Failover BGP servers?

[Yasen Atanasov]

 Hello,  Lets imagine I have 2 BGP servers announcing same subnet. BGP sessions 
with upstream are in different VLAN. Both servers have keepalived and all 
clients are using for default route the virtual address. The idea is if I need 
to upgrade/rebuild one of the servers the clients will still have connectivity 
trough the other server. Do I need to do something special in the bird config 
or just announcing the same net is enough?  Regards.  Yasen