Scaling the Spanning Tree Protocol
197
Once the priority is set, use the show spantree [vlan] command to see
the port priority. Notice that, because both ports are equal 100Mbps ports, the
switch will use the path cost to determine the forwarding or root port. The
priority won't be used unless the path costs are the same. If both ports have
the same priority, the interface f0/26 will be used because it is a lower port
number:
1900A# sh spantree 1
[output cut]
Port FastEthernet 0/26 of VLAN1 is Forwarding
Port path cost 10, Port priority 128
Designated root has priority 8192, address
00E0.3488.FC00
Designated bridge has priority 8192, address
00E0.3488.FC00
Designated port is 2, path cost 0
Timers: message age 10, forward delay 7, hold 1
Port FastEthernet 0/27 of VLAN1 is Blocking
Port path cost 20, Port priority 16
Designated root has priority 8192, address
00E0.3488.FC00
Designated bridge has priority 8192, address
00E0.3488.FC00
Designated port is 1, path cost 0
Timers: message age 10, forward delay 7, hold 1
1900A#
By changing either the port priority or the port cost, you can basically just
persuade the switch to use your chosen paths. However, there are some mis-
cellaneous other STP variables that you can change. We'll discuss those next.
Changing the STP Timers
The timers are important in an STP network to stop network loops from
occurring. The different timers are used to give the network time to update
the correct topology information to all the switches and also to determine the
whereabouts of all the redundant links.
The problem with the STP timers is that, if a link goes down, it can take
up to 50 seconds for the backup link to take over forwarding frames. This is
Copyright ©2000 SYBEX , Inc., Alameda, CA
www.sybex.com