The speed of Internet access through PPPoE dialup is slow

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The MTU of the PPPoE dialer interface is 1492 bytes. When the length of Layer 3 packets sent from this interface is larger than the MTU and non-fragmentation is configured, packets cannot be sent out and the Internet access speed is low. You can run the tcp adjust-mss value command on the dialer interface to modify the MSS (Max Segment Size) during TCP negotiation so that the packet length is shorter than the MTU of the dialer interface. Then packets can be properly processed and Internet access is smooth.
For example, change the MSS of the dialer interface to 1200.
<Huawei> system-view
[Huawei] interface dialer 0
[Huawei-Dialer0] tcp adjust-mss 1200

Other related questions:
Internet access is slow after a PPPoE dialup link on an AR router is switched over
Modify the TCP MSS value of the device, and set the state of the Dialer interface protocol after PPPoE dialup fails to Down for link switchover. Example: Modify the TCP MSS value. system-view [Huawei]interface Dialer 1 [Huawei-Dialer1]tcp adjust-mss 1200 Example: Set the state of the Dialer interface protocol after PPPoE dialup fails to Down. system-view [Huawei]interface Dialer 1 [Huawei-Dialer1]dialer number 1 autodial The causes are as follows: The TCP MSS value is used to control the maximum transmission load of TCP packets, and it is determined according to the negotiation between the client and server. Generally, it is set to 1460. Plus 20-byte IP header, 20-byte TCP header, and 8-byte PPPoE header, the MTU of TCP packets is 1508 bytes. However, the MTU of network device interfaces is 1500 bytes generally. Since TCP packets cannot be fragmented (according to DF flag in the IP header of the packets defined in the protocol), they will be discarded. As a result, Internet access slows down. Modify the TCP MSS value to enable the router to join in the MSS value negotiation process, and therefore avoid packet discarding during TCP transmission. The Dialer interface is a virtual service bearing interface of devices, and it is invisible on the device panel. This interface is snooping up, which causes switchover failures if the router attempts to switch over to a normal physical interface after dialup fails. Run the dialer number 1 autodial command to ensure that the state of the interface is Down after dialup fails and the route to the Dialer interface is canceled. Then, link switchover can be implemented.

When the device functions as a PPPoE client, why is the Internet access speed of users on the downstream LAN slow
Run the tcp adjust-mss value command in the dialer interface view to set the MSS of TCP packets on the interface. The recommended value of tcp adjust-mss is 1200 for PPPoE applications. The MTU of the PPPoE-enabled interface is 1492 bytes. When the MTU of outgoing Layer 3 packets on this interface is greater than 1492 bytes and the packets are configured as non-fragmented, packets cannot be sent out. The Internet access speed is slow. You can run the tcp adjust-mss value command on the dialer interface to change the Max Segment Size (MSS) value so that the packet length is smaller than the dialer interface. The packets then can be processed and this problem is solved.

AR router troubleshooting guide: An AR router cannot access the Internet through PPPoE dial-up connections
For the problem that an AR router cannot access the Internet through PPPoE dial-up connections, see the troubleshooting guide. For details, access the URL in the right column.

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