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I just got some already configured switches that are giving a lot of problems on the network with port shutdowns due to storm control policies.

Finally after dumping the running-config. I've saw extremely tight broadcast and multicast storm control policies, as low as 2 percent on a distribution switch, even on the trunk ports.

interface GigabitEthernet1/0/1
 speed 100
 storm-control broadcast level 2.00
 storm-control multicast level 2.00
 storm-control action shutdown
 no cdp enable
 spanning-tree portfast
!

Those are the configurations on all switchports of a 2960 model, and as I said, those guys act as a distribution switch, so some ports are plain access ports and others have switches attached to them.

The whole point here is: why using storm control on distribution switches and why using with this low threshold. I've seen a good practice being 5 to 10 percent of broadcast/multicast storm control, but never 2 percent. And why it was enabled on trunk ports? This makes no sense for me, since trunk ports will be receiving a lot more broadcasts then other ports.

Thanks in advance,

PS: Just in case, this network is huge and badly deployed, so it's a /20 network running with more than 2000+ devices, so I'm expecting huge broadcast traffic on the devices, which causes problem with this storm control policy.

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    Keep in mind that 2% of a gig link is 20Mbps of broadcast traffic, which is far more than should be expected on an access port. It may need to be adjusted on trunk ports depending on the network, but it may also be sufficient. I would have more concerns in figuring out why you regularly have more than 20Mbps of broadcast on these ports. Of course, I wouldn't recommend running a flat /20 network.
    – YLearn
    Aug 2, 2015 at 17:41

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The interface you are showing is a gigabit port that is configured to run at 100Mbps due to the speed 100. This means the port will shut down if there is more than 2Mbps of broadcast or multicast in any 1-second period, which given the size of the subnet is a very real possibility.

I would definitely be increasing these values on uplink ports considering the size of your network - start by increasing to 10.0 and seeing if this alleviates your issue.

A real broadcast storm is going to generate a lot more than 10Mbps, so setting the level higher is not that big of a deal.

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  • Well, it's a /20 segment using DHCP and a lot of devices (wired and wireless), so yes. There's a lot of broadcast happening. And since we're an university I think multicast is largely used. Unfortunately I don't have any monitoring system in place, I just got those switches.
    – Vinícius Ferrão
    Aug 3, 2015 at 0:38

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