Force Recalculation of Spanning Tree Protocol

Posted on 2011-05-03
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

    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 44

    Accepted Solution

    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

    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.
    LVL 6

    Assisted Solution

    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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