Asr 5500 System Administration Guide, Staros Release 21.4 - Cisco ASR 5000 Series Administration Manual

How Session Recovery Works
means that additional hardware may be required to enable this feature (see
Requirements, on page
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:
• Additional 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.
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 and billing
related information is checkpointed and recovered.
Session Recovery is available for the following functions:
• Any session needing L2TP LAC support (excluding regenerated PPP on top of an HA or GGSN session)
• ASR 5500 only – Closed RP PDSN services supporting simple IP, Mobile IP, and Proxy Mobile IP
• ASR 5500 only – eHRPD service (evolved High Rate Packet Data)
• ASR 5500 only – ePDG service (evolved Packet Data Gateway)
• GGSN services for IPv4 and PPP PDP contexts
• HA services supporting Mobile IP and/or Proxy Mobile IP session types with or without per-user Layer
3 tunnels
• ASR 5500 only – HNB-GW: HNB Session over IuH

ASR 5500 System Administration Guide, StarOS Release 21.4

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326).
Session Recovery
Additional ASR 5500 Hardware