Shamlink is flapping

Unanswered Question

I had configured a sham link between


iam getting the ospf neighbourship up but suddenly it is getting down.

Please share you views to clear this problem.Thanks in advance.


I have this problem too.
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jcozzupoli Thu, 12/17/2009 - 02:47

Hi Mahesh,

Can you please answer the following questions I have for you:

1. Please send us the conifgurations

2. What IOS is running on each router, especially on the PE routers? Is it the same on both ends?

3. Please show the output of "show ip ospf sham-links"

4. Please turn on "debug ip ospf adj" and "debug ip ospf events"

Once you answer these questions, i'm sure myself or someone here will be able to give you some advice.



Giuseppe Larosa Thu, 12/17/2009 - 05:00

Hello Mahesh,

as it happens for a point to point GRE tunnel you need to avoid recursive routing: the sham-link uses endpoints and this endpoints must be known by other means.

Configure the source and destination addresses of the sham-link   as a host route mask (
on the PE   routers that serve as the endpoints of the sham-link. The source and   destination IP addresses
must   belong to the VRF and be advertised by Border Gateway Protocol (BGP) to   remote PE routers. The
sham-link endpoint addresses should not be advertised by OSPF.

then of course your issue may be different and related to the IOS version on the PE nodes as noted by Joe.

Hope to help


amit.bhagat Fri, 12/18/2009 - 05:17

Hi Mahesh,

I think you will find your answers here-

NOTE: The Sham-link Endpoint Address must be advertised by BGP as VPN-IPv4 address; it must NOT be advertised by OSPF.

Also, remember to increase the cost of backdoor link and reduce the cost of sham-link otherwise, even though your sham-link will be UP, due to low-cost, backdoor link COULD be preferred.




delhi.pnr Thu, 08/09/2012 - 13:02

Before :

            |----------I BGP Session-------|


After : 

            |----------E BGP Session-------|



Nishant Thakur 


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