Specifications

Switching 8-37
Software Release 2.7.3
C613-03098-00 REV A
By default, all VLANs, and therefore all ports, belong to the default STP. To add
or delete a VLAN and all the ports belonging to it from any other STP, use the
commands:
add stp=stp-name vlan={vlan-name|2..4094}
delete stp=stp-name vlan={vlan-name|2..4094|all}
The default STP is disabled by default at switch start up, and STPs created by a
user are disabled by default when they are created. To enable or disable STPs,
use the commands:
enable stp={stp-name|all}
disable stp={stp-name|all}
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: the hellotime, forwarddelay, and maxage parameters. All
switches in the same spanning tree topology must use the same values for
these parameters, but can themselves be configured with different, and
potentially incompatible time intervals. 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.
The hellotime parameter, with a default of 2 seconds, determines how often
the switch sends hello messages containing spanning tree configuration
information if it is the Root Bridge, or is trying to become the Root Bridge in the
network. Setting a shorter value for hellotime than the default of 2 seconds
makes the network more robust; setting a longer time uses less processing
overhead.
The maxage parameter, with a default of 20 seconds, determines the maximum
time that dynamic STP configuration information is stored in the switch, before
it is considered too old, and discarded. The value can be set at approximately
two seconds for every hop across the network. If this value is too small, the STP
may sometimes configure unnecessarily. If it is too long, there can be delays in
adapting to a change in the topology, for instance when a fault occurs.
The forwarddelay parameter is used to prevent temporary loops in the
network occurring in the briefly unstable topology while a topology change is
propagated through the network. When STP is running in standard mode and
a port that has been in the Blocking state is to move into the forwarding state, it
must first pass through the listening and learning states. The forwarddelay
parameter determines how long a port remains in each of these intermediate
states before moving to the forwarding state in the active topology; that is, half
the time between when it is decided that the port will become part of the
spanning tree, and when it is allowed to forward traffic. When STP is running
in rapid mode, a port must pass from the discarding state through the learning
state to reach the forwarding state. In this case, the forwarddelay parameter
should be at least half the time it takes for a topology change message to reach
the whole network. A value that is too short risks the temporary creation of
loops, which can seriously degrade switch performance. A longer value can
result in delays in the network after topology changes. The default
forwarddelay value is 15 seconds.
The forwarddelay, maxage and hellotime parameters should be set according to the
following formulae, as specified in IEEE 802.1d:
2 x (forwarddelay - 1.0 seconds) >= maxage
maxage >= 2 x (hellotime + 1.0 seconds)