When the device functions as a PPPoE client, why is the Internet access speed of users on the downstream LAN slow

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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.

Other related questions:
The speed of Internet access through PPPoE dialup is slow
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

When an AR router is used as the PPPoE client, the Internet access rate of the LAN users connected to the device is low
In the dialer interface view, run the tcp adjust -mss value command to set the maximum TCP segment size of the interface. In scenarios of PPPoE application, it is recommended that the value used in the tcp adjust -mss command should be 1200. The maximum transmission unit (MTU) of the PPPoE interface is 1492 bytes. If the packets which are forwarded in Layer 2 and transmitted upward through the interface are larger than the MTU size and cannot be fragmented as configured, the packets cannot be sent, which results in the low Internet access rate of users. In this case, you can run the tcp adjust -mss command in the dialer interface view to modify the maximum segment size (MSS) value determined in the TCP negotiation phase, ensuring that the packet size is smaller than the MTU size of the dialer interface so that the packets can be processed properly and improving the Internet access rate.

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.

What should I do if I do not know the PPP authentication mode of the PPPoE server when the device functions as a PPPoE client
PPPoE authentication will fail if inconsistent PPP authentication modes are used on the PPPoE server and client. If you do not know the PPPoE authentication mode provided by the carrier, you are advised to configure both CHAP and PAP authentication modes on the device. The device will dynamically determine the authentication mode based on packets exchanged with the PPPoE server. Run the ppp chap user username command in the dialer interface view to configure a user name for CHAP authentication. Run the ppp chap password cipher password command in the dialer interface view to configure a password for CHAP authentication. Run the ppp pap local-user username password cipher password command in the dialer interface view to configure a user name and a password for PAP authentication.

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