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Title: Determining and setting up the correct MTU size

Article ID: 386


The Maximum Transmission Unit (MTU) feature of your Linksys router is an advanced configuration that allows you to determine the largest data size permitted on your connection.  Generally, if your MTU is too large for the connection, your computer will experience packet loss or dropping Internet connection.

This article will help you determine and set up the correct MTU size.  To know how, follow the steps below:

i. Determining the correct MTU size
ii. Setting up the correct MTU size

Determining the correct MTU size

To determine the correct MTU size for your network, you’ll have to do a specific ping test on the destination you’re trying to go to like using another computer or a web address.

For Windows computers, use the following command for the ping test:  ping [url / local server or IP address] –f –l xxxx

• For example: “ping www.yahoo.com –f –l 1472”

For Mac® computers, use the following command for the ping test:  ping [url / local server or IP address] –d –s xxxx

• For example: “ping www.yahoo.com –d –s 1472”

In determining your network’s MTU size, you’ll need to know the following information to understand the concept of using certain commands through the command prompt.

Ping:  a command to determine the connectivity between your computer and a particular address (within the Local network or the Internet).

URL or local address:  the web address or the ip address of the server you’re trying to check for speed connectivity.

-f:  a command to make sure that when you ping a certain address, it will not fragment the packet sent or received.

-l:  a command commonly known as a packet size switch.  This is the best command to help you determine the best MTU size for your network.

xxxx:  the packet size (value) which you will test.  You can start from 1472 until you can reach the exact packet size which will not result to the prompt “Packet needs to be fragmented but DF set.”  You may decrease the value by 10.

Here are the results that you may get after doing the ping test:

Four (4) replies received:  This means that the packet size entered is either within or the actual MTU size used within your network.

Destination net unreachable:  This means that there was no path or route to the destination or the address.

Request Timed Out:  This means that within the default wait time period (1 second), there was no response.

Packet needs to be fragmented but DF set:  This means that the packet size you entered is too high for your MTU value.

Bad parameter –f:  This means that you have typed the command incorrectly.

To do the ping test, follow the steps below according to the operating system that you are using:

Windows 8
Windows Vista / Windows 7
Windows XP 
Mac OS X®

NOTE:  The MTU of Linksys routers is 1500.

Windows 8

Step 1:
In the Charms bar, click the Search charm.

Step 2:
Select Command Prompt.

Step 3:
In the command prompt window, perform a special ping.  Enter “ping [URL] [-f] [-l] [MTU value]” and press [Enter].  Take note of the results.

NOTE:  In the image below, 1472 is the start point and the "yahoo.com" was used as the destination URL.

If you did not get a response, decrease the packet size by 10.  Enter “ping yahoo.com –f –l 1462” and press [Enter] Take note of the results.

Continue decreasing the packet size by 10 until you get the correct reply.

NOTE:  Once you get a reply, take 1452 and add 28 to it for you to get the correct MTU size.  In the example given above, 1452 is the proper value and the correct MTU size would be 1480 for the network you’re working with.

Windows Vista / Windows 7

Step 1:
Click the Pearl  button and enter “cmd” in the search field.  Then, press [Enter].

 

Step 2:
Perform a special ping.  Enter “ping [URL] [-f] [-l] [MTU value]” and press [Enter].  Take note of the results.

NOTE:  In the image below, 1472 is the start point and the "yahoo.com" was used as the destination URL.

 

If you did not get a response, decrease the packet size by 10.  Enter “ping yahoo.com –f –l 1462” and press [Enter].  Take note of the results.

 

Continue decreasing the packet size by 10 until you get the correct reply.

 

NOTE:  Once you get a reply, take 1452 and add 28 to it for you to get the correct MTU size.  In the example given above, 1452 is the proper value and the correct MTU size would be 1480 for the network you’re working with.

Windows XP

Step 1:
Click Start > Run.

 

Step 2:
Enter “cmd” in the Open field.  Then, click OK.

 

Step 3:
Perform a special ping.  Enter “ping [URL] [-f] [-l] [MTU value]” and press [Enter].  Take note of the results.

NOTE:  In the image below, 1472 is the start point and the "yahoo.com" was used as the destination URL.

 

If you did not get a response, decrease the packet size by 10.  Enter “ping yahoo.com –f –l 1462” and press [Enter].  Take note of the results

 

Continue decreasing the packet size by 10 until you get the correct reply.

 

NOTE:  Once you get a reply, take 1452 and add 28 to it for you to get the correct MTU size.  In the example given above, 1452 is the proper value and the correct MTU size would be 1480 for the network you’re working with.

Mac OS X®

Step 1:
Click the Spotlight  button at the upper-right corner of your screen.

Step 2:
Enter “Terminal” in the Spotlight field and select Terminal.

 

Step 3: 
Once the Terminal screen opens, enter “ping –d –s 1472 www.yahoo.com”.

NOTE:  You can change the host or website name to any other URL that you are trying to ping.

 

QUICK TIP:  Always start at 1472 and work your way down by 10 each time until you get a reply.

Step 4:
Press the [Return] key and take note of the results.

NOTE:  Since you got a reply, take 1472 and add 28 to it for you to get the correct MTU size.  In the example given above, 1472 is the proper value and the size would be 1480 for the network you’re working with.

Setting up the correct MTU size

After getting the correct MTU size, do the following:

Step 1:
Open a web browser like Internet Explorer or Mozilla Firefox.

Step 2:
Access the router’s web-based setup page.  Open any web browser (Internet Explorer, Mozilla Firefox, or Safari) and enter “192.168.1.1” on the Address bar then press [Enter].

 

Step 3:
You will be then asked for your log-in details.  Leave the User name field blank and enter “admin” on the Password field then click OK.

Step 4:
On the web-based setup page under Basic Setup, select Manual in the MTU dropdown box.

 

Step 5:
Enter the correct MTU size in the Size field.

 

NOTE:  The correct MTU size of 1480 is used as an example.

Step 6:
Click .

Other things that you can do

In case you are having network speed issues and changing the MTU settings did not help to resolve them, you can try the following:

 

 

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