MTU size

Posted on 2011-05-02
Last Modified: 2012-05-11
I wanted to know what the MTU size is and why it's important?  I presently have a client whose firewall (Sonicwall) is boucing.  When I called Sonicwall they were able to verify that their equipment is working properly; however, they suggested we contact the ISP so that we could verify the negotiating on their side as well as the MTU size.  They suggested that it should match up with the Sonicwall and vice versa.
Question by:ee4itpro
    LVL 22

    Expert Comment

    by:Matt V
    MTU controls the maximum packet size sent over the line.  If you are using a smaller MTU than your upstream provider, you would likely be fine but may be shorting yourself on speed.  If you are using a larger MTU than your upstream, you will see dropped packets.

    Ideally, you should match the upstream MTU, or be slightly smaller in some cases (such as DSL which has overhead).
    LVL 33

    Accepted Solution

    We've never had to match the MTU with the ISP's hardware. However, we have had to determine if we had the correct MTU. The default of the WAN is 1500. I wrote an EE article on configuring the proper MTU for your WAN.

    I've discovered that where the MTU is important is with internet solutions like Cox fiber or T-1 connections. It used to matter with cable internet, but I'm finding that less and less. But, to mattvmotas point, I doubt it will be the default of 1500 and will need to be less.

    Hope that helps.
    LVL 12

    Expert Comment

    MTU stands for Maximum Transmission Unit and determines the largest size a packet will be given in your network.  If a large packet is sent over the WAN and encounters a network with a smaller MTU, the packet will have to be broken down into smaller packets in order to be forwarded.  That overhead will slow down your overall transit speed.  Most of the time, if your MTU is equal to or slightly lower than your upstream connection (your ISP), you will be fine.  Occasionally, someone messes with the MTU, thinking that larger is better, and actually ends up slowing everything down.  digitap's article is a good one.  Additionally, you can do ping and traceroute tests, specifying the packet size, to see what is actually happening during your transmission.

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