4rmorris Thu, 08/14/2008 - 16:43

If you have your own IP range assigned and you're planning to connect to multiple service providers for redundancy, you need an AS number to talk BGP and properly advertise the path on the public internet.



Amin Shaikh Fri, 08/15/2008 - 01:09

Is there a cisco document ; explaining how to configure BGP for multiple service provider.

Can we use the service provider AS number.

4rmorris Fri, 08/15/2008 - 08:50

Start here:


And then if you connect multiple service providers you can use other features in other chapters of that doc to influence traffic flow.

If you use the SP AS number you can't really connect to multiple providers. You're either part of one AS or the other. If you talk to your provider, you may be able to use a private AS (>65000) and use that with both providers, and they would strip the private AS when they pass on the advertisement.

I'm not sure how this would affect Internet routing, as your routes would appear to originate in two different AS's at that point. It's worth a conversation with the SP.

Good luck,


Giuseppe Larosa Fri, 08/15/2008 - 05:37

Hello Amin,

you need an AS number when:

you have your own IP public address range

you are multihomed (multiple upstream ISPs advertising your IP prefixes)

AS numbers are becoming rare and RIPE will not release an AS number to a leaf AS that is not multihomed (I followed a similar case for one customer 5 years ago)

You can also be multihomed without having an AS number and without your own IP prefixes, if you get one address pool from each ISP and configure NAT on your border routers using route-maps.

Every provider expect to receive only traffic sourced by its address pool on its own link.

If you're interested on how to get an AS number visit for the instructions


Hope to help


Amin Shaikh Sat, 08/16/2008 - 00:39

Thanks Giusiar.

can you get me an example of a multihome without having AS number.

can I do load balance / failover with your option

Giuseppe Larosa Sat, 08/16/2008 - 00:58

Hello Amin,

my name is Giuseppe.

There are the so called private AS numbers in range 64512-65535 the last 1024 in the 16bit range of AS numbers.

You can use one of these private AS numbers to setup eBGP connections to two ISPs. Every ISP will provide an address pool for NAT from their own public IP block and you need to send to each provider traffic with a source address (after NAT) of its own address pool.

You need to define two address pools and to use NAT with a route-map to be able to manage this scenario.

As a reference for this scenario use the following link:


Hope to help


Amin Shaikh Sat, 08/16/2008 - 01:12

Thanks Giuseppe.

This is not failover/load balancing of services[web-server hosted on network].

( example webs server will not be failover if one link goes downm, since its a static NAT)

VPN tunnels will not failover since static peer is defined.


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