Allied Telesis SwitchBlade x3100 Series Manual page 388

Introduction
FORWARDDELAY (default 15 seconds) - This value is used in the convergence process to allow for
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propagation of hello messages through the network. The timer represents how long ports are in the listen-
ing and learning states. By using this delay, the network has time for all the switches to agree on the spanning
tree configuration. If the timer is too short, ports may reach the forwarding state before a stable topology
has been reached. This may result in network loops that serious degrade overall network performance. If
the timer is too long, it will cause unnecessary delays in enabling the ports for passing bearer traffic. (At the
default timer, the network will require at least 30 seconds for ports to transition from "blocking" to "for-
warding", since each port will spend 15 seconds in the "listening" state and 15 seconds in the "learning"
state. All switches in the same spanning tree topology must use the same values for these parameters. The
parameter values actually used by each switch are those sent by the root bridge, and forwarded to all other
switches by the designated bridges.
Each switch that participates in the spanning tree (i.e. each switch in the extended LAN) must use the same val-
ues for these timers; otherwise, the convergence process would be unpredictable and unstable. To ensure that
the timer values are consistent throughout the network, the timers for all the switches are set to values config-
ured for the root bridge, once the identity of the root bridge has been determined.
The recommended relationship between the timer values can be expressed using the following formulae:
MAXAGE >= HELLOTIME x (number of network "hops" in longest path through network)
MAXAGE >= 2 x (HELLOTIME + 1 second)
MAXAGE <= 2 x (FORWARDDELAY - 1 second)
A timer value, Migrate Time, is added for RSTP. It defaults to a constant value of 3 seconds. This value
Note:
is not user-configurable.
To modify the parameters controlling these time intervals, use the command SET STP and the appropriate
parameter.
4.5.3.11 The Priority Parameter
The value of the PRIORITY parameter is used to set the writable portion of the bridge ID, for example, the
first two octets of the (8-octet long) Bridge Identifier. The remaining 6 octets of the bridge IDs are given by the
MAC address of the switch. The Bridge Identifier parameter is used in all Spanning Tree Protocol packets trans-
mitted by the switch. The first two octets, specified by the PRIORITY parameter, determine the switch's prior-
ity for becoming the root bridge or a designated bridge in the network, with a lower number indicating a higher
priority. In fairly simple networks, for instance those with a small number of switches in a meshed topology, it
may make little difference which switch is selected to be the root bridge, and no modifications may be needed
to the default PRIORITY parameter, which has a default value of 32768. In more complex networks, one or
more switches are likely to be more suitable candidates for the root bridge role, by virtue of being more cen-
trally located in the physical topology of the network. In these cases, the STP PRIORITY parameters for at least
one of the switches should be modified. To change the STP priority value, use the command SET STP PRI-
ORITY=bridge-priority, where bridge-priority is 0..65535 for STP mode and 0..65535 in steps of 4096
in RSTP mode. To restore STP timer and priority defaults, use the command SET STP DEFAULT.
4-68
Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)
Spanning Tree Protocol (STP and RSTP)