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I have a couple test laptops connected to a C9300-24P. One laptop sends data as broadcast throughout the network and the other laptops collect and display the data. When running the test the ports that the laptops are on are showing a lot of pause inputs when doing a show interface giXXX. I know this has to do with flow control but I thought that we configure already covered it.

Port configuration is below:

switchport access vlan 25
switchport mode access
switchport block unicast
storm-control broadcast level bps 62000000
storm-control unicast level bps 1000000000
spanning-tree bpdufilter enable
spanning-tree bpduguard enable
spanning-tree guard root

Switch QOS policy allows for class-default set to bandwidth percent 100.

What am I missing to find why the port is causing so many pauses? These pauses are causing a drop in data and connection for the laptop which is unsatisfactory.

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2 Answers 2

Reset to default
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storm-control broadcast level bps 62000000

caps broadcast ingress to 62 Mbit/s (measured in 1-second intervals) - when the port exceeds that limit it is paused. Works as configured.

Any QoS policies are also enforced, but they do not override storm-control.

By the way: using that amount of broadcast traffic is very bad design. You should consider multicast.

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  • Switch is not sending storm control events in the log. Wouldn't it be throwing an error? Commented Jan 13 at 17:42
  • storm-control triggering isn't really an error. Have you tried debugging it? Commented Jan 13 at 18:22
  • Not yet. I do see storm control for other ports on other devices when it does trigger. But not on this one for all the PAUSES. PAUSES is caused by overwhelming the port on the switch or the computer not reading fast enough? Commented Jan 13 at 19:10
  • You mean it generates pause frames? That wasn't clear - pause frames are generated on the ingress port when the (or a) destination port is too busy to forward the ingress frames. Commented Jan 13 at 20:53
  • so the client is producing the Pause frame when it can't handle everything at once or is buffering? Commented Jan 13 at 21:00
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Pause inputs = Ethernet flow control (802.3x) being triggered

Heavy broadcast traffic is filling port buffers

Storm-control does not prevent pause frames

Flow control is likely enabled by default on switch or laptop NIC

Pause frames cause traffic slowdown and drops

Fix:

Disable flow control on the switch port

Disable flow control on the laptop NIC

Reduce broadcast traffic or use multicast/unicast

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