Why private network communication is delayed after the L2TP dialup is successful

L2TP encapsulation increases the length of IP packets. When the packet length exceeds the link MTU, the sender needs to fragment the packets, which are then reassembled by the receiver. Fragmentation and reassembly consume CPU resources. When many packets need to be fragmented, CPU resources will become insufficient, lowering the access speed and causing packet loss.

Therefore, the MTU value on the VT interface must be less than or equal to the encapsulation header length of L2TP packets (42 bytes with a serial number, and 38 bytes otherwise) subtracted from the MTU value on the physical outbound interface (1500 bytes by default). For example, when the MTU value on the physical outbound interface is 1500 bytes by default, and the encapsulation header length of L2TP packets is 42 bytes, the value must be less than or equal to 1458 bytes.

When the TCP packets encapsulated by L2TP exceed the link MTU, the ping operation to the private network will be delayed. In this case, web pages may not be displayed normally or remote logins may fail. You are advised to adjust the TCP MSS value on the VT interface to ensure that the length of TCP packets encapsulated by L2TP is less than or equal to the link MTU.

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