STP makes certain assumptions about its operational environment. These are the assumptions most relevant to this document:
Each link between the two bridges is bidirectional. This means that, if A directly connects to B, then A will receive what B has sent and B will receive what A has sent, as long as the link is up between them.
Each bridge that is running STP is able to regularly receive, process, and transmit STP Bridge Protocol Data Units (BPDUs), also known as STP packets.
While these assumptions appear logical and obvious, there are situations when they are not met. Most of these situations involve some sort of hardware issue; however, software defects may also lead to STP failures. Various hardware failures, misconfigurations, or miscabling cause the majority of STP failures, while software failures account for the minority. STP failures can also occur due to unnecessary additional connections that exist between the switches. VLANs go into a down state because of these additional connections. To resolve this problem, remove all of the unwanted connections between the switches.
When one of these assumptions is not met, one or more bridges might no longer receive or process the BPDUs. This means that the bridge (or bridges) will not be able to discover the network topology. Without knowledge of the correct topology, the switch can not block the loops. Therefore, the flooded traffic will circulate over the looped topology, consume all bandwidth, and bring down the network.
Examples of why the switches may not receive BPDUs include bad transceivers or Gigabit Interface Converters (GBICs), cabling issues, or hardware failures on the port, the linecard, or the Supervisor engine. One frequent reason for STP failures is a unidirectional link between the bridges. In such a condition, one bridge sends BPDUs, but the downstream bridge never receives them. STP processing can also be disrupted by an overloaded CPU (99 percent or more), because the switch is unable to process received BPDUs. BPDUs can be corrupted along the path from one bridge to the other, which also prevents proper STP behavior.
Aside from 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 caused by software issues. Such behavior might cause “slow-loops.” This means some packets are looped, but the majority of the traffic is still flowing through the network, because the links are probably not congested.
The remaining sections in this document provide guidelines to troubleshoot the most common STP-related issues
