# Stateful Line Module Switchover for LNS Sessions

In releases in which the stateful line module switchover feature is not available or in scenarios in which this behavior is disabled, a reload of the line module disconnects user sessions and disrupts traffic forwarding through it. In a network in which an E120 or E320 router that contains the Service IOA functions as the LNS device on one side of the L2TP tunnel, the LNS is the logical termination point of a PPP connection that is being tunneled from the remote system by the LAC. A LAC receives packets from a remote client and forwards them to an LNS on a remote network. All the tunneled sessions terminate on the LNS to provide enhanced performance during decapsulation and encapsulation of packets, and fragmentation and reassembly of tunneled packets. If the line module in the LNS that performs the traffic processing encounters a fault, such as a hardware or software error, all the active subscriber sessions are disconnected.

Stateful switchover of LNS sessions avoids subscriber disconnections during the switchover of the line module installed on the LNS device (tunnel server module or ES2-S1 Service IOA on ES2 4G LMs in this case). You can enable high availability for the line module pairs using the mode high-availability slot command in Redundancy Configuration mode. This command enables you to specify the slots in which the tunnel server line modules that you want to be configured as the primary and secondary modules reside. If HA is active between these modules, the secondary module becomes the primary when the assigned primary module fails. The newly active primary module retains all the subscribers that were active and were managed by the previously configured primary module without requiring the subscribers to be reconnected. The failure of the tunnel server module in the LNS device and the switchover from a defective module to a newly active primary module in a seamless, undisrupted manner for subscribers is transparent to the end users.

Line module high availability uses a 1:1 redundancy model to maintain subscriber sessions, and this functionality is supported only on E120 and E320 routers installed with ES2 4G LMs and Service IOAs. This feature is supported only for PPP-based stacks (such as L2TP, PPP, and IP) and not for other applications such as GRE.

The router uses the tunnel server module to increase the performance of packet processing by offloading the decapsulation and reassembly of packets to the tunnel server module. All the L2TP and PPP session data are downloaded to the tunnel server module to assist this operation. When the primary tunnel server module fails, either due to hardware or software error, subscribers are disconnected because of the PPP keepalive expiry mechanism and also because the forwarding path is not maintained. When stateful switchover for LNS sessions is enabled, you can provision another tunnel server module as the secondary module in 1:1 mode. When this feature is enabled, all the required session data is mirrored to the secondary module. Any session data change, such as session creation or deletion, is mirrored from the primary to the secondary module. The previously configured primary module, after it becomes operational, takes over the role of the secondary module.

#### Related Documentation

- Stateful Line Module Switchover Overview
- Preservation of Statistics During Stateful Line Module Switchover
- Application Support for Stateful Line Module Switchover