From: Pavel Bykov (slidersv@gmail.com)
Date: Sun Nov 09 2008 - 12:39:30 ARST
It all has to deal with why do we need to connect all areas to area 0, and
not to others, in a star-like topology. Because OSPF works "link-state"
online INSIDE an area, and it works basically as DISTANCE-VECTOR between the
areas (i.e. providing only vector and distance to the network, but not the
exact topological information that we see inside the area like routers and
links). Because it works as distance-vecotr between the areas, it would need
methods to stop routing loops like split-horizon, poison-reverse, etc.. So
instead it mandates star-like topology, where everything has to flow through
Area 0, and therefore loops will not be possible by design.
Virtual-link augments this, and the requirement is to have COMPLETE ROUTING
INFORMATION, which stub, nssa, totally stub or totally nssa cannot provide.
All these types do not have complete routing information and are more prone
to errors therefore, because packets are routed HOP-BY-HOP and you do not
have any guarantee that internal totally-stub-area router will not loop the
packet.
Cisco article has good explanation of it:
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9ee.shtml#using
The transit area cannot be a stub area, because routers in the stub area do
not have routes for 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. The routers
in the stub area do not have routes for specific external destinations.
So it is a design limitation, which is not actually limiting anything
because you can use tunnels, etc.
Hope this helps.
On Sat, Nov 8, 2008 at 4:33 PM, Sachin D <networker.ccie@gmail.com> wrote:
> Hi,
>
> Just wanted to understand why the Virtual link does not work with stub
> areas... If i configure it over stub area where does the LSA gets dropped.
>
> Appreciate if some one can help
>
> Regards
> Sachin
>
>
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-- Pavel Bykov ------------------------------------------------- Stop the braindumps! http://www.stopbraindumps.com/Blogs and organic groups at http://www.ccie.net
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