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Bronze

OSPF virtual links..

Hi,

Why OSPF virtual links cann't able to traverse stub areas?

Thanks in advance..

Regards...

Ashok.

With best regards... Ashok ----------- Pls kindly rate if helpful or answered your question.
5 REPLIES

Re: OSPF virtual links..

Basically OSPF virtual link will carry external LSAa and external LSAa can not exist in stub areas.

Gold

Re: OSPF virtual links..

Because a stub implies there is "no path through," while a virtual link implies "there is a path through." The two are contradictory. There is actually danger of routing loops if you could do this, since the routers in the middle of the stub area wouldn't have the information required to properly route the traffic through the stub area itself.

Russ.W

Bronze

Re: OSPF virtual links..

Hi Russ,

Thanks for the info. But could you please elaborate how it can create routing loops in stub areas?

Thanks and regards..

Ashok.

With best regards... Ashok ----------- Pls kindly rate if helpful or answered your question.
Gold

Re: OSPF virtual links..

Think of it like iBGP without synch, and without full routing running in the cloud. Okay, that probably didn't make it any simpler. :-) Let's try this:

A----B----C

If B is marked as a "stub," as in a stub area, it should only have one exit. Suppose C knows of some path through A, but B doesn't know about it (or it only pays attention to it's default exit point, as a good stub should do). Now, C forwards a packet to B, and B happens to be using C as it's default exit point--a loop.

In reality, the problem is more fundamental, though--stub means "there's only one exit point from this area," while virtual link means "there's a path through this area." They are in logical conflict with one another just in defining them.

:-)

Russ.W

Bronze

Re: OSPF virtual links..

Hi Russ,

I got it. Thanks again.

Regards...

Ashok.

With best regards... Ashok ----------- Pls kindly rate if helpful or answered your question.
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