From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sat Sep 27 2008 - 07:10:07 ART
Paul Cosgrove wrote:
>
>
> Bogdan Sass wrote:
>> paul cosgrove wrote:
>>> Hi Bogdan,
>>>
>>> So R3 has a RID of 150.1.3.3 and is advertising an E2 route for
>>> 158.1.0.0/24 with a forwarding address of 0.0.0.0. This forwarding
>>> address denotes itself (not a default route), so other routers learn
>>> that they need to send traffic to 158.1.0.0/24 via 150.1.3.3. SW4
>>> has to decide how it is going to get there. SW4 receives Type 1
>>> LSAs from R3 from both areas and must decide which to use.
>>>
>>> Have a look at section 16.4.1 in RFC2328 "External path preferences".
>>>
>>> - Intra-area paths using non-backbone areas are always the most
>>> preferred
>>> - The other paths, intra-area backbone paths and inter-area paths,
>>> are of equal preference.
>>>
>>> The first rule causes SW4 to select the path via area 38.
>>>
>>> SW3 does not receive an LSA about this prefix from R3, because of
>>> the no-redistribute option. So the only path it knows about is the
>>> default route it learns from SW4, and it sends the traffic right back.
>>>
>>> You need to loose the no-redistribute option on R3.
>> Thank you very much for your reply! (it explains very well what is
>> going on, so I finally have a solution to a problem that has been
>> bothering me for quite a while :) )
>> Unfortunately, I couldn't just remove the no-redistribute option
>> on R3 (as per the lab requirements - this problem was encountered in
>> one of the IE labs). I guess the only solution here would be to
>> filter the route going through area 38 on SW4. I managed to do that
>> with a route-map matching on the next-hop.
>> One more question, though: if the RFC specifies that the
>> intra-area non-backbone path (in my case, the path through area 38)
>> should be preferred, why does SW4 install both paths (backbone and
>> non-backbone) into the routing table? Is there a particular reason
>> for this, or is my router just... ignoring the RFC? :)
>>
>> O E2 158.1.0.0/24 [110/20] via 158.1.34.1, 00:00:01, FastEthernet1/13
>> [110/20] via 158.1.1.1, 00:00:01, Vlan110
>>
> I think SW4 is using the older RFC1583 rules. Try:
> no compatible rfc1583
>
Which interface is the RID for R3? Is it on the physical nic between R3
and R2? Just checked and even with RFC1583 rules (or at least my
reading of them) the cost to the forwarding address is still used to
differentiate between external paths. Based on your diagram, if you are
using a loopback I would not have expected this to happen.
Paul.
Blogs and organic groups at http://www.ccie.net
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