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AR Router Troubleshooting Guide

This Product Documentation provides guidance for maintaining AR Enterprise Router, covering common information collection and fault diagnostic commands, typical fault troubleshooting guide, and troubleshooting.
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Why Private Network Communication Is Delayed After the L2TP Dialup Is Successful?

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.

If a physical interface performs packet fragmentation again after the packet is fragmented on the corresponding VT interface, device performance degrades. To prevent this case, you are advised to set the MTU value of the VT interface to the range of 1400 to 1450.

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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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Updated: 2019-05-10

Document ID: EDOC1000079719

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