Cisco Cisco Packet Data Gateway (PDG) Fehlerbehebungsanleitung
Session Recovery
How Session Recovery Works ▀
Cisco ASR 5000 Series Enhanced Feature Configuration Guide ▄
OL-22982-01
How Session Recovery Works
This section provides an overview of how this feature is implemented and the recovery process.
Session recovery is performed by mirroring key software processes (e.g., session manager and AAA manager) within
the system. These mirrored processes remain in an idle state (in standby-mode), wherein they perform no processing,
until they may be needed in the case of a software failure (e.g., a session manager task aborts). The system spawns new
instances of ―standby mode‖ session and AAA managers for each active control processor (CP) being used. Naturally,
these mirrored processes require both memory and processing resources, which means that additional hardware may be
required to enable this feature (see the Additional Hardware Requirements section).
the system. These mirrored processes remain in an idle state (in standby-mode), wherein they perform no processing,
until they may be needed in the case of a software failure (e.g., a session manager task aborts). The system spawns new
instances of ―standby mode‖ session and AAA managers for each active control processor (CP) being used. Naturally,
these mirrored processes require both memory and processing resources, which means that additional hardware may be
required to enable this feature (see the Additional Hardware Requirements section).
Additionally, other key system-level software tasks, such as VPN manager, are performed on a physically separate
Packet Accelerator Card (PAC)/Packet Services Card (PSC/PSC2) to ensure that a double software fault (e.g., session
manager and VPN manager fails at same time on same card) cannot occur. The PAC/PSC/PSC2 used to host the VPN
manager process is in active mode and is reserved by the operating system for this sole use when session recovery is
enabled.
Packet Accelerator Card (PAC)/Packet Services Card (PSC/PSC2) to ensure that a double software fault (e.g., session
manager and VPN manager fails at same time on same card) cannot occur. The PAC/PSC/PSC2 used to host the VPN
manager process is in active mode and is 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 PAC/PSC/PSC2. In this mode, recovery is performed by using the mirrored
―standby-mode‖ session manager task(s) running on active PACs/PSCs/PSC2s. 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.
―standby-mode‖ session manager task(s) running on active PACs/PSCs/PSC2s. 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 PAC/PSC/PSC2 recovery mode: Used when a PAC/PSC/PSC2 hardware failure occurs, or when a
planned PAC/PSC/PSC2 migration fails. In this mode, the standby PAC/PSC/PSC2 is made active and the
―standby-mode‖ session manager and AAA manager tasks on the newly activated PAC/PSC/PSC2 perform
session recovery.
―standby-mode‖ session manager and AAA manager tasks on the newly activated PAC/PSC/PSC2 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 application cards to ensure task recovery.
task is paired together. These pairs are started on physically different application cards to ensure task recovery.
There are some situations wherein session recovery may not operate properly. These include:
Additional software or hardware failures during the session recovery operation. An example of this would be if
an AAA manager were to fail while the state information it contained was being used to populate the newly
activated session manager task.
activated session manager task.
A lack of hardware resources (i.e., PAC/PSC/PSC2 memory and control processors) to support session recovery.
Important:
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/billing related
information is checkpointed/recovered.
information is checkpointed/recovered.