Core issue
If the debug crypto isakmp command shows the "reserve not zero on payload 5!" error message, there is a pre-shared keys mismatch.
To confirm use command "show crypto isakmp sa". if the output displays "MM_Key_exchange" ,it mean's that phase 1 is getting stuck at key exchange. Reasons might be because of mismatch in preshare keys or wrong ip address for peer in cryptomap entry.
Resolution
For LAN-TO-LAN VPN using pre-shared keys, check the key in this command:
isakmp key ******** address 172.16.172.34 255.255.255.255 no-xauth no-config-mode
Note: The pre-shared key is designated by asterisks (***).
If the keys do not match, issue this command to remove the line:
no isakmp key ******** address 172.16.172.34 255.255.255.255 no-xauth no-config-mode
Re-issue the command with the correct pre-shared key.
