From: Bogdan Sass (bogdan.sass@catc.ro)
Date: Sat Sep 27 2008 - 04:28:57 ART
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
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).
Thank you once again for your help,
-- Bogdan Sass CCAI,CCNP,CCSP,JNCIA-ER Information Systems Security Professional "Curiosity was framed - ignorance killed the cat"Blogs and organic groups at http://www.ccie.net
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