Asante Technologies 35516 - Manual (Page 46)

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4.3.3 Configuring Spanning Tree Protocol (STP)
The Spanning Tree Protocol (STP) is part of the IEEE 802.1D standard. It provides for a redundant network without
the redundant traffic through closed paths. For example, in a network without spanning tree protocol, the same
message will be broadcast through multiple paths, which may start an unending packet-passing cycle. This in turn
causes a great amount of extra network traffic, leading to network downtime. The STP reduces a network like this,
with multiple, redundant connections, to one in which all points are connected, but where there is only one path
between any two points (the connections span the entire network, and the paths are branched, like a tree).
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
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.
Router(config)# spanning-tree ?
forward-time Set forwarding delay time
hello-time Set interval between HELLOs
max-age Maximum allowed message age of received Hello BPDUs
priority Set bridge priority
rapid Enable rapid convergence
Router(config)# spanning-tree
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 period 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)