Ethernet Circuit MTU

Posted on 2011-05-10
Last Modified: 2012-05-11

We're experiencing some wierdness with our network - a brief overview;

We have 2 sites (within 50 miles) using the same ISP, same firewall hardware.  MTU's on each device (switch, firewall etc) is set to 1500 but when we perform ping IP -f -l MTU we get fragmented packets for any MTU value over 1272.

Also, all sites we communicate with (32 sites worldwide) we're gettig the same response with over MTU 1272.

Is this normal? Should we be shouting at the ISP?

Clearly if the MTU is low, then we're getting less for our buck..?

Question by:jasonhamlett
    LVL 9

    Accepted Solution

    It's probably just the route. I mean every node that the transmission is going through is going to have and modify the MTU where possible according to the Path MTU Discovery 'protocol'. Meaning the entire route/path is going to be 'discovered' and adjusted to lowest MTU for any device that the packet will encounter. Forcing a 1500 would then of course become fragmented.
    1272 does seem low, but it's most likely a failsafe low. If you wanted to control the MTU outside your network you'd have to control every hop.

    Author Closing Comment

    got me thinking in the right direction...

    Write Comment

    Please enter a first name

    Please enter a last name

    We will never share this with anyone.

    Featured Post

    Maximize Your Threat Intelligence Reporting

    Reporting is one of the most important and least talked about aspects of a world-class threat intelligence program. Here’s how to do it right.

    Preface There are many applications where some computing systems need have their system clocks running synchronized within a small margin and eventually need to be in sync with the global time. There are different solutions for this, i.e. the W3…
    Configuring network clients can be a chore, especially if there are a large number of them or a lot of itinerant users.  DHCP dynamically manages this process, much to the relief of users and administrators alike!
    Sending a Secure fax is easy with eFax Corporate ( First, Just open a new email message.  In the To field, type your recipient's fax number You can even send a secure international fax — just include t…
    Illustrator's Shape Builder tool will let you combine shapes visually and interactively. This video shows the Mac version, but the tool works the same way in Windows. To follow along with this video, you can draw your own shapes or download the file…

    779 members asked questions and received personalized solutions in the past 7 days.

    Join the community of 500,000 technology professionals and ask your questions.

    Join & Ask a Question

    Need Help in Real-Time?

    Connect with top rated Experts

    12 Experts available now in Live!

    Get 1:1 Help Now