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Need help to identify Cisco 4500 Port STP flap trigger



A bit perplexed here. Replaced 10 switches with new switches connected C4500 via fiber.   All switches run MST0.  C4500 ports always link up and have no errors.  But all 10 C4500 fiber downlinks follow this cycle: about 30 seconds in "forwarding" state and passing traffic normally, for a moment port shows "no stp instance", and then it is cycles again "BLK-Learning-Forwarding" and so on. 

If the port never "link down" and has no errors how to identify what triggers STP restart on a port ?  

bpdufilter/portfast/access port - testing did not change anything. Still the same  "BLK-Learning-Forwarding" every minute or so. 



7 Replies 7

Thank you for information. I went through the article and most useful was "debug pm vp'.

Here is what is shows when the port allegedly goes down and STP restarts on port:

PORT CONFIGURED AS TRUNK:  pm_vp 1/4(1): during state forwarding, got event 7(trunk_remove)

PORT CONFIGURED AS ACCESS:  pm_vp 1/4(330): during state forwarding, got event 4(remove)

It feels like the STP is not what needs to be investigated. It is something else that flaps the port while its state is still "link up". When debug says "got event" I'm curious there that event came from.

I attached port info.

can you share 
show spanning tree summary <<<- of SW C4500

Here it is. As I mention in the other reply at this point I do not believe the STP is the cause but rather a consequence of some port drop/flap.


CORE2#show spanning-tree summary
Switch is in mst mode (IEEE Standard)
Root bridge for: none
Extended system ID is enabled
Portfast Default is disabled
Portfast Edge BPDU Guard Default is disabled
Portfast Edge BPDU Filter Default is disabled
Loopguard Default is disabled
PVST Simulation is enabled
Bridge Assurance is enabled
EtherChannel misconfig guard is enabled
UplinkFast is disabled
BackboneFast is disabled
Configured Pathcost method used is short (Operational value is long)

Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
MST0 1 0 0 11 12
---------------------- -------- --------- -------- ---------- ----------
1 mst 1 0 0 11 12

Bridge Assurance is enabled

Bridge Assurance only runs in RSTP or MST networks.  It makes sure that a neighboring switch does not malfunction and begin forwarding frames when it shouldn’t.  It does this by monitoring receipt of BPDUs on point-to-point links.  When the BPDUs stop being received, the port is put into blocking state (actually a port inconsistent state, which stops forwarding).  When BPDUs restart, the port resumes normal RSTP or MST modes.  This handles unidirectional links as well as the malfunction of a neighboring switch where STP stops sending BPDUs but the switch continues to forward frames.

there is issue with Link make BPDU stop received. 
check error counter and drop in STP stats.

cat#show interface g4/3 counters errors
cat#remote command switch test spanning-tree process-stats


I've just disabled Bridge Assurance and still observe the same problem. 

No "inconsistent" ports found in cli output or event log.

Counters errors - 0 for all interfaces

CORE2#remote command
% Invalid input detected at '^' marker



Show spanning tree interface detail

Check the bpdu send receive count is it increase?

If you can share here 


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