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Re: [OSPF] SPF calculation

On Aug 6, 2009, at 3:16 PM, Joakim Tjernlund wrote:



Acee Lindem <acee at redback.com> wrote on 06/08/2009 20:18:38:


Hello Joakim,


Hi Acee, I am back with my questions :)



On Aug 6, 2009, at 5:22 AM, 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?


No - if the Router-LSA doesn't have a link to the Network-LSA you
don't use it in the SPF calculation. If fails the bi-directional check.
The idea here is use some link until the remote end has updated its LSA 
to include the "back link" so the intervening router case will just
act as a substitute until next LSA update.
What I wonder is if this can cause more trouble than it solves, i.e what 
bad things can happen? Or won't it help at all?
I don't see that this achieves anything. If the router is reachable through an intra-area path, the SPF will find it. You don't want to do any second guessing - this will only create blackholes and routing loops. 

Thanks,
Acee




  Jocke