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I have a network switch that supports up to and including 1000 Mbps. If I have a Client A with 100 Mbps and Client B with 1000 Mbps connection, will:

  1. the connection between Aand B be limited to 100 Mbps, therefore the data sent is not buffered? Since each port negotiates with the selected/default speed, A would default to 100 Mbps and B would default to 1000 Mbps.

  2. the data sent be buffered? If this is the case, where is the buffer stored?

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The traffic between the two hosts will be limited to the speed of the slowest link.

Switches have a very small amount of buffering for situations like this. If the host with the faster link tries to stream a lot of data to the host with the slower link, many frames will be discarded, and ethernet has no facility for retransmission. You would depend on the upper-layer protocols (TCP or the application) to retransmit lost data.

Some hosts and switches support ethernet flow control, but implementation and support of this can be very spotty.

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  • When 1Gbps host -> 100Mbps host: Output queue (buffer) on switch will be utilized first to absorb oversubscription. Once full, frames will be dropped as Ron says. The buffer is tiny and fast - on-chip silicon - and is located in the switch itself.
    – Ted Quanstrom
    Mar 18, 2016 at 15:00

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