OSPF & ethernet link failure

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Jan 28th, 2009
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I have 2 links between 2 routers.

They have different OSPF costs.

The links are GE and each go via several switches so a failure at one end does not cause link to go down at the other end of the link.

If the Rx on one router fails this interface will immediately send LSAs to the other router via the remaining link.

Will the router (which is remote from the failure) immediately consider the link dead on the basis of the LSAs even though it's interface on the failed link is still up? or will it need to wait for dead time (40sec) before considering the link dead?

Regards, MH

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Giuseppe Larosa Thu, 01/29/2009 - 00:24
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Hello Mark,

different types of OSPF data structures (LSAs) are involved here:

R1 will send a new version of its Router LSA with the link with state down.

But between R1 and R2 there is the Network LSA of the two.

And also the OSPF neighbor state machine.

So I wouldn't expect the router R2 to be able to declare the R1 neighborship down before dead interval.

Hope to help


MARK HEUZENROEDER Thu, 01/29/2009 - 04:06
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Thankyou Giuseppe,

Now I've tried it on Dynamips.

It is indicating convergence in about 5 sec. so isn't waiting to dead-timer.

Interesting that with a direct ethernet connection between 2 routers (not via Dynamips switch) SHUT one i/f and the other one stays up.

Even under these circumstance OSPF converges in 5 sec.

Regards, MH


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