Changing ASNs - Gotchas??

Fri Oct 28 19:08:18 UTC 2005

On Fri, 28 Oct 2005, Flint Barber wrote:

> 	What are the real gotchas for changing ASNs that people have run
> into? There is a minor one in terms of route-registry timeliness, I can't
> update RADB until the change takes place and ISPs don't run their update
> scripts on my timetable. So I see there might be a gap. If someone knows a
> strategy there, I would be most appreciative. Beyond that, what extended
> trouble have engineers seen from remote networks?
> 	As background, I am changing an ASN at one location that has 5 ISP
> peers( all ranked in the top 20) connected to it over the weekend. I have
> made all of the arrangements with those ISPs the best I can, but am
> concerned some about propagation beyond them. The ASN was assigned about 60
> days ago by ARIN and I have added it to RADB so most filters should have
> been updated. Anyone know of trouble beyond what I have mentioned or have a
> strategy to ensure a successful migration to mitigate the gotchas??

Do you have and downstream customer BGP sessions to deal with, or just 
transits and peers?

There really isn't a nice, auto-magic way to guarantee that everything is 
OK after a change like this.  The best you can do immediately is establish 
a reasonable level of confidence.

What I'd suggest:
1) verify with each transit provider that they're hearing AND 
propagating the prefixes you're announcing to them as you get the 
sessions reconfigured.
2) verify you're receiving the prefixes you should be - should be pretty 
much the same as what you were receiving prior to the change
3) verify what you're announcing appears in the public route servers,
particularly those not located on networks you're directly connected to.
Also verify those prefixes are originated from the correct AS.

You may run into places with isolated reachability issues if the remote 
site does very aggressive BGP flap damping, but those  will correct 
themselves over time.

As long as 'the world' sees what you announce and vice versa, you should 
be in good shape.

As for the providers who generate filters based off of IRR data, some of 
those may have mechanisms to do some sort of a manual filter push to 
accommodate your needs.

jms