Force Recalculation of Spanning Tree Protocol

Posted on 2011-05-03
Medium Priority
Last Modified: 2012-05-11
We're seeing occasional high CPU utilization on our switches.  High enough that our Cisco 2950's are not responding to telnet/snmp and the like.  We're exploring the possibility that it's related to spanning-tree protocol, since we've got  >64 Vlans, which is over the 2950's max instance limit.

I'd like to test this theory by having the switches recalculate their STP information... I can't find a definitive answer on what it takes to make this happen.

How can I force a STP broadcast storm?

Question by:MU-IT
LVL 26

Expert Comment

ID: 35515267
You can first try disconnecting or shutting down the redundant trunks that are in forwarding state, causing the blocked trunks to switch to forwarding. You could also try doing a no spanning tree vlan vlanid to disable spanning for a particular vlan and then renenable. You could also try changing the root bridge for your STP instances.
LVL 47

Accepted Solution

Craig Beck earned 1200 total points
ID: 35516845
If you're running PVST or Rapid-PVST you should just be able to disconnect a redundant link and reconnect it.

If you're running Rapid-PVST and you aren't using spanning-tree portfast on edge ports you may be seeing excessive STP traffic.

Author Comment

ID: 35517924
We're not running any actual STP stuff, it's in place for the loopback detection.   Disabling/Enabling causes some packets, but not the high CPU situation.

Thanks for the help.

Assisted Solution

Wissam earned 800 total points
ID: 35604045
Also for your situation with >64 instance, i suggest configuring MSTP (which lead to having multiple vlan with single instance of spanning treet) so you can make your network with lower instances, e.g. 8 Instances with 8 vlans each...

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