All of the bridges (a switch is a complex bridge) on the network communicate with each other using special packets of
data called Bridge Protocol Data Units (BPDUs). The information exchanged in the BPDUs allows the bridges on the
network to do the following:
•
Elect a single bridge to be the root bridge
•
Calculate the shortest path from each bridge to the root bridge
•
Select a designated bridge on each segment, which lies closest to the root and forwards all traffic to it
•
Select a port on each bridge to forward traffic to the root
•
Select the ports on each bridge that forward traffic, and place the redundant ports in blocking states
5.4.1 Spanning Tree Parameters
The operation of the spanning tree algorithm is governed by several parameters. You can configure the following
parameters from global configuration mode: forward-time, hello-time, max-age, and priority.
Switch(config)#
spanning-tree mst?
forward-time Set forwarding delay time
hello-time Set interval between HELLOs
max-age Maximum allowed message age of received Hello BPDUs
mst Enable multiple spanning tree
priority Set bridge priority
rapid Enable rapid convergence
<cr>
Switch(config)#
Forward Time
After a recalculation of the spanning tree, the Forward Time parameter regulates the delay before each port begins
transmitting traffic. If a port begins forwarding traffic too soon (before a new root bridge has been selected), the
network can be adversely affected. The default value for Forward Time is 15 seconds.
Hello Time
This is the time between BPDUs transmitted by each bridge. The default setting is 2 seconds.
Maximum Age
Each bridge should receive regular configuration BPDUs from the direction of the root bridge. If the maximum age
timer expires before the bridge receives another BPDU, it assumes that a change in the topology has occurred, and it
begins recalculating the spanning tree. The default setting for Maximum Age is 20 seconds.
Note
: The above parameters (Hello Time, Maximum Age, and Forward Time) are constrained by the following
formula:
(Hello Time + 1) <= Maximum Age <= 2 x (Forward Delay – 1)
Priority
Setting the bridge priority to a low value will increase the likelihood that the current bridge will become the root bridge.
If the current bridge is located physically near the center of the network, decrease the Bridge Priority from its default
value of 32768 to make it become the root bridge. If the current bridge is near the edge of the network, it is best to
leave the value of the Bridge Priority at its default setting.
47
Asante IntraCore IC36240
User’s Manual
