Showing results for 
Search instead for 
Did you mean: 

BGP and same ASN number


Hi Team,

I have a doubt , I want to use same ASN number at different sites with different- different ISP.(consider i have public ip range )

consider I have a public server with ip at site A and and if i advertise same Public pool at B site , how return traffic know that if it has to come at site A or B.

or if some one from Internet want to access this public server , Is that request will come to on Site A router or Site B router . 



19 Replies 19

Rising star
Rising star


Are your two sites connected in any way? Or are they completely separate sites running the same AS?

Now, if you have for examle (, and advertise this network out to the network, you could do AS_PATH prepending to try and equal out AS_PATH lengths for both routes, but I wouldn't recommend doing it this way.

I'm assuming these are Provider Independent addresses, so you could advertise out specific prefixes with a /32, and use AS_PATH prepending as well, for kind of a "double shot" so to speak.

You could also split up the /24 into two /25s, with one /25 being in Site A and another being in Site B.

Hi John,

Thanks for reply

These sites are not connected and they are completly separate sites runing same AS.

Yes my public Pool is From APNIC and it is ISP provider independent .

i have ask my both ISP and they are saying that they can not split /24 pool in /25 ,with one at site A another at site B.

I basically looking for a DR solution and if my site A goes down then my public ip would be UP and work.

any suggestion ?




Thanks for the reply.

If you want to do this as a DR solution, this can be done pretty easy.

You can have the eBGP link going to your ISP that you want to use as backup, configure AS_PATH prepending, so it should never be used, for incoming traffic, unless the primary eBGP link goes down.

So you could do the following for example: (You can configure it out you like, this is jus an example)

access-list 1 permit

route-map ASPATH_PREPEND permit 10

match ip address 1

set as-path preped ASN ASN ASN

router bgp ASN

neighbor route-map ASPATH_PREPEND out

As long as your primary ISP eBGP peer doesnt' go down, the NLRI information from the Internet should always be taken, since it has a shorter AS_PATH

Feel free to research AS_PATH prepending to make sure this is something you want to implement, but this would be my suggestion.

Hi John,

Thanks a lot for help

i have done some research on As path prepend and i think this will work.

i will let you know as i will complete my testing on test environment.



Hi Prashant,

I had the exact same scenario.

I have my own, provider independent Public AS and a /23 Public IP.

We have 2 DC at 2 different locations with 2 Internet Routers at each site. Each receiving a FULL BGP table. So in a nutshell, I have 2 Datacenters ( San Diego and Phoenix), 4 ISP's (2 per DC),. Each site has redundant ISP's (eBGP to ISP and a iBGP between the 2 routers sharing the full internet routes) and I want to start advertising my new ARIN /23 IP via ANYCAST from the 2 DC's to all 4 ISP's.

DC # 2 is NOT live yet and I was wondering if I can use my same San Diego DC ARIN ASN and advertise my subnet?


Any feedback will be greatly appreciated.