EXP Computer S200 manual Spanning Tree Timers, Timer Parameters, Hello Timer, Max Age

	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 message on their root link
	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 intervals. These intervals
	are determined by the Hello Timer parameter configured in the Bridge 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.

Bridging 95

T0008-16F Release 5.2M