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High MTU Value

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High MTU Value

Hey Everyone

I Was informed by my Firewall this morning that my bewan adsl pci card could not set the maxium tranmission unit for my modem, the value that bt use is a maxium of 1500 before packets get fragmented i thought i read somewhere that plusnet recommend we set this value to 1492.

I was just wondering why plusnet have set there router value to accept packets this large since we can only transmit packets though the bt pipes at a value of mtu 1500(max)

My bewan card has auto config'd itself to a value of 32725 mtu as it was not possible to set it to the value of plusnets upstream router which was quite a lot higher.

my question is why not compile with the standards and whats to be gained for this, also will my connection degrade as a result of my packets hitting the bt routers with a mtu value of 1500 then fragmenting ?
2 REPLIES
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High MTU Value

Quote
I was just wondering why plusnet have set there router value to accept packets this large since we can only transmit packets though the bt pipes at a value of mtu 1500


Hi there,

Thats not quite right. You can set the MTU to the value which works best for you. Its no Plus net setting anything.

For a better understanding of MTU and how to make it work better for you see This article from kitz which should help greatly.
Community Veteran
Posts: 26,339
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Registered: 10-04-2007

High MTU Value

The size of the packets that get sent will be determined by the setting on your PC. The setting on the router only matters if it is smaller on than the PC setting which would cause packets to be fragmented by the router giving you a performance problem.

Setting the MTU on the router to larger than on the PC will not have a detrimental effect which is why it is normal to set this to 1500. I currently have my router set to this and my PC set to 1478 (Rwin 56720). A recent speed test I ran gave 1973.
jelv (a.k.a Spoon Whittler)
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