Multihoming BGP with Private and Public ASN

[howiekabam]

Hi,

I had a question regarding trying to multihome BGP with two different providers using private and public ASN. We currently have a private AS and wish to bring in a secondary ISP for redundancy. We recently received a public AS but current BGP peering is using a private ASN. What is the best way to implement the secondary link to our environment with a new ISP? Is it possible to run a private and public AS peering within the same BGP process to two different providers?

Thanks for any insight.

[paul driver · Accepted solution]

Hello

You can run a private ASN number to One ISP and be a neighour to a ISP with a  public ASN.

Private AS 65530 -

Public AS 200 -

The public ISP will peer to your router with ASN 200

router bgp 65530

neigbour 192.168.1.2 remote-as 65531

neigbour 20.20.20.22 remote-as 400

neigbour 20.20.20.22 local-as 200 (No-prepend) (Replace-as)

No-prepend = doesnt prepend the local-as number to your bgp table

Replace-as = prepends ONLY the local-as to the publlic ISP bgp table

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

[howiekabam]

Thanks Paul for your answer. Very helpful

[blau grana]

Hello Howard,

As pdriver wrote, it is possible to accomplish to run BGP like that. But I think that it will be better to migrate to your public AS if you have one.

If you peer with one ISP with private AS, this ISP has to replace your AS with some public, probably his own AS. If you run BGP with second ISP with public AS, no AS replacement is necessary.

- if you have public prefix f.e. 100.0.0.0/24 it would be advertised to internet from two ISPs but with different source AS in AS_PATH. This can possibly caused problems.

Best Regards

Please rate all helpful posts and close solved questions

[howiekabam]

Hi Blau

Yes I also thought about replacing the current peering with our new AS since we are multi homed to the same provider.  I should be able to change one peering, verify my route announcement, failover the traffic and change the second peering. Then bring up the redundant peering with my secondary provider. This will obviously take more work, so I am wonder what problems I might encounter if I were to use the local as command. I do understand that my AS is hidden from my private peering as it shows my ISPs AS in the AS PATH but it would be nice to understand the potential problems I may face doing it the way pdriver  suggest.

Thanks for your advice!

[blau grana]

Hello,

It is not very likely, but if you put together bad design with bad configuration it could happen that you will communicate with local host via internet links. BGP will choose best path via internet. This will never happen if you have one AS because BGP loop prevention would drop the prefix because of same AS in AS_PATH.

Best Regards

Please rate all helpful posts and close solved questions