User's Manual

6-4
Cisco 3200 Series Wireless MIC Software Configuration Guide
OL-7734-02
Chapter 6 Configuring Spanning Tree Protocol
Understanding Spanning Tree Protocol
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.
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.