Hello everyone,
I believe you can learn how to deal with analysis for user cannot get internet, but it can be pinging from my case
【Problem Description】
Customer feedback that pc can’t get some web page,But it can be pinging.
The topology like bellow:
PC---Switch 5700----- ISP L2 network ------NE40E------AR------Internet
【Problem Analysis】
Step 1 Do ping test to PC in NE40E, 1442 length packet is ok, but 1443 length is fail. Ping to switch also has this problem.
Between switch and NE40E, there exist a L2 network, it means packet length exceed 1443 will drop in the L2 network.
Step 2 Capture packets in NE40E and Switch when visit website baidu.
In NE40E there exist packets 1514 from server send to PC, and it will be drop in the L2 network, cause many retransmission.
Step 3 Modify MTU in NE40E and Switch.
When change MTU to 1400 in the sub-interface which connect to switch. Ping 1500 length packet to switch is success, but ping to PC remain fail.
Ping 1500 length to PC, and capture packets in Switch, find switch send the packets to PC with two fragments, but PC return with 1518 length packet.
Step 4 So we confirm that if the packets sizes more than 1443 length, isp l2 link will drop it, and switch will not fragmentation or reorganization when the packets was through the switch.
【Root Cause】
ISP l2 link drop the packets which is more than 1443 length
【Solution Description】
Change the MTU from the PC:
This is what I want to share with you, thank you!