Re: Sham-link Clarification

Nathan

This took me ages to study because there is so little out there in the way
of documentation with regards
to sham link with different areas.

I read a post from Marko which pointed me in the right direction and I will
give you the same advice.

http://www.2bccie.com/cisco-certification/ccie-sham-links-between-different-ospf-areas.html/comment-page-1

In your example you have a partitioned Area 0 scenario. The MPLS
Superbackbone is an Area 0 for the MPLS cloud.
You have introduced a separation between the backbone and the Area 0 on your
CEs. If you look for Area 0 routes for CE3 on PE1
you will not find them.

To fix this you can use virtual links for solve the partitioned Area 0.

Then remember that Sham links are created between identical areas.

Lab this up and see what happens.

On 10 June 2010 14:53, Nathan Richie <nathanr_at_boice.net> wrote:

> I am labbing up some various scenarios on MPLS and OSPF. From what I can
> tell
> at this point, a sham-link works great if both the sites are in the same
> OSPF
> area. However, from my what I can see in my results, it is not effective
> when
> the 2 sites are in different non-backbone areas.
>
> So here is my topology:
>
> PE1---------PE2
> | |
> | |
> CE1 CE2
> OSPF OSPF
> Area 10 Area 100
> | |
> | |
> CE3 CE4
> OSPF OSPF
> Area 0 Area 100
> | |
> | |
> CE5--------CE6
> OSPF
> Area 100
>
> CE1 always prefers the route to Area 100 via the backbone area. When I
> disable the link between CE5 & CE6, it will use the sham-link between PE1 &
> PE2. Even though the Sham-link metric is lower. I am assuming it is
> because
> of the requirement of OSPF to route inter-area through the backbone, but I
> could be wrong.
>
> When link between CE5 & CE 6 is enabled
> CE1#show ip route 2.2.2.2
> Routing entry for 2.2.2.2/24
> Known via "ospf 1", distance 110, metric 131, type inter area
> Last update from 22.22.0.34 on Vlan3, 00:00:17 ago
> Routing Descriptor Blocks:
> * 22.22.0.34, from 22.22.3.3, 00:00:17 ago, via Vlan32
> Route metric is 131, traffic share count is 1
>
> When link between CE5 & CE 6 is disabled
> CE1#show ip route 2.2.2.2
> Routing entry for 2.2.2.2/24
> Known via "ospf 1", distance 110, metric 4, type inter area
> Last update from 22.22.0.1 on Vlan5, 00:00:09 ago
> Routing Descriptor Blocks:
> * 22.22.0.1, from 22.22.0.1, 00:00:09 ago, via Vlan51
> Route metric is 4, traffic share count is 1
>
> Two questions:
>
> 1) Why is it choosing the higher metric link over the lower-metric
> sham-link?
>
> 2) Is there a way to route traffic through the sham-link instead of the
> CE5
> - CE6 link?
>
>
> Regards,
>
> Nathan Richie
>
>
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Received on Thu Jun 10 2010 - 15:11:31 ART