Controlling the Transmission of L2TP Hello Messages to L2TP Peers in Control Channels

In JunosE implementation, when a control channel is up, the E Series router that is acting as an L2TP access concentrator (LAC) or L2TP network server (LNS) transmits a Hello packet by default on a lapse of 60 seconds since the transmission or reception of the last control packet in the control channel. The Layer 2 Tunneling Protocol (L2TP) peer of the LAC or LNS is expected to acknowledge the reception of the Hello message with a Zero Length Body (ZLB) message. The LAC or LNS terminates the control channel if the Hello packet is not acknowledged. However, there could be some implementations where the peer is incapable of responding to the Hello messages with ZLBs, which causes the LAC or LNS to unintentionally terminate the control channel. In such cases, you can use the l2tp disable tunnel-hello command to disable the transmission of Hello messages from the LAC or LNS.

Note: When the transmission of Hello messages is disabled, the following effects are expected:

  • If a Point-to-Point Protocol (PPP) keepalive message is also disabled and the L2TP peer goes out of reach, the control connection cannot be closed at the LAC or LNS end because no explicit control message is received by the LAC or LNS from its peer. This causes the connection to use the LAC or LNS resources unnecessarily. If the maximum number of connections were in use before the peer went out of reach, the peer might not be able to reconnect after it comes up because all the L2TP resources are used up by the previous connections. The resultant behavior depends on the peer L2TP implementation.
  • JunosE Silent Failover (SF)–based control connection recovery is dependent on the transmission of Hello messages to synchronize sequence numbers with the L2TP peer after failover. With the transmission of Hello messages disabled, SF-based recovery cannot be performed.

To avoid the transmission of Hello messages to the L2TP peer:

Use the no version to restore the transmission of Hello messages.

Note: Execution of the no l2tp disable tunnel-hello command resumes the transmission of Hello messages in the tunnel and results in the termination of the control connection if the L2TP peer did not support the transmission of Hello messages.

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