Failover BGP servers?

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