1.Analyzed the configuration and confirmed that everything is fine. Trying to set up one test environment to verify it.
2.From the debugging information, I found the SSH client customer used is OpenSSH_6.9
Jul 17 2015 22:28:40.830.1+01:00 CPE-POLIZIA_LOCALE SSH/7/VERSION_RECEIVE:Version information received on VTY 1, version string:SSH-2.0-OpenSSH_6.9.
3.Using Cygwin linux environment to built ssh client. after that, test SSH service and faced same issue with cusotmer. There is big delay when S5700 campute DH shared key.
4.Debugging this case with RnD. And we found OpenSSH tool is using long bit key with 2068/4097. Since S5700 is low-end switch, the performance is not very strong and it takes long time to compute the shared key.
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<8192<8192) sent
debug1: got SSH2_MSG_KEX_DH_GEX_GROUP
debug2: bits set: 2068/4096
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
5.Based on above analysis, we provide one solution: Change the key algorithm on SSH client.
Add below command in file /root/.ssh/config or /etc/ssh/ssh_config
After that, we tested it and there is only 2~3 seconds delay. The problem is solved.