Solved: SPANTREE-2-LOOPGUARD_BLOCK

Eloy Pascal · ‎11-22-2006

I have encountered these two Log messages and the Giga port on the Switch blocked as

if it had port security. The port is directly connected to the Core Lan Switch.

Nov 22 08:41:13: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEt

hernet0/2 on VLAN0001.

Nov 22 08:41:13: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port Gigab

itEthernet0/2 on VLAN0001.

What may be the reason? What should i check?

amit-singh · ‎11-22-2006

Hi Eloy,

The above log messages on the switches indicates that the spanning tree message age timer has expired, because no BPDUs were received from the designated bridge. Because this condition could be caused by an unidirectional link failure, the interface is put into blocking state and marked as loop guard inconsistent in order to prevent possible loops from being created.

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat4000/12_1_19/message/emsg.htm#1186228

Use the show spanning-tree inconsistentports command to review the list of interfaces with loop guard inconsistencies. Determine why devices that are connected to the listed ports are not sending BPDUs. One reason could be that they do not run the Spanning Tree Protocol; in this case, you should disable loop guard in the inconsistent interfaces or start the spanning tree protocol on the other side of the links, depending on the context. Another reason could be a failure in the cable: If the link has a failure that makes it unidirectional (you can transmit but you cannot receive), you should replace the old cable with a new cable.

HTH, Please rate if it does.

-amit singh

View solution in original post

amit-singh · ‎11-22-2006

Hi Eloy,

The above log messages on the switches indicates that the spanning tree message age timer has expired, because no BPDUs were received from the designated bridge. Because this condition could be caused by an unidirectional link failure, the interface is put into blocking state and marked as loop guard inconsistent in order to prevent possible loops from being created.

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat4000/12_1_19/message/emsg.htm#1186228

Use the show spanning-tree inconsistentports command to review the list of interfaces with loop guard inconsistencies. Determine why devices that are connected to the listed ports are not sending BPDUs. One reason could be that they do not run the Spanning Tree Protocol; in this case, you should disable loop guard in the inconsistent interfaces or start the spanning tree protocol on the other side of the links, depending on the context. Another reason could be a failure in the cable: If the link has a failure that makes it unidirectional (you can transmit but you cannot receive), you should replace the old cable with a new cable.

HTH, Please rate if it does.

-amit singh

Eloy Pascal · ‎11-22-2006

FYI.

Show interface of Floor Switch

!

interface GigabitEthernet0/1

description Hacia Core

switchport trunk encapsulation dot1q

switchport mode trunk

flowcontrol receive desired

udld port

end

Show interface of CORE Lan Switch

!

interface GigabitEthernet4/2

description Trunk Piso3.1

switchport trunk encapsulation dot1q

switchport mode trunk

logging event link-status

udld port

flowcontrol send desired

rmon native

rmon collection stats 11 owner config

Any other advice?