[PATCH, RFC 3/3] BGP: Don't unconditionally bounce connection for remote AS changes.

[Lennert Buytenhek]

On Thu, Mar 09, 2017 at 04:23:05PM +0100, Ondrej Zajicek wrote:

> > If we are using the secondary remote AS mechanism, we don't necessarily
> > need to restart an already established BGP connection if the remote AS
> > for a BGP session changes, as long as the AS number that the peer is
> > currently connected with is still in the list of AS numbers that we
> > accept from the peer.
> > 
> > This commit will avoid reconfiguring a BGP protocol if there is a
> > corresponding connection in OpenConfirm or Established state and the
> > only change to the protocol configuration was related to remote AS
> > numbers and the AS number that the peer is currently connected with is
> > still a permitted remote AS number.  This allows adding a secondary
> > remote AS number to a BGP protocol without bouncing an already
> > established BGP session for this protocol.
> 
> Well, the simple way to avoid reconfiguration in this case is just to move
> 'remote_as2' field at the end of bgp_config, like 'check_link' or 'bfd'.

Well, e.g. if remote_as is 12345 and the currently established peer AS
is 12345, and you change remote_as from 12345 to 23456 and remote_as2
from 0 to 12345, you want to avoid doing the reconfigure.

And if you keep remote_as at 12345 and set remote_as2 to 23456 by
specifying the ASes the other way around, you would avoid the reconfigure
if you leave remote_as2 outside the memcmp() field, but then when the
other side completes its maintenance and you reestablish a connection
with remote AS 23456, you would then set remote_as from 12345 to 23456
and remote_as2 from 23456 to 0, and that would then unnecessarily
trigger a reconfigure as well because remote_as changes.