Private ASN for iBGP and public ASN for eBGP?

[facundo.ag]

Hi, we are in the process of building a small carrier and we would like some
suggestions regarding BGP. We already have a public ASN and a /22 pool.

Right now we are using the private ASN 65000 for our iBGP sessions, and we are
about to establish an eBGP session with the first of our providers, but,

Would be correct to keep the private ASN for our iBGP sessions and use our
public ASN for the eBGP session? Is there any advantage to keep private AS for iBGP sessions?

Or is better to use the public ASN for the entire network (iBGP and eBGP)?

Thanks!

[Josh Sprang]

I may need a little more information to make a total valid response..  Can you provide more info such as what you are doing right now at your edge?  Private ASN to single provider?

Typically you want to use the same ASN through and through due to routing policies such as local pref get lost when going between ASN.   Just make sure you have a full view in your iBGP (full mesh, Route Reflector, etc.)  Also BGP loop prevention may get caught when going exit to exit if it crosses 65000 unless you turn it off with allowasin

Also doing eBGP from <your public asn to ASN:6500> can cause complexity.  Plus you need to make sure you have the "remove-private-as" feature on your exit points to providers.

I know you can use the local-as feature to help with the migration. 

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13761-39.html

[facundo.ag]

Thanks for the quick reply! Very useful and informative.

A little more info,

The private ASN for iBGP was a decision made by a company that is advising us, at least until we got a public ASN and IP pool, and now that we have them they told us to keep the ASN 65000 for iBGP and use the public ASN only for eBGP, and this is what sounds really strange to me.

Right now at the edge we have a default route to our provider (and we don't have any client yet, but expecting to have in the following weeks).

As we are about to establish the eBGP session with our first provider (to whom we have the default route right now, and we will have a second provider soon), we want to make the best choice regarding the private ASN for iBGP.

It seems clear from your answer that the way to go is to change the private ASN to the public ASN in the route reflectors and routers PE (we have two of each, but we expect to have at least 8 PE more during the following year).

As you point out, all the information I saw on the web about having two ASN in the same network was because of migration, purchase or similar, nothing like that for a network built from the ground up. 

I hope my explanation confused no more, sorry for my english, not a native speaker.

Thanks!

[chishtyssaa1]

Hii, I have the same problem. Did you get any valid response.