Cisco Cisco IOS Software Release 12.4(22)XR
33
Cisco Packet Data Serving Node Release 5.1 for Cisco IOS Release 12.4(22)XR1
OL-20782-01
Session Redundancy Infrastructure
The PDSN Session Redundancy feature provides user session failover capability to minimize the impact
of a PDSN failure on the mobile user experience. The PDSN uses a 1:1 redundancy model, with a
standby present for every active PDSN. The active PDSN sends state information to the standby PDSN
for synchronization on an as-needed basis. When a PDSN failure occurs, the standby PDSN has the
necessary state information to provide service to all existing sessions. It then takes over as the active
PDSN and services user sessions, thus providing session redundancy. When the previously active PDSN
comes back online, it assumes the role of standby for the now active PDSN, and receives state
information for all existing sessions from the newly active PDSN.
of a PDSN failure on the mobile user experience. The PDSN uses a 1:1 redundancy model, with a
standby present for every active PDSN. The active PDSN sends state information to the standby PDSN
for synchronization on an as-needed basis. When a PDSN failure occurs, the standby PDSN has the
necessary state information to provide service to all existing sessions. It then takes over as the active
PDSN and services user sessions, thus providing session redundancy. When the previously active PDSN
comes back online, it assumes the role of standby for the now active PDSN, and receives state
information for all existing sessions from the newly active PDSN.
Under normal operating conditions, the active and standby PDSN pairs are two separate PDSN images
that have identical configurations. They share one or more HSRP interfaces, which are used by all
external entities to communicate with them. The active PDSN synchronizes session data to the standby
PDSN based on events described below.
that have identical configurations. They share one or more HSRP interfaces, which are used by all
external entities to communicate with them. The active PDSN synchronizes session data to the standby
PDSN based on events described below.
Session Events
When a new user session needs to be established, the PCF first sets up an A10 connection to the active
PDSN using the HSRP address known to the PCF. The MN then sets up a PPP connection with the active
PDSN using the A10 tunnel. Once the call is in a stable state (the PPP session is successful), the active
PDSN then synchronizes relevant state information to the standby PDSN. The standby then duplicates
the actions of the active PDSN with regards to the A10 connection and the PPP session, and awaits
further updates from the active. When any of the other events as listed below occurs, the active PDSN
sends state information to the standby.
PDSN using the HSRP address known to the PCF. The MN then sets up a PPP connection with the active
PDSN using the A10 tunnel. Once the call is in a stable state (the PPP session is successful), the active
PDSN then synchronizes relevant state information to the standby PDSN. The standby then duplicates
the actions of the active PDSN with regards to the A10 connection and the PPP session, and awaits
further updates from the active. When any of the other events as listed below occurs, the active PDSN
sends state information to the standby.
In order to minimize the loss of accounting data in the event of a failover, a periodic accounting update,
with configurable frequency will run on the active PDSN. Every periodic update for a session will trigger
a synchronization sent to the standby PDSN, which will update its accounting data. Only counters and
attributes that undergo a change on the active PDSN are synchronized to the standby periodically.
Information since the last accounting synchronization point will be lost. Also, in order to ensure that the
latest information is correctly conveyed to the billing system, the standby unit will never send out any
accounting records to the AAA server. The records are always sent from the active unit.
with configurable frequency will run on the active PDSN. Every periodic update for a session will trigger
a synchronization sent to the standby PDSN, which will update its accounting data. Only counters and
attributes that undergo a change on the active PDSN are synchronized to the standby periodically.
Information since the last accounting synchronization point will be lost. Also, in order to ensure that the
latest information is correctly conveyed to the billing system, the standby unit will never send out any
accounting records to the AAA server. The records are always sent from the active unit.
Session events that lead to a synchronization are:
•
Call Setup
•
Call teardown
•
Flow setup
•
Flow teardown
•
Dormant-Active transition
•
Handoff
•
A11 Reregistrations
•
Periodic accounting synchronization
•
PPP renegotiation
Active PDSN Failure
In the event that the standby PDSN detects that the active PDSN has failed (using HSRP), it then takes
over as the active PDSN. Since all external entities, including PCFs, AAA servers, and HAs are
configured to communicate with the PDSN pair only using the HSRP addresses, once the standby PDSN
takes over those addresses, they are unable to detect a failure. All stable calls also have their state
synchronized to the standby; therefore the standby is able to start forwarding user traffic once it takes
over as the active PDSN. Since all external entities, including PCFs, AAA servers, and HAs are
configured to communicate with the PDSN pair only using the HSRP addresses, once the standby PDSN
takes over those addresses, they are unable to detect a failure. All stable calls also have their state
synchronized to the standby; therefore the standby is able to start forwarding user traffic once it takes