12-29-2003 06:17 AM - edited 03-02-2019 12:35 PM
Hi,
Why OSPF virtual links cann't able to traverse stub areas?
Thanks in advance..
Regards...
Ashok.
12-29-2003 06:39 AM
Basically OSPF virtual link will carry external LSAa and external LSAa can not exist in stub areas.
12-29-2003 07:59 AM
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
12-29-2003 08:07 PM
Hi Russ,
Thanks for the info. But could you please elaborate how it can create routing loops in stub areas?
Thanks and regards..
Ashok.
12-30-2003 08:28 AM
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
12-30-2003 09:15 PM
Hi Russ,
I got it. Thanks again.
Regards...
Ashok.
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