A disabled interface performs these functions: • Discards frames received on the interface • Discards frames that are switched from another interface for forwarding • Does not learn addresses • Does not receive BPDUs How a Device or Port Becomes the Root Device or Root Port If all devices in a network are enabled with default spanning-tree settings, the device with the lowest MAC address becomes the root device. Figure 2: Spanning-Tree Topology Switch A is elected as the root device because the device priority of all the devices is set to the default (32768) and Switch A has the lowest MAC address.
However, because of traffic patterns, number of forwarding interfaces, or link types, Switch A might not be the ideal root device. By increasing the priority (lowering the numerical value) of the ideal device so that it becomes the root device, you force a spanning-tree recalculation to form a new topology with the ideal device as the root. When the spanning-tree topology is calculated based on default parameters, the path between source and destination end stations in a switched network might not be ideal. For instance, connecting higher-speed links to an interface that has a higher number than the root port can cause a root-port change. The goal is to make the fastest link the root port.
For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on Switch B (a 10/100 link) is the root port. Network traffic might be more efficient over the Gigabit Ethernet link. By changing the spanning-tree port priority on the Gigabit Ethernet port to a higher priority (lower numerical value) than the root port, the Gigabit Ethernet port becomes the new root port. Spanning Tree and Redundant Connectivity You can create a redundant backbone with spanning tree by connecting two switch interfaces to another device or to two different devices. Spanning tree automatically disables one interface but enables it if the other one fails.
If one link is high-speed and the other is low-speed, the low-speed link is always disabled. If the speeds are the same, the port priority and port ID are added together, and spanning tree disables the link with the highest value. Figure 3: Spanning Tree and Redundant Connectivity, on page 8 shows redundant connectivity on a spanning tree topology. Configuring Spanning Tree Protocol How a Device or Port Becomes the Root Device or Root Port Configuring Spanning Tree Protocol 7