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How does a network switch prioritize the incoming ethernet frames that all have the broadcast MAC address as a receiver address?

So, we have the following situation:

  • a 10GE network switch with 3 Ports A, B and C
  • Traffic on Port A and B is bidirectional and Port C works only as a output port
  • All ethernet frames from Port A and Port B have the receiver address: 0xFF FF FF FF FF FF

What happens now on port C in the following situations:

  1. Two frames (one from A and one from B) arrive simultaneously and should both be forwarded to port C
  2. Bandwidth on port A and on port B is above 5 Gbps, but should be forwarded to port C

How does the switch prioritize here?

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  • Switches have tiny FIFO (First in, first out) queues, and they will drop frames exceeding the queue for an interface. Some switches can have (fairly minimal compared to IP) QoS that can be configured on the switch to give priority to some frames.
    – Ron Maupin
    Sep 30 at 14:05
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How does a network switch prioritize the incoming ethernet frames that all have the broadcast MAC address as a receiver address?

By default, a switch doesn't prioritize frames at all, broadcast or unicast.

It's simply first come, first served with excess frames being dropped when they cannot be forwarded or queued.

Two frames (one from A and one from B) arrive simultaneously and should both be forwarded to port C

From the perspective of the switch, the two frames are not received exactly simultaneously and one is queued after the other (again, with excess being dropped).

Of course, if your switch supports that, you can prioritize using different queues and weighted scheduling by 802.1Q classes, DSCP flags, VLANs, source/destination IP addresses or ports, ...

On some switches I've seen strange behavior that one port is prioritized over the other, depending on port number, ACLs being present or other config details - so your mileage may vary.

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