Cisco CISCO1401 Software Guide - Page 106

Understanding Spanning Tree Protocol Chapter 8 Configuring Spanning Tree Protocol Election of the Spanning-Tree Root All bridges in the Layer 2 network participating in STP gather information about other bridges in the network through an exchange of BPDU data messages. This exchange of messages results in these actions: • The election of a unique spanning-tree root for each spanning-tree instance • The election of a designated bridge for every LAN segment • The removal of loops in the network by blocking Layer 2 interfaces connected to redundant links For each VLAN, the bridge with the highest bridge priority (the lowest numerical priority value) is elected as the spanning-tree root. If all bridges are configured with the default priority (32768), the bridge with the lowest MAC address in the VLAN becomes the spanning-tree root. The bridge priority value occupies the most significant bits of the bridge ID. When you change the bridge priority value, you change the probability that the bridge will be elected as the root bridge. Configuring a higher value decreases the probability; a lower value increases the probability. The spanning-tree root is the logical center of the spanning-tree topology. All paths that are not needed to reach the spanning-tree root from anywhere in the network are placed in the spanning-tree blocking mode. BPDUs contain information about the sending bridge and its ports, including bridge and MAC addresses, bridge priority, port priority, and path cost. STP uses this information to elect the spanning-tree root and root port for the network and the root port and designated port for each LAN segment. Spanning-Tree Timers Table 8-1 describes the timers that affect the entire spanning-tree performance. Table 8-1 Spanning-Tree Timers Variable Hello timer Forward-delay timer Maximum-age timer Description Determines how often the bridge broadcasts hello messages to other bridges. Determines how long each of the listening and learning states last before the interface begins forwarding. Determines the amount of time the bridge stores protocol information received on an interface. Creating the Spanning-Tree Topology In Figure 8-1, Bridge 4 is elected as the spanning-tree root because the priority of all the bridges is set to the default (32768) and Bridge 4 has the lowest MAC address. However, because of traffic patterns, number of forwarding interfaces, or link types, Bridge 4 might not be the ideal spanning-tree root. By increasing the priority (lowering the numerical value) of the ideal bridge so that it becomes the spanning-tree root, you force a spanning-tree recalculation to form a new topology with the ideal bridge as the spanning-tree root. Cisco Aironet 1400 Series Wireless Bridges Software Configuration Guide 8-4 OL-4059-01