%SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port =========== what could be the reason???

[bibin.chaithanya1]

i'm getting this errors..

b 7 2016 01:59:19.526 PET: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/1 on VLAN0752.
Feb 7 2016 02:00:46.055 PET: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/1 on VLAN0390.
Feb 7 2016 02:00:46.088 PET: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/1 on VLAN0390.
Feb 7 2016 02:00:48.093 PET: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/1 on VLAN0702.
Feb 7 2016 02:00:48.135 PET: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/1 on VLAN0702.
Feb 7 2016 02:01:10.239 PET: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/1 on VLAN0702.
Feb 7 2016 02:01:10.273 PET: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/1 on VLAN0702.
Feb 7 2016 02:02:50.910 PET: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/1 on VLAN0510.
Feb 7 2016 02:02:50.936 PET: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/1 on VLAN0510.
Feb 7 2016 02:03:37.224 PET: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/1 on VLAN0600.
Feb 7 2016 02:03:37.241 PET: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan600, changed state to down
.Feb 7 2016 02:03:37.257 PET: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/1 on VLAN0600.
.Feb 7 2016 02:03:37.274 PET: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan600, changed state to up
.Feb 7 2016 02:03:42.274 PET: %SYS-6-LOGGINGHOST_STARTSTOP: Logging to host 10.8.0.76 port 514 started - reconnection
.Feb 7 2016 02:03:49.303 PET: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/1 on VLAN0702.
.Feb 7 2016 02:03:49.337 PET: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/1 on VLAN0702.
.Feb 7 2016 02:03:52.298 PET: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/1 on VLAN0511.
.Feb 7 2016 02:03:52.348 PET: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/1 on VLAN0511.
PELABSWI94#

[InayathUlla Sharieff]

Hi,

1)LoopGuard: Spanning Tree Loop Guard helps to prevent loops when you use fibre links. Fibre links have a transmit and receive connector. If one of these links fails it's possible that interfaces that are currently in "blocking" mode go to forwarding. This might cause a loop. Loop guard will ensure that if a blocked interface no longer receives BPDUs from the other side that it will be shut down to prevent a layer 2 loop.

Taking 3 switchs as a example: connecting in a triangle.

     SW1

___|________

|                |

Sw2 T0/1---T0/2 SW3

Hence consider the above topology one of the link will be block.

think SW1 is the Root bridge hence the port T0/2 will be in blocking state.( To have the loopfree topology)

It works similar to UDLD feature. The Sw2 and Sw3 is connected through fiber cable, One end would be tx and other end would be rx.

As you know that blocked port would be recieving the BPDU's. what happens when it stopped reciving the BPDU's? (Considering the example that there is some issue with the fiber cable and Tx  is haiving issue hence port T0/2 is not reciving it hence it waits for the max age timer to expire after which the port transition from Blocking to forwarding mode which is not supposed to hence there would be loop. Hence when you confiugre the loopguard/udld then the port would go blocked.

Hence it is layer 1 cable issue STP would not be able to detect it automatically, hence you would use the loopguard feature.

Action plan:

=========

Could yiu please check what is connected to port G0/1 ? If possible try to replace the cable and also check the port status at other end..

Regards

Inayath

[husain.tarik]

Superb .

[Rolf Fischer]

Hello,

just an addition to what Inayath already said.
Apparently Gi0/1 is a non-designated port (Root-, Alternate- or Backup-Port) and it stopped receiving BPDUs for several VLANs while the link status was still up and no other change of the spanning-tree topology was detected. LoopGuard prevents the port under such conditions from transitioning to the forwarding state (and potentially forming a loop) by putting it to the loop inconsistent state until it receives a new BPDU on that port.
This can be caused by a unidirectional link (feature of choise: UDLD) but also by STP configuration errors or misoperations, which UDLD could not detect.

I agree on the action plan but it might be necessary to review the whole spanning-tree topology and -configuration if you can't solve the problem that way. The device name indicates its a lab environment; you may want to enable spanning-tree logging on all switches during troubleshooting to gain a better understanding of what's going on.

Useful document: STP Enhancements using Loop Guard and BPDU Skew Detection


HTH
Rolf

[paul driver]

Hello

I would agree with the other statements with an addition suggest Enable UDLD and disable any error recovery feature set 

If this is indeed an UDLD issue This could negate multiple stp tcns occurring stable you lan  plus give you time to check the physical layers for issues

res

paul