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New Member

OSPF sham-links

Hi,

I have MPLS backbone (area 0) and two locations also in area 0, which have a backdoor link. Do I need to setup sham-link between these two sites if they are in the same area?

Thanks,

Petr

4 REPLIES
Cisco Employee

Re: OSPF sham-links

You need to setup a sham-link if you want the traffic between the two sites to prefer the mpls backbone rather than the backdoor link. Otherwise the routes learnt via the mpls backbone will be seen as inter-area at best and traffic will be forwarded via the backdoor as routes learnt via the backdoor are seen as intra-area and therefore preferred

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
New Member

Re: OSPF sham-links

but I have all sites in area 0 as well as backbone. So I thought that all routes will be intra-area type and don't need to use sham-links. Then do I still need to setup the sham-links?

Petr

Re: OSPF sham-links

Hi Petr,

yes you still need sham-links, if you want to use the MPLS backbone as primary path. The reason is, that you have LSA type 3 inserted by the PE into your OSPF. This means you will get INTER-area routes even if area 0 is everywhere. And a normal OSPF router will always prefer INTRA-area (LSA1+2) over INTER-area no matter what the metric is. Thus your backdoor links will always be used, as long as they have connectivity.

OSPF sham-links "pretend" to be an INTRA-area link between PE routers across your MPLS MBGP backbone. This way you get INTRA-area routes and metric decides which path is prefered. All you might need then is to adjust your interface metrics to achieve the desired traffic flow.

Hope this helps! Please rate all posts.

Regards, Martin

Cisco Employee

Re: OSPF sham-links

Petr,

Updates received from the MPLS core will always be seen as inter-area or external unless you configure a sham-link between the PEs.

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
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