If you connect on a 10/100M port an application device(video streaming); on 10/100M port Jumbo frames aren't supported. So the MTU size is standard 1518, if my Gigabit uplinks ports support jumbo frames, then in this case the forwarded/routed frames size stays the same (1518) I suppose? Or become the frames remaped in frames of 9018??
Are jumbo frames only usefull if your application maps the packets into jumbo frames (>1518), and you connect your application device direct on your Gigabit interface, which support Jumbo frames?
one example of Jumbo frames (sometimes called baby-giant) are 802.1q or ISL frames sent on trunks. There are 4 or 30-bytes headers added to the original frames, so the total frame length can exceed the 1518 byte limit.
IMHO, frames are never remaped to greater ones (in the case of IP fragmentation once fragmented the packet is never completed back until it reaches the target device, e.g.).
If you have a core network built with Ethernet and you have a few FDDI groups connected to it, you will need jumbo frames.
FDDI supports a MTU of 4000 bytes and thus requires fragmentation when going over standard ethernet. Jumbo frames solves that.
Or if you have a few large file servers, jumbo frames can really ease the load on CPUs. With standard 1500 bytes packets, you would pump about 80kpps through your network. 80kpps creates quite a cpu load on every party involved (client, server, switch, routers). By using 9k frames you would lower the number of packets to about 14kpps. Quite a difference.
But just as you mention, if you use your gigabit ports strictly for uplink from a 10/100 switch, I see no apparent reason for using jumbo frames.
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