I have a PIX 501 firewall in my company's network. When customers from outside come inside the company and they trying to
connect to their sites via Cisco VPN client, the PIX 501 stops the VPN connection.
It depends on which version of PIX you are running.
This is assuming that the PIX firewall is passing through IPSec tunnel:
Here is a sample configuration for PIX version 6.x:
Here is sample configuration for PIX version 7.x and above:
Hope that helps.