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I have a 7100 series Arista passthrough switch. In my configuration, the switch takes as input 1G Ethernet, and routes it to a 10G output.

In this 1G to 10G configuration, is the Arista switch still acting as a cut-through switch, given that information can go out faster than it comes in?

Randomblue
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2 Answers2

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When speed changes, all switches function in 'store-and-forward' mode.

The reason is that the bit timing of a frame is very strict. The switch must emit 10 billion bits per second; it can't emit 1 bit-time of data + 9 bit-times of idle to make up the slack. The receiving side will consider the idle bit-times as a 'dropped carrier' and drops the whole frame.

Hence, the full frame must be stored before it begins transmitting at a higher speed.

pepoluan
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Changing speed from slower to faster forces the frame to be stored and forwarded, as you can't space out bits or have a concept of fragments in ethernet.

This is reflected in the accepted answer, however not all speed changes force store-and-forward switching: Changing speed from faster to slower still allows for cut-through switching, as you should have (more than) enough bits to start transmitting a the slower speed. So while you do have to use buffering to store parts of the frame due to speed mismatch, you are not required to accept the entire frame before starting transmission on the egress interface.

osbjmg
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