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Tuesday, November 12, 2013

Using the BGP ASN from multiple locations

Can the same AS number be used by an organization at two different sites to peer with two different ISPs? 
The scenario is as per what is shown below: 

ASN 1111-a ( XXX---ASN YYY---ASN 1111-b (

The AS number is 1111. The organization using it at location A to advertise network to the ISP peer. At location B the organization is is using the same ASN to advertise to another ISP. There is no peering between the two AS 1111 routers as they are physically separated.

Will this work?

Some Notes:

You need to advertise separate public ip address blocks out of the two sites.
If you try to advertise something more specific that a /24 you may have problems with summarization of your routes at the ISP level.
So it can work in real world if you have at least two /24 public ip address blocks registered with your AS number

Without knowing the details of the remote site routes it is difficult to say what is happening when something goes wrong.

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