As a general best practice, I would always make sure that configured MTUs on adjacent switch/router interfaces match across a network. If not, fragmentation can occur, thereby degrading performance. On the other hand, the packet may not be fragmented if the DF bit is set.
That having been said, I want to focus on a specific scenario involving JUMBO MTUs in a server farm.
The access layer switches are set globally to 9208, but the distro/core switches are set globally to 9216. While I would avoid this if possible, i want to delve a bit deeper than a canned thought.
As I see it, this can cause a problem, but not necessarily. For one, if there are no applications that are leveraging Jumbo frames in the first place, you will not have an issue. Also, since the access layer switches are the ones with the lower MTU, there will not be any fragmentation. If, however, it was the other way around and the access switches had the higher MTU, then I can see fragmentation issues occuring. The caveat again being of course that there is an app that is leveraging Jumbo frames in the first place.
i actually see it as you do. as long as all devices support at least the 9208 frame size on the jumbo frame way there shouldnt be a problem. cause the clients which could but not necessarily would produce a jumbo frame are the ones connected to the access layer switch, thus the max. frame size would be 9208.
-if there is a switch on the way which doesnt support the the jumbo frame size, the frame will be dropped.
-if there is a router on the way which doesnt support the framz size and the DF bit is not set, the frame will be fragmented.
-if there is a router on the way which doesnt support the frame size and the DF bit is set, the frame will be dropped.
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Fragmentation happens when the egress link's MTU is less than the ingress link's MTU.
Since many switches set MTU globally, a different situation can arise. Devices sharing the same link can have different MTUs. From smaller to larger isn't a problem, but the converse is as the packet wouldn't be fragmented but instead dropped.
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