Cisco CISCO1401 Software Guide - Page 105

Chapter 8 Configuring Spanning Tree Protocol Understanding Spanning Tree Protocol The bridge maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge ID, consisting of the bridge priority and the bridge MAC address, is associated with each instance. For each VLAN, the bridge with the lowest bridge ID becomes the spanning-tree root for that VLAN. Bridge Protocol Data Units The stable, active spanning-tree topology of your network is determined by these elements: • The unique bridge ID (wireless bridge priority and MAC address) associated with each VLAN on each wireless bridge • The spanning-tree path cost to the spanning-tree root • The port identifier (port priority and MAC address) associated with each Layer 2 interface When the bridges in a network are powered up, each bridge functions as the STP root. The bridges send configuration BPDUs through the Ethernet and radio ports. The BPDUs communicate and compute the spanning-tree topology. Each configuration BPDU contains this information: • The unique bridge ID of the wireless bridge that the sending bridge identifies as the spanning-tree root • The spanning-tree path cost to the root • The bridge ID of the sending bridge • Message age • The identifier of the sending interface • Values for the hello, forward delay, and max-age protocol timers When a bridge receives a configuration BPDU that contains superior information (lower bridge ID, lower path cost, and so forth), it stores the information for that port. If this BPDU is received on the root port of the bridge, the bridge also forwards it with an updated message to all attached LANs for which it is the designated bridge. If a bridge receives a configuration BPDU that contains inferior information to that currently stored for that port, it discards the BPDU. If the bridge is a designated bridge for the LAN from which the inferior BPDU was received, it sends that LAN a BPDU containing the up-to-date information stored for that port. In this way, inferior information is discarded, and superior information is propagated on the network. A BPDU exchange results in these actions: • One bridge is elected as the spanning-tree root. • A root port is selected for each bridge (except the spanning-tree root). This port provides the best path (lowest cost) when the bridge forwards packets to the spanning-tree root. • The shortest distance to the spanning-tree root is calculated for each bridge based on the path cost. • A designated bridge for each LAN segment is selected. The designated bridge incurs the lowest path cost when forwarding packets from that LAN to the spanning-tree root. The port through which the designated bridge is attached to the LAN is called the designated port. • Interfaces included in the spanning-tree instance are selected. Root ports and designated ports are put in the forwarding state. • All interfaces not included in the spanning tree are blocked. OL-4059-01 Cisco Aironet 1400 Series Wireless Bridges Software Configuration Guide 8-3