From: Felix Nkansah (felixnkansah@gmail.com)
Date: Sat Nov 08 2008 - 14:52:37 ARST
Hi Sachin,
In short, it has to do with the behavior of OSPF which requires other areas
of the domain to be connected to the backbone area.
As such, data from one area to another must transit through the backbone
area.
A virtual link serves as a "point-to-point" extension of the backbone area,
through which other areas can send their transit data.
Unfortunately, virtual links do not tunnel the transit data, only the
routing updates are. The transit data is sent natively by the area in which
it lies.
The routers in the stub area do not have routes for specific external
destinations. Because data is sent natively, if a packet destined for an
external destination is sent into a stub area which is also a transit area,
then the packet is not routed correctly.
To overcome this limitation when the backbone is 'extended' in a stub area,
we need a 'link' that would tunnel both the routing updates and the transit
data itself, in the case of data going to external destinations.
GRE tunnels are the ones known to solve such a problem. So you would have to
use GRE tunnels if you want to extend the backbone through a stub.
I guess it's not a short explanation afterall. Maybe others can make it
clearer in case this doesn't help.
Happy studies!
Felix Nkansah, CCIE
Blogs and organic groups at http://www.ccie.net
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