBRs. rfc 2328 doesn't not talk about the multiple ASBR scenario. It explains the rules in section 16.4 assuming multiple paths to a single ASBR. But if there are multiple paths to multiple ASBRs, should the rules be simply be extended or is there something more to it. Please explain. Thanks in advance. regards, Shashidhar On 2/28/07, Acee Lindem <[EMAIL PROTECTED]> wrote: > Hi Shashidhar, > > Are you testing with multiple ASBRs or multiple paths to a single ASBR? > > I suspect it is the latter. In this case, RFC 2328 (or 1583 for that > matter) isn't completely clear as to whether the selection rules > extend to a single ASBR path lockup. Given where we are in OSPFv2 > life cycle and being the pragmatic WG chair that I am, I'd say this > is an implementation choice. Personally, I've worked on > implementations doing it both ways. > > Besides, everyone should be using the new ASBR selection rules by > now and for OSPFv3 these are the only rules. > > Thanks, > Acee > > On Feb 28, 2007, at 6:43 AM, techmail technology wrote: > > > Hi All, > > > > I have one interesting question regarding the behavior of certain > > routers (including CISCO), when rfc1583Compatibility is enabled. (By > > default it is disabled). > > > > According to rfc2328, it clearly says in section 16.4, when > > rfc1583Compatibility is enabled, the preference rules mentioned in > > 16.4.1 should not be applied in choosing the best path to ASBR. > > Instead it should select the path based on the cost only i.e whichever > > path has the lowest cost should be considered as the best path, and if > > there are multiple low cost paths to ASBR, then choose that path > > through highest AREA ID, when considered as an unsigned integer. > > > > With this background, I tested the behavior of CISCO series 2651 IOS > > software release 12.4. I was surprised with the CISCO's behavior. > > Instead of choosing a low cost path when rfc1583compatibility is > > enabled, it continued to select the path to ASBR based on the > > preference rules (i.e path through non-backbone intra-area path). > > > > I know that rfc2328 uses preference rules specified in 16.4.1 to avoid > > formation of routing loops, in rfc1583 implementation in certain > > topologies as explained by Richard Woundy in rfc2178. But still > > network provider can have a topology where there is no chance of such > > loops forming, so he may want to use rfc1583 behavior to select the > > low cost path to ASBR, CISCO doesn't seem to be allowing that. > > > > Is there any specific reason why CISCO doesn't allow such behavior, I > > couldn't immediately find the reason, why CISCO(as well as other > > implementations like IP Infusion ZebOS) is behaving like this. If > > there are multiple routers behaving similarly, but not the way rfc2328 > > has specified, then there must be some reason. > > > > Does any one know the reason to this behavior ? > > > > regards, > > Shashidhar > > > > _______________________________________________ > > OSPF mailing list > > [email protected] > > https://www1.ietf.org/mailman/listinfo/ospf > > _______________________________________________ OSPF mailing list [email protected] https://www1.ietf.org/mailman/listinfo/ospf
