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Dec 07

Failure Reasons for Troubleshooting STP

Troubleshooting STP

Spanning Tree Protocol’s (STP) main function is to stop broadcast storms and loops by preventing traffic from flowing through redundant links between devices. The problem arises when STP fails to do so. Mostly algorithm failures lead to bridging loops and it is extremely difficult to troubleshoot because of the lack of visibility. Some of the reasons for which the STP can fail are:

  1. Spanning Tree Convergence: When bridges and switches are transitioning into a forwarding or blocking state, the BPDUs can get lost causing a blocked state to change into a forwarding state. This makes the algorithm fail.
  2. Duplex Mismatch: When there is a mismatch in the duplex mode between two devices it can cause collisions resulting in bridging loops.
  3. Unidirectional Link: When a device sends packets but does not receive any response packets from the other device, the connection becomes one way and has an uncollaborated setup.
  4. Corrupted Packets: It is the most common form of disruption when the during transit the packets become corrupted and coverts the blocking ports to forwarding ports that confuses STP. This happens due to mismatch in duplex modes, using unsuitable cables, or the cable is too lengthy.
  5. Resource Errors: If the CPU resources of the device are overburdened, they cause problems while transmitting BPDUs which could result in the vulnerability of the STPs prevention capabilities.

The process to start troubleshooting for Spanning Tree Protocol requires knowledge of the topology of the bridge network, location of root bridge, and blocked ports. The following options can help to troubleshoot the STP.

  1. Diagnose Bridging Loop: The bridging loop issue can be diagnosed if you find all the users on one bridge domain got connection problems at the same time. Another method is to capture traffic from a flooded link and check for its matches all over.
  2. Restore Connectivity Quickly: This can be resolved by disabling all the ports that provided redundancy to the network. Try disabling the blocking ports first one at a time and check whether the connectivity has been restored or not. Once you find the faulty port causing the loop, the problem will be solved.
  3. Check Ports: To troubleshoot ports, you need to check a few things like –
  • Check the blocked ports to see if it is receiving the BPDUs and how many BPDUs were received. The command show spanning-tree bridge-group provides information regarding the same.
  • Check for a duplex mismatch on the ports using the command show interfaces[interface-number] status. It shows the speed and duplex status on the port or interface.
  • Sometimes large amounts traffic overwhelms the port causing BPDUs to get lost. To check port utilization use command show interfaces.
  • Due to packet corruption, the packets don’t reach their destination in the correct condition. So, the command show interfaces show an error counter which indicates if any unusual activity has occurred.
  1. Check for Resource Errors: These errors can be checked by using the command show processes cpu which allows you to see CPU utilization and can decide how overstretched it is.

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