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I always thought it was one of those legacy things because the max frame size for frame relay was 9000 Bytes. But in theory, a frame for Ethernet can be 64K I believe but that is not really practical of course for reasons mentioned in other threads here about re-trans.

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Probably a bigger reason to use MTA less than 64k (9k-16k) is than 64k frames can quickly fill up small buffers in switches and routers and memory allocator in OS kernels may also be optimized for lower values.

Historically, like 20, 22 years ago it was a LOT more costly to put some high speed ram in 24/48 port gigabit Ethernet switches for port buffers. Now we live in an era of high speed cheap memory so even throwing gobs of ram at rib and fib on routers meant to take multiple full bgp tables isn't so costly.

Jumbo frames are used only in LAN (mostly DC LAN) where packet loss is low and retransmits are rare.

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