Election Of The Root Bridge - Cisco Catalyst 4500 Series Software Configuration Manual

About STP
•
•
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:
•
•
•
•
•

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 23-3
Table 23-3
Variable
hello_time
forward_time
max_age
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
23-4
The identifier of the transmitting port
Values for the hello, forward delay, and max-age protocol timers
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.
describes the STP timers that affect the performance of the entire spanning tree.
Spanning Tree Protocol Timers
Description
Determines how often the switch broadcasts hello messages to other
switches.
Determines how long each of the listening and learning states last before the
port begins forwarding.
Determines the amount of time that protocol information received on a port
is stored by the switch.
Chapter 23 Configuring STP and MST