Sham-link Clarification

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
Received on Thu Jun 10 2010 - 09:53:19 ART