We have BGP between our network and our customers networks and so far each connection uses private AS numbers (on our end we use a different AS number for each connection)

We recently got a public AS number for a connection to a provider. Now that we have this public AS number, should we be using that internally between our own routers (i.e. when deploying new BGP sessions between devices etc) - or should we continue using Private AS numbers?

From what I would gather, we would use the one AS number (our public AS number) to represent our organisation (internally between our routers and externally) where we can (and this would be the public AS number). Is this correct/best practice?

I had a quick look at RFC 1930 which states:

 The classic definition of an Autonomous System is a set of routers
 under a single technical administration, using an interior gateway
 protocol and common metrics to route packets within the AS, and
 using an exterior gateway protocol to route packets to other ASes.
 Since this classic definition was developed, it has become common
 for a single AS to use several interior gateway protocols and
 sometimes several sets of metrics within an AS.  The use of the
 term Autonomous System here stresses the fact that, even when
 multiple IGPs and metrics are used, the administration of an AS
 appears to other ASes to have a single coherent interior routing
 plan and presents a consistent picture of what networks are
 reachable through it.

    To rephrase succinctly:

 An AS is a connected group of one or more IP prefixes run by one
 or more network operators which has a SINGLE and CLEARLY DEFINED
 routing policy.
  • sounds like ebgp and ibgp Aug 6, 2018 at 3:17
  • yeah but my question was around should we use the public as number (now that we have it) for ibgp as well as ebgp
    – jmurphyau
    Aug 7, 2018 at 4:19

2 Answers 2


There are two common solutions:

  • Use your public AS on the network edge with upstream providers and keep private AS inside your network, then make some peers between the two. This way you can define easily your "internal" and your "external" network, but it may be prone to complexity and errors if your network is large.

  • Use your public AS everywhere, but you should change all iBGP sessions at the same time to prevent routing problems. You may have to schedule a downtime for this. And on the customer side, you can use "local-as" or stuff like that on those sessions until they are ready to make the change with you.


The ASN doesn't really NEED to be the same throughout but routers only support having a single AS instance at a time. You can peer with multiple ASNs (different remote networks) but YOU can only have one instance defined on your router at a time. Personally, I'd use your public ASN thought-out. Alternatively, you COULD switch to using something else internally such as EIGRP or OSPF (possibly adding redistribution into the mix) and leave BGP for your edge.

Technically, you can peer between private and public ASNs, so you could mix them together. I don't know if there really is a "best practice" answer to this - it's mostly just up to the admin and if you want to maintain separate ASNs.

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