Controlling the Handling of L2TP Hello Packets in Fast Path

When the E series router acting as an LNS receives L2TP hello requests from the L2TP access concentrator (LAC), the LNS processes them by using an active switch route processor (SRP) module and returns L2TP hello responses to the LAC. When a unified ISSU operation is performed on the LNS deployed in an environment with more than 16,000 L2TP tunnels, the L2TP hello request may be timed out and lost for the ES2 10G UPLINK LM, ES2 10G LM, and ES2 10G ADV LM modules as these modules take more than 80 seconds to reload. This causes the L2TP tunnels established over ES2 10G UPLINK LM, ES2 10G LM, and ES2 10G ADV LM modules to be terminated.

To avoid such unexpected termination of L2TP tunnels, you can enable the handling of L2TP hello packets in fast path by using the l2tp fastpath-hello-processing command in the LNS. When the handling of L2TP hello packets in fast path is enabled, following steps are performed in the LNS during the unified ISSU operation:

  1. Active SRP module informs the ES2 10G UPLINK LM, ES2 10G LM, and ES2 10G ADV LM modules acting as access line modules to divert L2TP hello requests to ES2 4G LMs that act as Tunnel Service line modules (TSMs). The SRP module passes the details about all active TSMs to the access line modules.
  2. Access line modules forward the hello requests received from the LAC to the TSMs in a round robin manner for load balancing.
  3. TSMs process the received hello requests in their forwarding layer and forward the hello responses to the LAC directly from their forwarding layer.

Once the unified ISSU operation is completed, the active SRP module processes the L2TP hello requests.

Note:

  • This feature is supported only on the E120 or E320 router with ES2 10G UPLINK LM, ES2 10G LM, and ES2 10G ADV LM modules acting as access line modules. This feature is not supported for ES2 10G ADV LM acting as TSM.
  • If an ES2 4G LM acting as TSM goes down during unified ISSU, the round robin mechanism is not modified dynamically.
  • The L2TP hello request is processed only by the ES2 4G LM acting as shared tunnel server or the ES2 4G LM with ES2-S1 SERVICE IOA.

To enable the handling of L2TP hello packets in fast path:

Caution: This feature may cause the L2TP tunnel hello packet to be dropped during peak tunneled data traffic conditions.

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