Capturing traffic through VPN tunnel

Hi all,

we have a VPN tunnel between a customers and our firewall (ASA 5510). Now we would like to start a capture to know if traffic is coming through:

access-list acl_cust permit ip CUSTOMERIP OURIP
access-list acl_cust permit icmp CUSTOMERIP OURIP
capture cap_cust type isakmp access-list acl_cust interface outside

but this doesn't give any data in the capture. We also tried without the 'type isakmp' but still, no data is recorded..

What are we doing wrong?

Thanks!