This may help:
Caveat "CSCsj30582"
http://www.cisco.com/en/US/docs/ios/12_4t/release/notes/124TCAVS.html
Symptoms: A Cisco IOS router that is running ZPF (Zone-based Policy Firewall) intermittently drops ESP packets even when it is configured to pass them. This causes traffic over an IPsec VPN tunnel through this router to fail intermittently, although the tunnel is up and phase 1 (isakmp) and phase 2 (ipsec) SAs have been established. If the router is configured to log dropped packets, it will log a %FW-6-DROP_PKT syslog message for these packets.
Conditions: This symptom is observed on a Cisco IOS router that is enabled with ZPF (Zone-based Policy Firewall) and that is configured to pass the ESP traffic based on a "match access-group" policy, where the access list has entries to permit the ESP traffic specifically from one host to another.
For example:
class-map type inspect match-any cm-esp match access-group 100
policy-map type inspect in2out class type inspect cm-esp pass
access-list 100 permit esp host 10.0.0.2 host 10.1.1.2 access-list 100 permit esp host 10.1.1.2 host 10.0.0.2
Workaround: Configure the access list so that the source is "any", for example:
access-list 100 permit esp any host 10.1.1.2 access-list 100 permit esp any host 10.0.0.2
First Alternate Workaround: Use the classic Cisco IOS firewall instead of ZPF; that is, use "ip inspect".
Further Problem Description: If an explicit deny rule is added to the above example, for example:
access-list 100 permit esp host 10.0.0.2 host 10.1.1.2 access-list 100 permit esp host 10.1.1.2 host 10.0.0.2 access-list 100 deny esp any any
Then the show access-list command will indicate that the dropped packets are hitting the deny rule, although they should match one of the permit rules:
Router# show access-lists 100
Extended IP access list 100 10 permit esp host 10.0.0.2 host 10.1.1.2 (999 matches) 20 permit esp host 10.1.1.2 host 10.0.0.2 (999 matches) 30 deny ip any any (1 match)
