Allied Telesis SwitchBlade x3100 Series Manual page 387

Spanning Tree Protocol (STP and RSTP)
In LAN environments where bridges are in use that are operating different revision levels of STP, all the bridges
must be configured to use compatible path cost value ranges. This will either require the older STP revision
level bridges to be reconfigured to use the ranges specified in the newer standard, or the bridges with newer
STP revisions will need to be configured to utilize the ranges from the older standard. The range of path cost
values available from the older STP standard may be insufficient to support the data rates available in newer
bridges.
The default PATHCOST values and the range of recommended PATHCOST values depend on the interface
speed (as indicated above). If the path cost for an interface is not explicitly set, it will vary as the speed of the
interface varies. Setting the path cost to a larger value on a particular interface is likely to reduce the traffic over
the LAN connected to it. This may be appropriate if the LAN has lower bandwidth, or if there are reasons for
limiting the traffic across it. To modify the STP interface path cost, use the command:
SET STP INTERFACE
If the path cost of an interface has been explicitly set to a particular value, it can be returned to its self-adjusting
default path cost and priority, using the following command:
SET STP INTERFACE={type:id-range|id-range|ifname-list|ALL} DEFAULT
Each interface also has a path cost, which is used if the interface is the root interface for the STP on the switch.
The path cost is added to the root path cost field in configuration messages received on the interface to deter-
mine the total cost of the path to the root bridge. To modify the STP interface path cost, use the command:
SET STP INTERFACE={type:id-range|id-range|ifname-list|ALL} PATHCOST=path-cost
The range of the path-cost value for STP mode is 1..65535. For RSTP mode, it is 1..200000000.
Note:
To display STP interface information, use the command:
SHOW STP INTERFACE[={type:id-range|id-range|ifname-list|ALL}]
4.5.3.10 STP Timer Control Parameters
The Spanning Tree Protocol uses three configurable parameters for the time intervals that control the flow of
STP information on which the dynamic STP topology depends:
HELLOTIME (default 2 seconds) - This value determines how often the switch sends hello messages if it is
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the root bridge, or if it is trying to determine the root bridge identity in the network. Setting a shorter value
makes the network more robust, in that network changes can be detected more rapidly. Setting a longer
value reduces network traffic and processing overhead.
MAXAGE (default 20 seconds) - This value determines the maximum "age" of dynamic spanning tree con-
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figuration information (e.g. the root bridge ID, designated ports, and root ports). If this information has not
been refreshed by hello messages before the timer expires, the information is discarded and the spanning
tree must reconverge. If this timer is too short, the spanning tree will undergo reconvergence unnecessarily,
resulting in network outages. If the timer is too long, the spanning tree may be slow to react to changes in
network topology.
Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)
Introduction
4-67