From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sat Sep 27 2008 - 06:51:36 ART
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
> Also, could you please elaborate a little on the "set the
> forwarding address" part? What interface is the one that should be set
> as a multiaccess network type in order for this to happen? If I read
> this (
> (http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009405a.sh
> ) correctly, it should be the redistributed interface (150.1.0.0/24).
> However, this is a PPPoFR interface, and I am not allowed to enable
> OSPF on it (as per the lab requirements).
If you cannot run ospf on the PPPoFR interface (and keep in mind you
only need it running on R3, not the other router), then you could
instead alter the cost of the default route injected into A38 by R4.
e.g. area 38 nssa default-cost 10.
This will make the default route less preferred than the one advertised
by R3.
>
> Thank you once again for your help,
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