ip mtu / interface mtu

I am having an issue on a provider network in which they do not pass any ip packets over 1400 bytes. This has caused big problems with network drives that are mapped over the WAN and with email (connectivity to the Exchange server) because packets over 1400 bytes are being dropped sporadically (about 50-60% of the time).

After opening a ticket with the provider, we decided that we would alleviate our problems by setting our serial interface IP mtu to 1400 for all of our remote sites. It seemed to help initially, however we are having similar problems again.

I am a level-4 engineer and I understand this stuff pretty well, however I am having a problem understanding the MTU in relation to the interface. In the Cisco IOS, there is an interface MTU setting and an IP MTU setting. I understand that the second one is a protocol MTU and relates to layer-3, while the interface MTU command seems to be more related t a layer-2 framing perspective. Additionally, even though I have set the IP MTU to 1400 bytes, I can source an ICMP off of the ethernet interface of my router and specify a size of 1500 bytes and it will go, even though I have set my serial side lower.

I am not having much luck finding decent documentation on Cisco's website; any further understanding from anyone out there would be great.

V. Evans

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V. Evans
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Stig Meireles Johansen Senior Consultant

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Provides the following advice:

MTU is the Maximum Transmission Unit of general frames entering/exiting the interface.

IP MTU is the MTU for IP packets entering/exiting the interface.

The difference is there because of the different features you can add (ie. run IPX etc.)

If you have MTU-problems, try setting "ip tcp adjust-mss 536" on your inside interface... you could always adjust to a bigger value.. use a ping with different sizes to see which size gets through..

If you use the microsoft-ping, remember that the "-l size" parameter (-s for unix-version) is the payload of the ICMP-packets, so you should subtract 28 bytes (20 bytes IP header + 8 bytes ICMP header) from the total.. If you ping with a size of 1472, this would equal a IP-packet with MTU of 1500.

The MSS is the Maximum Segment Size, ie the data payload of TCP-packets.. this should be (normally in a ethernet-environment) 1500

- 20 bytes IP header - 20 bytes TCP header = 1460 bytes. The minimum IP MTU which must be supported on the 'net is 576 bytes, which gives an MSS of 536.

If you are running PPPoE, remember to take this into account.

Troubleshooting MTU Size in PPPoE Dialin Connectivity:

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Hope this helps,

Brad Reese BradReese.Com Cisco Repair Service Experts

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