Traffic Storm-Control: a tiny detail to keep in mind

When you configure Traffic Storm Control on Cisco switches ports using bandwidth percentage as threshold level, be aware that this percentage will be calculated on the basis of negotiated bandwidth not the hardware port type. In other words, you may have 1000BaseTX interface configured something like this:

interface GigabitEthernet1/0/6
 storm-control broadcast level 1.00
 storm-control action trap

And you think that port will start dropping broadcast traffic when it reaches 10Mbps rate (1% of 1000Mbps). That is true if it gets negotiated at actual 1000Mbps. However, if for some reason the speed gets negotiated to let’s say 100Mbps drops will start at 1Mbps rate which is 1% of 100Mbps.

The documentation says “The traffic storm control level is a percentage of the total available bandwidth of the port” This available bandwidth can be seen in show interface command output.

When the port is at 1000Mbps:

#sh int gi1/0/27 | inc Giga|BW
GigabitEthernet1/0/27 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 6cfa.8953.fd1b (bia 6cfa.8953.fd1b)
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,

When it is at 100Mbps:

#sh int gi1/0/29 | inc Giga|BW
GigabitEthernet1/0/29 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 6cfa.8953.fd1d (bia 6cfa.8953.fd1d)
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,