Support for dual AS and AS migration

[Tore Anderson]

* Lennert Buytenhek <buytenh at wantstofly.org>

> Somewhat related to this, I wrote this patchset last year:
> 
> 	http://trubka.network.cz/pipermail/bird-users/2017-March/thread.html#11084
> 
> The idea was to let you configure two AS numbers that a remote peer
> can connect to us with, so that the remote peer can be reconfigured
> to use a different AS number without the local machine needing to
> reconfigured at the same time.
> 
> (The converse case, where the local machine will reconnect with a
> secondary AS number if connecting with the primary AS number fails is
> more involved, and in my use case, I control both endpoints anyway,
> so this was sufficient.)
> 
> If there's interest in this patchset I can dust it off and try
> resubmitting it.

Hi Lennert,

In case you do decide to work on this again, I'd suggest you take a
look at the FRR / Cumulus feature where you can configure a
peer/peer-groups ASN to be the string «external». This means to accept
the peer to be in any arbitrary ASN (except for the local one).

I've found this to be a very convenient feature, and it'd be nice to
see it in BIRD too.

https://docs.cumulusnetworks.com/display/DOCS/Border+Gateway+Protocol+-+BGP#BorderGatewayProtocol-BGP-ConfiguringBGPPeeringRelationshipsacrossSwitches

Tore