Manual

Cisco Packet Data Serving Node (PDSN) Release 2.0

Redundancy and Load Balancing

12.3(11)T

• The actual PDSN selection and load-balancing procedures are similar to the R1.1 implementation;

however, different record tables are used.

• Auto-configuration of a new PDSN controller added to the cluster—The new controller must be

configured as such, and must be configured as a member of the HSRP group of routers. As a

consequence, the new controller (standby) automatically downloads member and session records

from the active controller. The active controller updates the standby as needed, so that records are

synchronized.

• Auto-configuration of the controllers when a new member is added to the cluster—The new member

registers with the active controller, which updates the standby controller.

• Redundancy—All controllers in the cluster maintain session and load information for all members.

This provides redundancy for availability, and, in case of a controller failure, session and

load-balancing information is not lost.

Redundancy

Cluster redundancy is based on the premise that only one PDSN might fail at any given time. Two

controllers are configured as an HSRP group: One controller is active, the other standby. Controllers

have redundancy and members have load sharing.

Load Sharing

Cluster member loadsharing is an N+1 scheme. If a member fails, the established sessions will be lost,

but the overall group capacity allows sessions to be re-established with the other group members.

Additionally, redundancy is also enhanced because cluster members no longer have to be network

neighbors.

Controllers exchange information over an ethernet link. Controllers and members exchange information

over a unicast interface link where members address messages to the HSRP group address of the

controllers. The members in a PDSN cluster do not need to be network neighbors; they can be attached

anywhere in the IP network.

Adding an additional controller to a cluster is simplified by auto-configuration of the controller in the

cluster. This is possible by configuring the additional controller for HSRP. The newly-added controller

will automatically synchronize with the active controller. Similarly, when a new member is added to the

cluster, auto-configuration for the member occurs in all cluster controllers.

PDSN Cluster Member Selection

Selection of a cluster member by the controller is based on a load factor. Load factor is a computed value

by session load and CPU load on a member. The controller attempts to assign sessions to a member that

has smallest load factor so that data connections are evenly distributed over members in the cluster as

much as possible.

If an A11 Registration Request is received indicating a handoff, a member that is already serving the

session is selected by the controller.