Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 228

Understanding How STP Works Chapter 17 Configuring STP and MST • The shortest distance to the root bridge is calculated for each network device based on the path cost. • A designated bridge for each LAN segment is selected. This is the network device closest to the root bridge through which frames are forwarded to the root. • A root port is selected. This is the port providing the best path from the bridge to the root bridge. • Ports included in the spanning tree are selected. Election of the Root Bridge For each VLAN, the network device with the highest bridge ID (the lowest numerical ID value) is elected as the root bridge. If all network devices are configured with the default priority (32768), the network device with the lowest MAC address in the VLAN becomes the root bridge. 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 switch will be elected as the root bridge. Configuring a higher value increases the probability; a lower value decreases the probability. The STP root bridge is the logical center of the spanning tree topology in a Layer 2 network. All paths that are not needed to reach the root bridge from anywhere in the Layer 2 network are placed in STP blocking mode. BPDUs contain information about the transmitting bridge and its ports, including bridge and MAC addresses, bridge priority, port priority, and path cost. STP uses this information to elect the root bridge for the Layer 2 network, to elect the root port leading to the root bridge, and to determine the designated port for each Layer 2 segment. STP Protocol Timers Table 17-2 describes the STP protocol timers that affect STP performance. Table 17-2 STP Protocol Timers Variable Hello timer Forward delay timer Maximum age timer Description Determines how often the network device broadcasts hello messages to other network devices. Determines how long each of the listening and learning states last before the port begins forwarding. Determines the amount of time protocol information received on an port is stored by the network device. Creating the Spanning Tree Topology In Figure 17-1, Switch A is elected as the root bridge because the bridge priority of all the network devices is set to the default (32768) and Switch A has the lowest MAC address. However, due to traffic patterns, number of forwarding ports, or link types, Switch A might not be the ideal root bridge. By increasing the priority (lowering the numerical value) of the ideal network device so that it becomes the root bridge, you force an STP recalculation to form a new spanning tree topology with the ideal network device as the root. 17-4 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY OL-11439-03