Feng,

  If you decide to maintain a single path per ASBR, you'll
  have to recalculate Dijkstra in all areas when the best
  ASBR path goes away. This is more load than maintaining
  available paths on a per-area basis and choosing the best
  one whenever the path set changes.

--
Alex Zinin
Thursday, August 23, 2001, 12:58:11 PM, Xie, Feng wrote:
> Hi, Nikhil:
>       It is still not very clear to me why we need to store multiple paths
> to an ASBR. Please correct me if I am wrong. My understanding is that an
> ospf router only needs to keep the best route to an certain ASBR according
> to the rules specified in RFC2328 section 16. In implementation, you can
> choose to keep all routes to an ASBR and then choose the best one every time
> an external prefix is processed in SPF calculation. Or you can only keep the
> best route to an ASBR. Every time you receive a different route to the same
> ASBR, you compare it with the one you keep. If better, replace the old one.
> For me, the second one seems better.
>      It is possible that multiple ASBRs advertise the same external prefix.
> Since there is a best route to each ASBR, you do need to use the spec in
> Section 16 to choose the best among them.
> Feng Xie
> -----Original Message-----
> From: Nikhil Sama [mailto:Nikhil.SamaNEXSI.COM]
> Sent: Thursday, August 23, 2001 1:58 PM
> To: OSPFDISCUSS.MICROSOFT.COM
> Subject: Re: About ASExternal calculation
> Alex,
>         Actually you do keep multiple routes to a router destination.
>         For instance, if an ASBR is reachable in multiple area's via
> different paths, you must store a routing table entry for each path
> regardless of cost.
>         It is only in the case of IP routes that you store only the "best
> route/routes" to a destination.
> Nikhil
> "Xie, Feng" wrote:
> Hi, Alex:
>     Usually, we only keep the best route to a destination. As you said, at
> any time, there may be only one entry for an ASBR with multiple next hops
> due to equal-cost multipath. My understanding is that what described in the
> RFC is a process to build the best route to an ASBR when you have choice
> among multiple routes. It does not mean that an implementation has to keep
> all possible routes to an ASBR.
> Feng Xie
> -----Original Message-----
> From: alex lerin [ mailto:alexlerinYAHOO.COM <mailto:alexlerinYAHOO.COM> ]
> Sent: Thursday, August 23, 2001 5:43 AM
> To: OSPFDISCUSS.MICROSOFT.COM
> Subject: About ASExternal calculation
> Hi..
> I have a doubt in ASExternal Calculation...
>   In ASExternal calculations of RFC2328, Section
> 16.4,3rd point 3rd paragraph,it says that if the
> forwarding address is zero,th packet should be sent to
> the ASBR itself.Also it says that we have to prune for
> multiple entries.
>   My doubt is that how can we prune when there is
> always only one entry for the ASBR in the routing
> table.Because whenever we get a destination ,we check
> for its entry in the routing table and compare the
> path type etc.,and if thay are equal cost we only add
> the next hop entry to the route and not a different
> entry.So there is no possibility for multiple entries
> for the same ASBR in the routing table.
>   If I am wrong tell me when can there be more than
> one entries for the same ASBR in the routing table?
>   Thanks in advance
>   Alex
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> Nikhil Sama                     E-Mail: Nikhil.Samanexsi.com
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