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We have ordered a 100 Mbps leased line and did not specify our requirement to have a minimum of 1530 MTU supported. The telco now sent us a feedback that they do not support 1530, but only 1515.

Did anyone experience something similar? And what can be the rational behind for the Telecom provider not supporting higher MTU. What kind of hardware is deciding this in the case of a leased line service?

Edit: line protocol is Ethernet; also I corrected: it is a 100 Mbps we ordered. Hope it clarifies. Thanks for your help.

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The standard MTU for ethernet is 1500. Jumbo frames are required for larger MTUs. What line protocol are you using. Omitting address information could get you an additions 16 bytes or so. – BillThor Aug 31 '10 at 5:35
Why do you have a 1530 MTU requirement??? Packet fragmentation should take care of packets that are larger than 1515 (which is an odd number too). – Chris S Aug 31 '10 at 20:30

In all likelihood, one or more segments of the leased line are tunnelled through some other technology (e.g. IP, ATM, Frame Relay). Adding up all the different networking layers tends to create this sort of problem. I've experienced this directly, and companies generally discover this "minor detail" from the telco only after signing multi-year contracts that they can't pull out of. Changing every single device in your network to support a smaller-than-standard MTU is a nightmare, and should be avoided if at all possible.

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And some segments are probably tunneled inside tunnels inside tunnels, each chipping away at MTU. – Joris Jan 26 '11 at 9:32
I guess the lesson is if you need a MTU over 1500 make sure that is specified in the contract you sign. – Peter Green Jan 15 at 14:42

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