NAT acceleration is a feature that is available on Asus branded routers. It is also known as hardware acceleration in some cases by other branded routers. It can also be named as CTF (Cut-Through Forwarding) and FA (Flow Accelerator) This is a general guide that can potentially help you troubleshoot the issues that you may have with your wired or wireless network.

Please note that the language and information written here is intended for the average basic user. We will not go into detail on how and why NAT acceleration work, but rather some observations and guidelines on how you can make the best out of this feature.

What is NAT Acceleration

NAT Acceleration is a set of special designed software rules with hardware features created to speed up internet connections. They are usually required for peak speed when you have incoming internet speed above certain level. You typically do not need it for internet speed below 100 mb/sec.

The NAT acceleration or hardware acceleration option are usually found under the LAN settings for your router. At the time of writing, there are two levels of the NAT acceleration, some models of routers allow you to set the the level. Below is what NAT acceleration actually does and how it accelerates your speed.

Level 1: CTF (Cut Through Forwarding): Software optimization technique to accelerate NAT traffic.
You may need this option if your internet provider is offer you speed above 100 mb /sec. You generally need this option if you want to achieve peak speeds especially for speed above 200 mb/sec.

CTF or Cut-Through Forwarding is achieved by the router starting to send out transmission frames as soon as it receives its destination. However, the router relies on the end device to tell it whether the data is corrupted for resend. This restriction can cause problems with a few common home uses.

When you have CTF or level 1 NAT Acceleration disabled, the router will “store” the entire frame before sending it out to its destination. This holding period may require more router’s CPU use.

Level 2: Level 1 (CTF) + FA (Flow Accelerator): Hardware NAT acceleration mechanism design for accelerating wired DHCP and Static IP connections.
You will need Flow Accelerator option to fully take advantage of internet provider’s Gigabit service is offered.

Flow Accelerator is achieved through special hardware related designs.

When You Should Use or Enable NAT Acceleration

You should typically enable NAT acceleration or Cut-Through Forwarding when you have internet speed above 100 mb/sec. You will typically only see a difference for speed above 200 mb/s.

You should generally set the option as Auto or ON, unless you need to use features that directly conflict with NAT acceleration.

Potential NAT Acceleration Issues and Conflicts

NAT Acceleration, specifically the CTF Cut Through Forwarding portion can conflict with a few other common features. You should consider disable NAT acceleration or turn it to off if you experience any of the problems below.

QoS Restrictions
QoS or Quality of Service features come in direct conflict with the implementation of NAT Acceleration or Cut-Through Forwarding. Quality of Service options let your router “prioritize” the type of traffic that will be transmitted first. However, it requires to router to actually hold some data and make that choice.

NAT acceleration removes that possibility by starting the transmission as soon as it receives the destination. Below is a quick list of NAT acceleration and QoS level allowed.

No HW or NAT Acceleration = Traditional QoS
Level 1 (CTF) = Adaptive QoS Only
Level 2 (CTF + FA) = No QoS Allowed

Port Forwarding Not Compatible
NAT Acceleration is reported to not be compatible with port forwarding feature. This means that when you have the NAT Acceleration, you will typically unable to host gaming sessions from your home. So you will not be able to host any game servers such as Minecraft, MMO, or First person shooters since players will not be able to connect to it.

IP Traffic Monitoring and Parental Control
Traffic Monitoring and Parental Control require the router to examine the type of traffic. This means that it is not compatible with the CTF feature as CTF transmits the data almost as soon as it receives the proper destination.

Issues with Mirroring of Streaming Devices
In some cases, CTF can cause choppiness in the mirroring of stream devices such as Apple TV, Chromecast, and VoIP. This is because the data transmitted may be corrupted and routers “resend” those data. However, your end point devices show those corrupted data since they are designed to be latency sensitive.

NAT Acceleration ON or OFF Conclusion and Recommendation

You should generally leave the option as Auto for the NAT Acceleration in Asus Router. Since it does improve the peak connection speed of your home network’s internet service.

However, you should consider turning it off if you require any of the following features: Port Forwarding, QoS, IP Traffic Monitor, and Parental Control.

If your mirroring of video streaming devices have errors often and appear to be choppy, consider testing the effect of having an “disabled” NAT Acceleration or CTF.

To get the peak speed offered by your internet provider while having the features you need, consider combining two different routers with different settings to create your suitable network.


NAT Acceleration On or Off — 13 Comments

  1. Port forwarding is supported for hardware acceleration… well not really supported but they can peacefully coexist i guess.

    My ASUS router has port forwarding enabled and CTF+FA enabled

  2. Definitely enable this if you have a fast connection! My measured download speeds dropped from 125 Mbps to 20 Mbps when I disabled NAT acceleration on an ASUS RT-87U router!

  3. CTF must be disabled if you have a Cisco IP phone. Took me almost two months to figure out why my Cisco phone cannot connect to the corporate Cisco server. CTF On was the cause.

  4. Enabling this option on my ASUS RT-AC68U fixed my throughput problem, and port forwarding still functions. Right now I have AT&T Gigapower service, and I get symmetrical >900 Mbps when I’m plugged directly into the AT&T provided PACE 5268AC modem/router, but I’m a nerd and like the Tomato firmware on my ASUS. The only problem is that I’m forced to use the PACE because it’s the only fiber modem available for AT&T’s service. So, because the PACE doesn’t have a bridge mode, I’m forced to use the DMZ option. So with the WAN port of my ASUS plugged into one of the ethernet ports of the PACE, I had the PACE put the ASUS into a DMZ; allowing the WAN of the ASUS to be assigned the same external IP address as the PACE. Everything functioned perfectly, except the throughput was an abysmal ~250Mbps (I know, cry me a river right?) as a consequence of the double NAT that occurs with this configuration. After some searching, it turns out that enabling CTF was all I needed to do to resolve my issue. Thanks for the info!

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