Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

When ospf sham link will be up?

1.When ospf sham link will be up? What conditions are required excluding that /32 endpoint address is known via bgp? I found cisco sham link up only needs /32 endpoint address is known via BGP,however i feel that if sham link is up another conditions is needed,it is in "area area-id sham-link source-address destination-address" command area-id field should be area related PE VPN !

2. in latest draft"OSPF as the PE/CE Protocol in BGP/MPLS VPNs",there exists attribute "129 for Sham Link Endpoint Addresses",can someone tell me how to appear this attribute in BGP vpn route if you configure cisco 2600 router.


Re: When ospf sham link will be up?

I'm not certain I understand the first question--why should the srea id be related to the PE VPN for a sham link? If the right SOO and target are set in redistribution from OSPF into BGP, the route to the other end of the sham link will only be inserted in the correct vrf, and I'm not certain we need the area id fields to match? Or maybe I just missed what you are asking.

On the second question, I'm not certain we support the 129 attribute yet. Why would you want it, if you can advertise the endpoint of the sham link in normal BGP?


New Member

Re: When ospf sham link will be up?

Thanks Russ.In order enable ospf prefer vpn backbone route over backdoor route between two sites,the latter usually is intra-area route,It's necessary that sham link is treated as a intra-area link.So,sham link and pe/ce link should be in the same area.If I make configurations on pe1 as "area 1 sham cost 10",and on pe2 as "area 2 sham cost 10"£¬do you think this sham link will be up?which area will this sham link belongs to?

The second question,when auto mode is used to config a sham link,pe1 uses 129 attribute to tell pe2 that a prefix is the sham link endpoint address.If manual config mode is used,pe2 can get pe1's endpoint address via config,but pe2 cann't get pe1's area of a sham link,so use bgp to distribute endpoint address is still necessary,and in bgp update,sham link endpoint address should have a 129 attribute.

How do you think of these question?Discussing is expected!


Re: When ospf sham link will be up?

The sham link will not come up in this situation (or it shouldn't). Both ends need to be in the same area, since it's an intraarea link, like a normal serial unnumbered link:

A sham-link represents an intra-area (unnumbered point-to-point) connection between PEs. All other routers in the area see the sham-link and use it to calculate intra-area shortest path first (SPF) routes to the remote site.

As for the area ID, we put that in the extended community 0x8000.


CreatePlease to create content