[c-nsp] BGP AS Number Change

[Andrew Melton]

I have two routers which are IBGP neighbors of each other, and EBGP
neighbors with upstream providers.  I need to change the ASN runnig on the
boxes with the least impact to either the IBGP or EBGP processes.  After
pouring over the config, it seems that there are at least two options.  I am
looking for some advice and best practice guidance, and ultimately, choosing
a method to make the change.

The basic config is:

Existing AS = 36113
New AS = 25113

RTA Config

<snip>
router bgp 36113
neighbor 172.16.213.6 remote-as 36113
neighbor 172.16.213.6 description IBGP Peer
neighbor 172.16.213.6 version 4
neighbor 4.4.4.4 remote-as 4
neighbor 4.4.4.4 description EBGP Peer
neighbor 4.4.4.4 version 4
</snip>

RTB Config
<snip>
router bgp 36113
neighbor 172.16.213.5 remote-as 36113
neighbor 172.16.213.5 description IBGP Peer
neighbor 172.16.213.5 version 4
neighbor 5.5.5.5 remote-as 5
neighbor 5.5.5.5 description EBGP Peer
neighbor 5.5.5.5 version 4
</snip>

Option 1:  My thought is, to either use the 'neighbor x.x.x.x local-as' and
'neighbor x.x.x.x remote-as' commands on the IBGP peers to use a private AS,
i.e. 65213.
For example:
RTA
neighbor 172.16.213.6 remote-as 65213
neighbor 172.16.213.6 local-as 65213
RTB
neighbor 172.16.213.6 remote-as 65213
neighbor 172.16.213.6 local-as 65213

This should cause IBGP updates in either direction to append the private AS
65213 to the public 36113, making the AS Path 65213 36113.
The EBGP neighbors on each router need 'remove-private-as' to suppress the
65213 in the EBGP updates:
RTA
neighbor 4.4.4.4 remove-private-as
RTB
neighbor 5.5.5.5 remove-private-as

Once this is done, IBGP updates are now from the private AS 65213.  At this
point, as long as the EBGP peer is configured to accept updates from either
the new public AS 25113 or the old public AS 36113, the routing process on
RTA can be changed, and the IBGP routes exchanged will continue.  If RTA is
changed before RTB, then the updates in one direction would be 65213 25113
and in the other direction would be 65213 36113.

Once the routing process ID has been changed from 'router bgp 36113' to
'router bgp 25113', the private AS config could either be left in place or
removed.


Option 2:  Would be to use the same 'neighbor x.x.x.x local-as' command on
EBGP neighbors, to append the new public AS to the EBGP updates.  Then, the
EBGP AS path would be 25113 36113, whereas the IBGP path would be 36113.

This would allow time to coordinate the new AS updates with the EBGP
neighbors.  Once the rest of the world is seeing the prefixes coming from
the new ASN 25113, the routing process ID on each router could be changed
from 'router bgp 36113' to 'router bgp 25113' and the IBGP neighbors would
come up with the new ASNs.

After the IBGP routes have been exchanged with AS path 25113, the 'neighbor
x.x.x.x local-as 25113' can be removed, and everything will be back to
normal.

Are there reasons why one of these might not work?  Better ideas to
accomplish this?

Thanks...