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Re: [OSPF] SPF calculation
Dave Katz <dkatz at juniper.net> wrote on 06/08/2009 20:24:25:
Hi Dave :)
>
> If there is no back link, there is no link, and SPF moves on. If the
> network is partitioned, somebody should fix it.
Yes, this problem exist for a short while, until the remote
route has updated it's LSA.
>
> Ignoring the bidirectional test rule can lead to loops and black
> holes, particularly if other implementations are following the rules.
hmm, can you be more specfic?
Consider this example:
R1 R2 R3
| | |
------- N1
Destination is R3 and R2 is the calculation router. R2 cannot find a backlink
so it falls back to the intervening router case and uses the nexthops
from the "parent", hoping that one of them will redirect IP frames to
R3.
Jocke
>
> --Dave
>
> On Aug 6, 2009, at 8:02 AM, Joakim Tjernlund wrote:
>
> > "Joel M. Halpern" <jmh at joelhalpern.com> wrote on 06/08/2009 15:38:47:
> >>
> >> I am not sure what you are asking. A link advertised in OSPF may
> >> only
> >> be used if it is advertised in both directions.
> >
> > Exactly, so you may encounter the scenario below when links are going
> > down/up until all routers has recalculated everything.
> >
> >> Therefore, it appears taht the case you are describing can not occur.
> >
> > It can, so when it does you can either just give up or try
> > to do the best you can until you find a back link.
> >
> > Jocke
> > PS.
> > Please keep the ospf list on the CC: line.
> >
> >>
> >> Yours,
> >> Joel
> >>
> >> Joakim Tjernlund wrote:
> >>> In "16.1.1. The next hop calculation" one have:
> >>> In the second case, the parent vertex is a network that
> >>> directly connects the calculating router to the
> >>> destination
> >>> router. The list of next hops is then determined by
> >>> examining the destination's router-LSA. For each link in
> >>> the router-LSA that points back to the parent network,
> >>> the
> >>> link's Link Data field provides the IP address of a
> >>> next hop
> >>> router. The outgoing interface to use can then be
> >>> derived
> >>> from the next hop IP address (or it can be inherited from
> >>> the parent network).
> >>>
> >>> Suppose that one cannot find any links that points back, is it a
> >>> good
> >>> idea to treat this case as a intervening router:
> >>>
> >>> If there is at least one intervening router in the
> >>> current
> >>> shortest path between the destination and the root, the
> >>> destination simply inherits the set of next hops from the
> >>> parent.
> >>> That is, just inherit the next hops from its parents?
> >>>
> >>> Jocke
> >>>
> >>> _______________________________________________
> >>> OSPF mailing list
> >>> OSPF at ietf.org
> >>> https://www.ietf.org/mailman/listinfo/ospf
> >>>
> >>
> >
> > _______________________________________________
> > OSPF mailing list
> > OSPF at ietf.org
> > https://www.ietf.org/mailman/listinfo/ospf
> >
>
>
>