provides an overview of how this feature is implemented and the recovery
The Session Recovery
feature provides seamless failover and reconstruction of subscriber session
information in the event of a hardware or software fault within the system
preventing a fully connected user session from being disconnected.
Session recovery is
performed by mirroring key software processes (for example, session manager and
AAA manager) within the system. These mirrored processes remain in an idle
state (standby-mode) wherein they perform no processing, until they may be
needed in the event of a software failure (for example, a session manager task
The system spawns new instances of "standby mode" session and AAA managers for each active control processor (CP) being used. These mirrored processes require both memory and processing resources, which means that additional hardware may be required to enable this feature (see Additional ASR 5x00 Hardware Requirements).
Other key system-level software tasks, such as VPN manager, are performed on a physically separate packet processing card to ensure that a double software fault (for example, session manager and VPN manager fails at same time on same card) cannot occur. The packet processing card that hosts the VPN manager process is in active mode and reserved by the operating system for this sole use when session recovery is enabled.
There are two modes of session recovery.
Task recovery mode: Wherein one or more session manager failures occur and are recovered without the need to use resources on a standby packet processing card. In this mode, recovery is performed by using the mirrored "standby-mode" session manager task(s) running on active packet processing cards. The "standby-mode" task is renamed, made active, and is then populated using information from other tasks such as AAA manager. In case of Task failure, limited subscribers will be affected and will suffer outage only until the task starts back up.
Full packet processing card
recovery mode: Used when a packet processing card hardware failure occurs, or when a planned packet processing card migration fails. In this mode, the standby packet processing card is made active and the "standby-mode" session manager and AAA manager tasks on the newly activated packet processing card perform session recovery.
Session/Call state information is saved in the peer AAA manager task because each AAA manager and session manager task is paired together. These pairs are started on physically different packet processing cards to ensure task recovery.
There are some
situations wherein session recovery may not operate properly. These include:
software or hardware failures occur during the session recovery operation. For
example, an AAA manager fails while the state information it contained was
being used to populate the newly activated session manager task.
A lack of
hardware resources (packet processing card memory and control processors) to
support session recovery.
After a session
recovery operation, some statistics, such as those collected and maintained on
a per manager basis (AAA Manager, Session Manager, etc.) are in general not
recovered, only accounting and billing related information is checkpointed and