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

__________________________________________________
Do You Yahoo!?
Make international calls for as low as $.04/minute with Yahoo! Messenger
http://phonecard.yahoo.com/ <http://phonecard.yahoo.com/>

--
Nikhil Sama                     E-Mail: Nikhil.Samanexsi.com
Routing Protocols Group         Ph:  408 579 5880
Nexsi Corp.                     Fax: 408 579 5701
1959 Concourse Drive            Ph: (M) 310 428 9191
San Jose, CA 95131.             www.nexsi.com
Home:
1240 Dale Av., Apt 16           Ph: 650 237 9055
Mountain View, CA 94040

• Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]