Diameter redundancy is supported on multiple levels:
High Availability (HA)
This refers to control plane redundancy with dual Control Plane Modules (CPMs) in a single chassis configuration. User sessions for Diameter applications are fully synchronized between CPMs.
However, the TCP connections with peers are not synchronized and after a CPM switchover, the TCP connections need to be re-established. Once the TCP peering connections are re-established, communication on the user level (NASREQ/Gx/Gy) is resumed without the loss of any user sessions.
peer and server failover
An existing user session can be rerouted to an alternate peer or server if the currently used peer or server fails.
multi-chassis redundancy
Diameter sessions are synchronized on the application level (through ESM if Gx and NASREQ) between two redundant SR nodes. Only one of those two nodes maintain a peering connection with the agent or server. As a result, the agent or server interacts with the redundant pair through a single peering connection.