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| 4.3 |  | Troubleshooting Switched Ethernet Networks |
| 4.3.1 |  | Troubleshooting the Spanning Tree Protocol |

The Spanning Tree Protocol (STP) is designed to allow a logical loop free network in a switched network featuring redundant links. This is achieved by strategically blocking some ports from forwarding data. STP is a straightforward protocol, and has been in use for many years.

STP is likely to fail when there is a problem with the exchange of Bridge Protocol Data Units (BPDUs). The key parameters for STP operation are the HELLO time and the MAX AGE time. The HELLO time determines the frequency with which BPDUs are sent. The MAX AGE time determines how long a port will wait since the last BPDU was heard before deciding that a link is no longer connected to a switch, and is therefore no longer part of a loop.

Symptoms of STP failure and consequent switching loops include:

- Unusually high backplane utilization due to forwarding frames at line speed.
- Rapid address re-learning, as the loop will allow a switch to see the same source address entering on multiple ports.
- Rapidly incrementing frame counters on the affected ports as they receive frames at line speed.
- Extremely poor link performance, high latency on ping replies, and TCP timeouts, as this traffic has to compete with broadcast frames caught in the loop.
- Broadcast storms within a Layer 2 domain. Since broadcasts will be perpetuated by a loop, their effect will be felt throughout the switched network.

Any network problem that prevents BPDUs from being received within the MAX AGE time will cause an STP topology recalculation. Ports that need to be blocked will be placed in a forwarding state and a switching loop occurs.

A switching loop is more serious than a routing loop because unlike low end routers, even a modest switch can forward broadcast frames at the full line speed. This can fully saturate a link and further prevent BPDUs from being exchanged. Thus, the loop is maintained.

How is it possible for the switch to stop receiving BPDUs while the port is up? The most obvious answer is that the STP has been turned off. Another common cause is the unidirectional link. A link is considered unidirectional when:

- Links are up on both sides of the connection.
- Local side is not receiving the packets sent by remote side.
- Remote side receives packets sent by local side.

Consider the scenario in Figure , the arrows indicate the flow of STP BPDUs.

During normal operation, bridge B is designated on the link B-C. Bridge B sends BPDUs down to C, which is blocking the port. The port is blocked while C is seeing BPDUs from B on that link.

Now, consider what happens if the link B-C fails in the direction of C. C will stop receiving traffic from B, but B will still receive traffic from C.

C will stop receiving BPDUs on the link B-C, and will age the information received with the last BPDU. This will take up to 20 seconds, depending on the maxAge STP timer. Once the STP information is aged out on the port, it will transition from blocking state to listening, learning, and eventually to forwarding STP state. This will create a forwarding loop, as there will be no blocking port in the triangle A-B-C. Packets will cycle along the path (B still receives packets from C) taking more and more bandwidth until the links will be filled up completely bringing the network down.

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