ASA implicit rule

When using the packet tracer utility in a Cisco ASA, sometimes a reason for a dropped packet is an "Implicit Rule". For example:

asa-5550-edge# packet-tracer input dmz810 tcp 172.29.12.34 587 10.100.20.50 587 detail

Phase: 1
Type: ROUTE-LOOKUP
Subtype: input
Result: ALLOW
Config:
Additional Information:
in   10.100.20.0     255.255.255.0   dmz

Phase: 2
Type: ACCESS-LIST
Subtype:
Result: DROP
Config:
Implicit Rule
Additional Information:
 Forward Flow based lookup yields rule:
 in  id=0x2bf97148, priority=11, domain=permit, deny=true
        hits=79960, user_data=0x5, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
        src ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=0
        dst ip/id=0.0.0.0, mask=0.0.0.0, port=0, tag=0, dscp=0x0
        input_ifc=dmz810, output_ifc=any

Result:
input-interface: dmz810
input-status: up
input-line-status: up
output-interface: dmz
output-status: up
output-line-status: up
Action: drop
Drop-reason: (acl-drop) Flow is denied by configured rule

The Implicit Rule in the ASA packet tracer means that either the implicit deny statement in an Access-List (ACL) is being met, or the rule that states that traffic can only flow from a higher security level interface to a lower security level interface is met. For more information about security levels, take a look at ASA security levels.