What it does

By default an ASA does stateful inspection of all traffic. It must see the entire conversation to be able to set up the connection and pass the traffic. If traffic is asymmetric, such that the ASA only sees traffic in one direction, the packets will not be passed. Additionally, even if the traffic is symmetric and a new connection is established, subsequent fast path packets will be inspected for things such as TCP sequence number randomization, TCP normalization, and other checks.

In order to address specific asymmetric traffic issues, or just targeted performance problems, the tcp-state-bypass feature may be used to “skip” or bypass these checks. I’ve jokingly referred to this tool as a way to “make a firewall not be a firewall.”

How to do it

First define an access list to match the interesting traffic. Create a class-map that uses that ACL, and apply it to the global policy-map (or that of a specific interface). Here you can set the connection options. I’d recommend you also set some limits on this traffic so that it doesn’t consume excessive resources that the inspection would otherwise prevent.

access-list tcp_bypass permit ip any
access-list tcp_bypass permit ip any
class-map tcp_bypass
 description TCP traffic test network
 match access-list tcp_bypass
policy-map global_policy
 class tcp_bypass
  set connection per-client-max 10000
  set connection timeout idle 0:00:15
  set connection advanced-options tcp-state-bypass

Cisco has a nice document on this feature.


Note that this will only be used for “new” traffic. If you are doing an A/B test, make sure you clear the existing connection(s) after applying the policy.

Don’t use this for everything. It is a targeted tool for solving specific network topology issues. Make sure you understand where to tactically apply this tool.


Note that the connection table will show the b flag for any connections using this feature.

lab-5585-1# conf t
lab-5585-1(config)# access-list tcp_bypass permit ip any
lab-5585-1(config)# access-list tcp_bypass permit ip any
lab-5585-1(config)# class-map tcp_bypass
lab-5585-1(config-cmap)#  description TCP traffic test network
lab-5585-1(config-cmap)#  match access-list tcp_bypass
lab-5585-1(config-cmap)# policy-map global_policy
lab-5585-1(config-pmap)#  class tcp_bypass
lab-5585-1(config-pmap-c)#   set connection per-client-max 10000
lab-5585-1(config-pmap-c)#   set connection timeout idle 0:00:15
lab-5585-1(config-pmap-c)#   set connection advanced-options tcp-state-bypass
lab-5585-1(config-pmap-c)#  exit
lab-5585-1(config-pmap)# exit
lab-5585-1(config)# cle conn address
1 connection(s) deleted.
lab-5585-1(config)# show conn address
47 in use, 715 most used
TCP tcp tcp, idle 0:01:10, bytes 293223484, flags b

Use cases

Here are a couple of real-world situations where I’ve used this feature:

  • For a network topology where the firewalls were terminating IPsec tunnels over the Internet to AWS and Azure as a backup for Direct Connect and ExpressRoute circuits. In this topology, these were the only devices available with the crypto processing to handle the traffic. But, in certain failure modes there was a potential for asymmetric traffic. Specifically targeting the internal VPC/VNet networks with this feature solved the issue.

  • For a video editing platform that had network performance issues. The storage backend was served from a SMB fileserver, but it had suboptimal performance though the firewall. With an ACL matching the client(s) and server, throughput as measured by the client was increased by 46%. This was a simple targeted workaround to help with performance until the network could be upgraded.