Manual

Cisco Packet Data Serving Node (PDSN) Release 2.0
Redundancy and Load Balancing
47
12.3(11)T
Load Balancing
A controller maintains load information for all members in the cluster in order to perform PDSN Cluster
Member selection. This load information is transferred from the members to the controller under the
following conditions:
• at periodic intervals.
• when a session is established or dismantled in a member. In this case, the periodic timer is restarted.
• requested from the members by the controller.
The session and member records are synchronized between the active and standby controllers as needed.
Since both active and standby controller maintain session and load information for all the members of
that cluster, failure of a controller does not result in the loss of any session or load information.
Intelligent PDSN Selection and Load Balancing (Peer-to-Peer)
The Cisco Intelligent PDSN Selection (Peer-to-Peer) feature allows you to group a number of Cisco
PDSNs into clusters that can exchange session information for performance and load-balancing
purposes. Each Cisco PDSN in a group maintains a table that contains information for the entire group. Using
PDSN clusters, minimizes inter-PDSN handoff, provides intelligent load-balancing, and ensures high
availability.
To distribute session information, each PDSN sends a broadcast to the Mobile IP multicast address when
a session is created or ended. The IP address of the originating PDSN and the MSID are encoded in the
Mobile IP messages. Each PDSN in a group updates its session table upon receiving the broadcast.
When a session request is received from the PCF by the Cisco PDSN, the PDSN checks its own session
list for an existing session, and also checks session lists within its PDSN group. If it determines that a
session exists with another PDSN, it redirects the PCF to that PDSN. This redirection helps to avoid
dropping the IP address and, thereby, avoids dropping any existing communication.
If the session does not exist with any other PDSN, the receiving PDSN uses a load-balancing mechanism
to determine the appropriate PDSN to use for session establishment. With load balancing, the receiving
PDSN looks for the least utilized PDSN in the entire cluster. If the number of active PPP links on that
PDSN is some factor less than the number of PPP links on the receiving PDSN, the request will be
forwarded. The factor for determining whether the PPP link is forwarded is calculated as a percentage
(number of active PPP links vs. total number of possible PPP links).
Load Balancing
For a new packet data session, one PDSN may direct a connection request to another less “loaded” PDSN
within the cluster by proposing the address of that PDSN to the PCF. Such redirection of A10 connection
requests is performed among lesser loaded PDSNs in a round-robin manner. In PDSN software releases
prior to Release 1.1, the load balancing threshold was implemented in terms of a session count
differential. Starting in Release 1.1, the threshold is configured in terms of a load factor—the ratio of
number of sessions supported and total session capacity of the PDSN. In future releases, other factors
(such as QoS, session throughput considerations, CPU load, memory utilization) might also be
considered as parameters used to determine of load factor of a PDSN.