Disable ASA TCP State tracking (make my ASA a router)

Recently had to use this (useful?) feature to help a customer with an asymmetric traffic flow via an ASA. The ASA would only see the out bound traffic from a host with the return traffic going via an alternative path not visible to the ASA.This simple diagram is an example of the flow and with default ASA configs in place the TCP state tracking would block tcp traffic (ICMP ect worked fine). Asym2

Green is outbound via ASA, Orange is from ASA to destination host and blue is return traffic back from destination not going via ASA. To work around this Cisco gives us this little option we can disable with a policy map TCP State Bypass Feature

Doing this we can tell the ASA not to track the TCP state between hosts we define as an ACL as usual. This probably isnt a feature to be deployed without an understanding of how it may affect other functionality with the ASA.

To configure.

Define ACL to match traffic to exempt from state tracking:

access-list tcp_state_bypass extended permit tcp
access-list tcp_state_bypass extended permit tcp

Define class map that matches on our ACL:

class-map tcp_bypass_class
 description "Helping with asymmetric flows"
 match access-list tcp_state_bypass

Define a policy map that does something with our class map:

policy-map tcp_bypass_policy
 class tcp_bypass_class
  set connection advanced-options tcp-state-bypass

Finally we apply this to our interfaces to make the policy live:

service-policy tcp_bypass_policy interface IN1
service-policy tcp_bypass_policy interface IN2

Now our traffic can pass through asymmetrically without the ASA dropping it AKA being a router? 🙂 We all secretly know tracking state is a waste of time anyway? 😛



  • Does tcp_state_bypass actually disable connection tracking or merely disable the requirement for connection matching of non-SYN packets? My previous experiments determined it to be the latter in which case resources are not saved by the use of this feature.

    I’m not aware of any feature of the ASA that allow you to permit connections without creating a tracking entry which is a shame as this makes the ASA vulnerable to state exhaustion attacks using things such as DNS and NTP traffic to legitimate servers.

Leave a Reply