Bridging 95
T0008-16F Release 5.2M
Spanning Tree Protocol Entity (STPE)
Spanning Tree Timers
Introduction If the spanning tree converges to a final topology (it usually does, but
misconfiguration as discussed below can cause instability and lack of convergence),
the topology is maintained by timed messages initiated by the root bridge and sent
out its designated links. Subsequently, bridges receive the me ssa ge on the ir root li nk
and in turn pass the message along the spanning tree by transmitting it on their
designated links.
Timer Parameters The root bridge message has timer parameter values that all bridges should copy and
use. These timers are:
• Message Age
• Max Age (Bridge Parameters Record)
• Hello Time (Bridge Parameters Record)
• Forward Delay (Bridge Parameters Record)
Notice that the last three are parameters configured for each bridge. Once the root
bridge is determined, however, all the other bridges use the value in the root bridge
initiated message rather than their own configured values. The Max Age and Hello
Time are the two principal timers used by the spanning tree protocol for detecting a
fault condition.
When selecting values to configure these two parameters, consider the fact that the
bridge network is, when X.25 WAN circuits are used, overlaid onto an underlying
network which has its own timers and recovery procedures. It is important that the
two networks do not interfere with each other’s protocols, especially where timer
considerations are involved.
Hello Timer Hello messages are sent by the Root Bridge at specific time i ntervals . These interva ls
are determined by the Hello Timer parameter configured in the B ri dge Parameters
record.
If the bridges in the network receive these Hello messages, then this indicates to the
bridges that the Root Bridge is functioning and the path from the bridge to the Root
Bridge is functional as well.
If a bridge does not receive a Hello message from the Root Bridge within the time
allowed by the Max Age Timer parameter, then that bridge begins the process of
recalculating the spanning tree for the network.
Max Age The Max Age is a configurable parameter on the Bridge Parameters Record. This
parameter indicates to a bridge when to discard information about the Root Bridge
and the link to the Root Bridge.