1. Check whether the link between the PC and server is normal.
Ping the IP address and domain name of the server on the PC. For example, if the IP address of the server is 10.1.1.1, run the ping 10.1.1.1 command. If the ping operation succeeds, use the tracert tool to trace the path. If the path is normal, the link between the PC and server is normal.
2. Check the device configuration, neighbor status, and routing table. The result shows that they are normal.
3. Check whether the MTU value is set properly.
Ping the IP address of the server on the PC and set the DF bit to 0 (indicating that packets are not fragmented). After the ping operation is performed several times, the packets with a maximum of 1468 bytes can be pinged successfully. The MTU value may be improper. On the original network where packets are not fragmented, the maximum number of bytes in packets that can be pinged successfully is 1472. After MPLS VPN is deployed, a 4-byte MPLS label is added to packets. Therefore, the maximum number of bytes is 1468.
On PEs at both ends, increase the interface MTU and MPLS MTU values to 1520. The packets with a maximum of 1468 bytes can be pinged successfully. The customer consulted the carrier and learned that the MTU value of the carrier's transmission device is limited.
4. Check whether the TCP MSS value is proper.
Through packet obtaining and analysis, the TCP Previous segment lost alarm is generated, indicating that some TCP packets are lost. The total packet length (MSS + TCP header + IP header) is greater than the MTU value of the link. Generally, a TCP connection does not allow packets to be fragmented (the MSS value is negotiated), and the DF bit is set. However, after the enterprise deploys MPLS VPN, MPLS labels are added to data packets. As a result, the MSS value plus all the header lengths exceed the MTU value of the transmission link, causing packet loss. Then HTTP applications fail to be accessed.
5. Change the TCP MSS value.
Run the tcp adjust-mss 1452 command on the interfaces connecting PEs and CEs to decrease the TCP MSS value. After the TCP MSS value is changed, the PC can access server web pages through HTTP.