Flow control support is limited unless all equipment can use it. If a 10 GbE host and 1 GbE host are connected to a switch, the one host can overwhelm the other.

Assuming flow control is not supported. In general, how does the switch control sending too fast or does it not do anything to control it? Is it left to the protocol layer, such as TCP having its own flow control algorithms as compared to UDP having no flow control?

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Some switches will have some (but not a lot of) buffers for this situation. Basically, the switch will just drop frames which it cannot send from a high-speed to a low-speed link.

That's why you normally use lower speed ports for access ports, and the faster ports for uplink ports. Cisco recommends, at most, a 20:1 access to uplink bandwidth ratio (add up the bandwidth of all the access ports and compare it to the uplink speed); for every 20 Gb of access ports, you need at least 1 Gb in the uplink.

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