From: Andrew Lissitz \(alissitz\) (alissitz@cisco.com)
Date: Wed Nov 02 2005 - 21:49:43 GMT-3
Hey Buddy,
Here is a live example, I have not done the hub and spoke labs like
several others on this mail list have:
CE ---bgp---PE ---(ISP Cloud)--- PE---bgp---CE
Each CE runs AS 65000 and shares routes with the PE. The PEs share
routes via iBGP. The remote PE shares routes with the remote CE, and
the CE sees the routes from AS 65000.
What is BGP to do? It sees its own AS number and realizes there is a
problem.
Solution: Either PE changes the AS number with as-override or the CE
allows its own AS number to come in via the allowed-as command. The
number @ the end is how many times the CE will allow it's own AS number
to be present in the path string of the incoming route information.
Concerning your gut feelings (btw ... hope you are not writing on empty
stomach), number one sounds good, but with number 2, you are essentially
saying that this command will override bgp split horizon. This is not
what it will do, if a route is already coming in, and it contains the
BGP's AS number in the path, then let this in. Not whether or not to
advertise to other peers. I have not seen this command change BGP split
horizon behavior ...
BGP best path selection still occurs, it is just that the routes will
not be rejected because of loop detection. I have not seen multiple
routes being accepted as best paths... Can multiple routes exist without
the BGP multipath command?
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jongsoo
Sent: Wednesday, November 02, 2005 7:33 PM
To: C&S GroupStudy; FORUM
Subject: "neighbor allowas-in" command ( SP CCIE)
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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