Of the four switches in Figure 22-15, the administrator decides that Switch A
is the most centrally located in the network and is the least likely to be moved
or redeployed. For these reasons, the administrator selects it as the root bridge
for the spanning tree. The administrator configures Switch A with the highest
priority and uses the default priority values for Switch B, Switch C, and
For all switches, the administrator also configures ports 4–17 in Port Fast
mode because these ports are connected to hosts and can transition directly
to the Forwarding state to speed up the connection time between the hosts
and the network.
The administrator also configures Port Fast BPDU filtering and Loop Guard
to extend STP's capability to prevent network loops. For all other STP
settings, the administrator uses the default STP values.
To configure the switch:
1 Connect to Switch A and configure the priority to be higher (a lower
value) than the other switches, which use the default value of
console(config)#spanning-tree priority 8192
2 Configure ports 4–20 to be in Port Fast mode.
console(config)#interface range gi1/0/4-20
3 Enable Loop Guard on ports 1–3 to help prevent network loops that might
be caused if a port quits receiving BPDUs.
console(config)#interface range gi1/0/1-3
console(config-if)#spanning-tree guard loop
4 Enable Port Fast BPDU Filter. This feature is configured globally, but it
affects only Port Fast-enabled access ports.
console(config)#spanning-tree portfast bpdufilter
complete the configuration.
on Switch B, Switch C, and Switch D to
Configuring the Spanning Tree Protocol