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Sham Links

If I am configuring my PEs to support shamlinks for CEs that have a non mpls redundant connection to each other, do I have to have a connection on the PE in the same OSPF area as the one I am creating the sham link for ?

In other words, if the CEs have there redundant link in area 1, but the CE-PE link is in area 0, could I simply configure the sham link as " area 1 shamlink x.x.x.x y.y.y.y cost A " even if there is no area 1 on the PE for that vrf ?

I was trying to do this in the lab but my sham link was always down and I could not provide MPLS connectivity for the customer.

Any help ?

Herbert.

2 ACCEPTED SOLUTIONS

Accepted Solutions
Cisco Employee

Re: Sham Links

Herbert,

Yes the link has to be in the same area if you want the traffic to traverse the MPLS cloud rather than the non mpls redundant connection. Otherwise the intra area route via area 1 will always be preferred over the inter area route via area 0.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Re: Sham Links

Hello Herbert,

as Harold pointed out, a sham link will not work in your scenario. But basically your problem has nothing to do with MPLS VPNs at all. The routing decision in your case is made by an area 1 internal router R1, which also gets the same prefixes through an ABR.

R1 does not "know" anything about MPLS VPNs, it will just do, what a normal OSPF router does: prefer intra area over inter-area independant of metric.

So either place your PEs in the same area with R! and use a sham-link, or use floating static or redistribution (with filters!) across the backdoor link.

Hope this helps! Please rate all posts.

Regards, Martin

2 REPLIES
Cisco Employee

Re: Sham Links

Herbert,

Yes the link has to be in the same area if you want the traffic to traverse the MPLS cloud rather than the non mpls redundant connection. Otherwise the intra area route via area 1 will always be preferred over the inter area route via area 0.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Re: Sham Links

Hello Herbert,

as Harold pointed out, a sham link will not work in your scenario. But basically your problem has nothing to do with MPLS VPNs at all. The routing decision in your case is made by an area 1 internal router R1, which also gets the same prefixes through an ABR.

R1 does not "know" anything about MPLS VPNs, it will just do, what a normal OSPF router does: prefer intra area over inter-area independant of metric.

So either place your PEs in the same area with R! and use a sham-link, or use floating static or redistribution (with filters!) across the backdoor link.

Hope this helps! Please rate all posts.

Regards, Martin

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