Jan 10

Troubleshooting The STP


Spanning Tree Protocol, which is a Layer 2 protocol, passes data back and forth to seek out how the switches are organized on the network and takes all the data, and uses it to make a logical tree. A part of the data STP receives describes how all the network switches are interconnected.

Spanning Tree Protocol prevents problems caused by loops on a network and when redundant loops are planned on a network, it deals with cleanup of network changes or failures. Spanning Tree Protocol detects and stops broadcast storms caused by loops on the network. Broadcast storms caused by loops can slow down or completely stop traffic on the network, but STP can prevent loops by making sure that only one path between each set of switches is active.

There are several things that could fail, maybe switches do not seem to be giving expected output or something wrong can happen due to misconfiguration. Various hardware failures, wrong cabling cause the most of STP failures, while software failures are minor issue. STP failures may also occur because of unnecessary connections that exist between the switches and when VLANs get into a down state due to additional connections. To resolve this problem, remove all the unwanted connections between the switches.

Beside the forwarding loops, when no ports are blocked, there are situations when only certain packets are incorrectly forwarded through the blocking ports. In most cases, this is often caused by software issues. Such behavior might cause “slow-loops.” this suggests some packets are looped, but most of the traffic remains flowing through the network, because the links are probably not congested.

One frequent reason for STP failures could be a unidirectional link between the bridges. In such a condition, one bridge sends BPDUs, but the downstream bridge never receives them. STP processing may also be disrupted by an overloaded CPU because the switch is unable to process received BPDUs. BPDUs are often corrupted along the trail from one bridge to the alternative, which also prevents proper STP behavior

Depending on the problem and therefore the situation, troubleshooting involves:

  • Loss of connectivity to, from, and thru affected network regions
  • High CPU utilization on routers connected to affected segments or VLANs which can cause various symptoms, like routing protocol neighbor flapping.
  • High link utilization.
  • High switch backplane utilization.
  • Syslog messages that indicate packet looping within the network.
  • Syslog messages that indicate constant address relearning or MAC address flapping messages
  • An increasing number of outputs drops on many interfaces

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