How to Optimize an MTU Cable for RoadRunner

By Skip Shelton

The MTU, or Maximum Transmission Unit, is the maximum size, in bytes of data, of packets sent across the network. When the MTU packet-size setting is too high, packets of data are fragmented to send one full packet and one partial packet. Sending fragmented packets decreases the overall efficiency of the connection. Even when Roadrunner cable's broadband speeds are high, inefficient MTU settings degrade the performance of the transfers. Optimize your MTU settings by identifying the optimal byte size of the packets, and adjusting your router settings to the exact MTU size that's optimal for your connection.

Step 1

Connect a PC to the cable modem linked to the Roadrunner service, if not already connected. Open the "command prompt" window available in Microsoft Windows systems. The command prompt is accessed by typing "cmd" in the "Run" functionality from the Start menu.

Step 2

Type or copy the command "ping --f --l 1492" at the prompt. Press Enter. This command sends a packet of data that is 1,492 bytes in size to and displays whether or not the packet is fragmented or lost in the process. This is called a "ping test."

Step 3

Wait for the command to be executed. The ping test responds with a "Reply from..." or a "Packet needs to be fragmented" response. "Reply from..." indicates the packet was not fragmented and the MTU setting may be too high. If the packet response is "Packet needs to be fragmented" the MTU size is too low, requiring fragmenting of data packets.

Step 4

Retype the ping test at the command prompt and replace the "1492" value with one that is higher if the response was "Packet needs to be fragmented," or lower if the response was "Reply from...."

Step 5

Repeat Step 4 until the optimized setting is identified. The optimal value is the lowest possible packet size sent without fragmentation.

Step 6

Calculate the optimal MTU size by adding 28 to the final optimized packet value. Adding 28 to the final number accounts for the IP/ICMP headers sent with packets in a transmission unit. This new number is the optimized MTU for your Roadrunner cable connection.

Step 7

Log in to the router as the administrator. Adjust your router's MTU settings to the number calculated in Step 6. Specific steps to access and change the MTU settings are dependent on the brand, model and firmware installed. Consult the help menu or documentation of the router for specific steps. Save the new settings.