STP loops

Unanswered Question
Oct 12th, 2008


I would like to know what are the step to find and resolve STP issue in 6500 sereis switches.

Thanks in advance

I have this problem too.
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Giuseppe Larosa Sun, 10/12/2008 - 02:25

Hello Vishwa,

I imagine that now you have a mix of native IOS C6500 (the one that you have just converted in the other thread) and one or more C6500 in Hybrid mode.

First of all, verify that:

a) all switches agree on the STP mode STP type to be used in the campus network (PVST+ or Rapid PVST+ in your case)

CatOS and Native IOS could default to different STP modes and this has to be fixed.

b) consider one specific vlan : verify that all switches agree on the root bridge id for this vlan. If this doesn't happen you can have problems.

c) if a mismatch exist in root bridge-id find out where is the border between devices that have elected root1 and devices that have elected root2 this should lead you to the troubled link

Hope to help


vishwancc Sun, 10/12/2008 - 03:45

Hi Giuseppe,

Thanks for your reply .

The conversion went fine and till now i have not faced any issue.

This is just a query incase the issue comes up.

b)When a spanning tree loop will occur the network will be choked and if there are 100 vlan in the network working on each of them is not feasible , so i was wondering if there are some other quick ways.



The question is not whether we will die, but how we will live.

Giuseppe Larosa Sun, 10/12/2008 - 05:07

Hello Vishwa,

I would suggest to implement broadcast storm-control we use it and it help you to be able to access switches.

Actually, the right term is bridging loop: STP tries to provide a loop-free topology it doesn't create loops on its own.

Other features that can help:

bdpu guard on access ports using portfast

loop guard and root guard on inter-switch links

UDLD on fiber-based links.

When a real loop happen the signs are clear: main cpu usage rise to 80-100% and without any help devices become unresponsive to telnet.

Deploying the safety measures of STP and UDLD and the different forms of storm control (broadcast, multicast, unknown unicast ) help to give you time to shut down troubled link.

Storm control will discard traffic that will increment the output drops on the interface and can rise syslog message that can be seen on NME like HP Openview and OVO.

For example we limit to 1% broadcast traffic out GE ports.

Otherwise the only solution is to have someone unplug fibers or cable to break the loop and regain access to devices. And sometimes we had to do this way.

(a misconfiguration on a etherchannel caused the problem between two C6500)

Hope to help


vishwancc Wed, 10/15/2008 - 20:05




The question is not whether we will die, but how we will live.


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