Re: Sham-link Clarification

Make sure that your domain IDs are the same.

On Jun 10, 2010, at 9:53 AM, Nathan Richie 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
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html

Blogs and organic groups at http://www.ccie.net
Received on Thu Jun 10 2010 - 09:57:22 ART