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| 4.3 |  | Troubleshooting Switched Ethernet Networks |
| 4.3.4 |  | Troubleshooting incorrect STP root configuration |

Switches have become the basic building block of even the simplest networks. They form a seamless replacement for hubs, and come with STP enabled by default. Link redundancy can be implemented without any switch configuration being necessary. In fact it is possible to construct a highly redundant switched network without ever having heard of the Spanning-Tree Protocol (STP).

Poorly configured Layer 2 designs usually result from a lack of knowledge of the Spanning Tree Protocol, or a misunderstanding of its operation.

Examine the switched network topology in Figure . At first glance, it appears to be an efficient design. A few pings to devices in the network reveals full connectivity. However, upon closer inspection the traffic flow in this topology reveals suboptimal traffic patterns. To really analyze the network topology it is necessary to look at the spanning tree states of each of the ports. In Figure it is apparent that the network has unfortunately chosen the Marketing department switch as the STP root.

Spanning tree has placed the Data Center port leading to the Telephone Sales Center in a blocking state. Consequently the majority of traffic must take the longest path to the Data Center. With the Data Center at one end of the Spanning Tree, the link between the switch in Reception and Data Center must carry the entire traffic load of the organization.

The symptoms of suboptimal Layer 2 design are usually those of congestion and are often only noticed when traffic levels increase. In a worst case, the following are symptoms of poor Layer 2 design:

- Lost packets
- High latency
- Poor throughput

Troubleshooting suboptimal Layer 2 design can be a challenge. Traditional tools such as traceroute operate at Layer 3 and will reveal nothing about the Layer 2 topology. It is a common error to overlook Layer 2 as the culprit and incorrectly troubleshoot the higher layers when faced with congestion caused by poor Layer 2 design. Another common mistake made by inexperienced network administrators and engineers is to assume that there is simply a lack of bandwidth and thus the solution chosen is a link bandwidth upgrade.

Take the network administrator view of the network. If complaints are received from the Telephone Sales Center that network performance is lacking, which link would he recommend be upgraded? Instinctively the inexperienced administrator orders the 100 Mbps between the Telephone Sales Center and the Data Center upgraded to Gigabit Ethernet. Embarrassingly, the upgrade will have no affect on the performance of the network.

The correct course of action is to set the Data Center switch as the STP root. This will allow the traffic flow as shown in Figure . It is possible that an upgrade to the link is still required but at least the full benefits of the upgrade will now be realized.

When considering a Layer 2 topology and STP, keep in mind the following:

- By default the STP root will be the switch with the lowest MAC address.
- The switch from which the majority of traffic is centered should be the STP root.
- Switches which provide access ports for server farms or the Internet gateway should be given consideration as the STP root. Often this is the distribution layer switch.
- The physical topology can be misleading. It must be determined which ports are blocking in order to trace the traffic path through the network.

Tools do exist that allow the mapping of the Layer 2 topology. When dealing with large complex networks or when required to work regularly with unfamiliar networks, these tools can be of great benefit. For simpler, familiar, networks, simply documenting the STP states within the LAN would be sufficient.