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18-4
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 18 Configuring STP and MST
About STP
• The identifier of the transmitting port
• Values for the hello, forward delay, and max-age protocol timers
When a switch transmits a BPDU frame, all switches connected to the LAN on which the frame is
transmitted receive the BPDU. When a switch receives a BPDU, it does not forward the frame but
instead uses the information in the frame to calculate a BPDU and, if the topology changes, initiate a
BPDU transmission.
A BPDU exchange results in the following:
• One switch is elected as the root bridge.
• The shortest distance to the root bridge is calculated for each switch based on the path cost.
• A designated bridge for each LAN segment is selected. it is the switch closest to the root bridge
through which frames are forwarded to the root.
• A root port is selected. it 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 switch with the highest bridge priority (the lowest numerical priority value) is
elected as the root bridge. If all switches are configured with the default priority value (32,768), the
switch with the lowest MAC address in the VLAN becomes the root bridge.
The spanning tree root bridge is the logical center of the spanning tree topology in a switched network.
All paths that are not required to reach the root bridge from anywhere in the switched network are placed
in spanning tree blocking mode.
A spanning tree uses the information provided by BPDUs to elect the root bridge and root port for the
switched network, as well as the root port and designated port for each switched segment.
STP Timers
Table 18-3 describes the STP timers that affect the performance of the entire spanning tree.
Table 18-3 Spanning Tree Protocol Timers
Variable Description
hello_time Determines how often the switch broadcasts hello messages to other
switches.
forward_time Determines how long each of the listening and learning states last before the
port begins forwarding.
max_age Determines the amount of time that protocol information received on a port
is stored by the switch.