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Spanning-Tree blocked State

avorobyev
Level 1
Level 1

Hi!

I've got Cisco 881 connected to the network in a branch office.

Sometimes (may occur once a week or three times a day) I see STP starts blocking port:

rtr#show spanning-tree

 VLAN1 is executing the ieee compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, address 649e.f33b.7dfa
  Configured hello time 2, max age 20, forward delay 15
  We are the root of the spanning tree
  Topology change flag not set, detected flag not set
  Number of topology changes 2 last change occurred 01:50:39 ago
          from FastEthernet3
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 1, topology change 0, notification 0, aging 300

 Port 4 (FastEthernet3) of VLAN1 is blocking
   Port path cost 19, Port priority 128, Port Identifier 128.4.
   Designated root has priority 32768, address 649e.f33b.7dfa
   Designated bridge has priority 32768, address 649e.f33b.7dfa
   Designated port id is 128.4, designated path cost 0
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   BPDU: sent 502925, received 1807

It recovers only by shutdown/unshutdown port (or cable plug out/in).

If there were loop, this state should not recover after shutdown, but it does.

There are switches behind cisco, but they are not manageable.

 

Any ideas where is the problem and how to auto-recover from the blocking STP state?

 

 

2 Replies 2

Mark Malone
VIP Alumni
VIP Alumni

Hi you need to trace where the change is occurring at layer 2 , use this command below and follow the path of the change , so if as below my example on my vss the change had just occurred rather than a week ago i would do a show cdp of po122 and jump onto that switch and then run the command again , this will help you trace the fault port source, somethings changing in the network thats automatically causing STP to put your port into blocking state to prevent a loop from occurring, you need to find the source of the problem , could be a faulty cable , incorrect config or a bad nic somewhere or bug.

If you cant use this command the whole way through you will need to check each switches stp settings and config , and look for any errors miss-configs

Your router lan port should never go into blocking for obvious reasons its blocking the network at layer 2 if it occurs , check first whats the settings on the directly connected device and what the logs are in relation to stp, if this problem only recently started you may just have a faulty device port

what are the switches you have connected off the 881,

Hello

Its does indeed suggest just that and FYI it can recover if that possbile loop is now not applicable to the current stp topology.

A suggestion would be , Before you reenable this port- investigae why it was shutdown in the first place.

 

STP is a loop prevention protocol and as such it seems in this case for some reason ( not posted) a once forwarding port has not become block to stop a loop.

What you haven’t posted is what type of error disablement it is -But i can see it activatin on the stp root bridge for that vlan
 

It is possible due to

- Designated port transitioning into a root port when root guard is active

 

But it can be other reasons  also.

- Due to an topology change resulting due to an non direct or direct link failure -
 - Access port receiving bpdus when bpduguard is enabled

- UDLD issue
 
Can you post the log buffer or the type of stp error thats occuring?

sh log
sh int x/x status err-disabled

 

Lastly - regards Marks good recommendation, Please be aware--
You will see access ports stp transitioning very frequently if you don’t have portfast enabled and as such this could lead you away from the actually root of the problem.


res

Paul


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Kind Regards
Paul
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