Few service providers mandate that an SBC register to the service provider before accepting or invoking calls. An FQDN manages the servers on the service provider side for more reliable call management.

The DNS SRV feature is used to identify the highest priority server resolved to this FQDN so that Avaya SBC can register to that server. For Geo-Redundant deployments, a distributed Avaya SBC HA pair can point to different servers based on priority and how you administer the DNS SRV feature on Avaya SBC. Multi-server Avaya SBC configurations also support the DSN SRV feature, where the FQDN stays the same and the credentials on each server for the Register message vary on each server.

Avaya SBC sends a Register message based on the administered refresh time interval. Based on resiliency designed on the Service Provider side, if the priority of these servers change, Avaya SBC then switches its Register message. If Avaya SBC detects a server failure on the Service Provider side based on a heartbeat mechanism such as OPTIONS, Avaya SBC sends an UnRegister message to the failed server, followed by a Register message to the next highest priority server. Avaya SBC then initiates the call to the registered server. When Avaya SBC switches registration, it rejects new calls. Any calls that were active on the now unregistered Service Provider trunk server remain connected on the Avaya SBC when this failure occurs.

Avaya SBC does not support load balancing during failures and selects one priority server for registration. Avaya SBC can be configured to register with multiple servers so that at time of registration, the system uses different credential that are specific to that service provider.