System Operations When Stateful Line Module Switchover Is Enabled

Line modules on E120 and E320 routers comprise three components—forwarding controller (FC), interface controller (IC), and the input/output adapter (IOA). The IC operational image runs in the IC section and the FC operational image runs in the FC section. Each line module can be connected to multiple IOAs, although the IOA does not contain an active control processor and has no operational image running on it. The IOA has a control path to the IC, which is used to configure and preset the hardware on the IOA. The IOA has a data path between the FC and the IOA external connections. After the hardware in the IOA is configured, it transfers data between the FC and the IOA external connections.

Line module high availability or stateful line module switchover behavior refers to providing this functionality of uninterrupted connectivity for subscribers by switching over to the operational image running on the IC that is active on the secondary line module of a pair of modules enabled for high availability. The applications on the secondary line module recover to their original states by reconstructing data from the preserved mirrored storage containers to a stable state. After the secondary line module is equipped to take over the load (services) of the primary line module, the switchover of subscriber traffic occurs on the secondary line module. The secondary line module begins to operate normally, although certain users might experience some subscriber data outage.

The design architecture used for E120 and E320 routers causes the packets that are designated for the SC to be first sent to the IC using a direct memory access (DMA) method. These packets are then forwarded to the SC over the internal 100 Mb Ethernet channel. Similarly, packets that are destined to be transmitted from the router are first sent to the IC over the Ethernet channel and then sent from the IC to the FC using a DMA operation. During the switchover period, until the secondary line module becomes operational, this communication channel from the IC is not active. The amount of time taken for the operational image on the secondary line module to start fully functioning can take up to several seconds. During this period of completion of the switchover process, applications on the SC handle this timeout. Applications that are impacted by this outage are routing protocols or protocols that are time-critical. For example, SRP-based TCP and UDP services are preserved across a switchover, which enables applications, such as Telnet, FTP, SSH, and SNMP, to operate seamlessly.

When the high availability functionality for line modules is active, subscriber sessions are maintained whenever a software or hardware fault is detected on the primary line module. A brief interruption occurs in the subscriber data traffic during the time that the secondary line module takes over as the primary line module. Until the newly functioning active line module receives an acknowledgment from the standby module after a stateful switchover, the updates in routing tables are not sent to the SRP module. Such a phased system of transfer of updates from the active module to the SRP module reduces any out-of-synchronization problems that occur in the transmission of packets between the IC and SC.

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