From: Jongsoo (bstrt2004@gmail.com)
Date: Wed Nov 02 2005 - 21:33:05 GMT-3
I am trying to understand this command will allow to receive MP-bgp
vpn routes with the same ASN.
If I see usage guide in CCO, it says
##################################
Usage Guidelines
In a hub and spoke configuration, a PE router readvertises all
prefixes containing duplicate autonomous system numbers. Use the
neighbor allowas-in command to configure two VRFs on each PE router to
receive and readvertise prefixes are as follows:
One Virtual Private Network routing/forwarding instance (VRF)
receives prefixes with ASNs from all PE routers and then advertises
them to neighboring PE routers.
The other VRF receives prefixes with ASNs from the customer edge (CE)
router and readvertises them to all PE routers in the hub and spoke
configuration.
You control the number of times an ASN is advertised by specifying a
number from 1 to 10. "
################################################
In my gut feeling, basically, this command seems allow two things, 1)
receive BGP routes with its own ASN from PE or CE, ( normal behavior
of BGP blocks BGP route with its own ASN in order to prevent loop) and
2) advertize iBGP routes to iBGP peers ( normal behavior of iBGP is
not to advertize iBGP routes to any iBGP peers).
What seems interesting is this feature will creates a lot of redundant
routes but the length of AS path will quickly determine the best
routes so that there won't be any loop...
This will be a perfect command to make hub and spoke topology to work...
The biggest question I have now is " am I right or wrong?".
Someone please correct me if I am wrong.
Thanks
Jongsoo
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