3-14
System Configuration
Under the “System Configuration” tab, you can change the spanning tree settings and view the
Root Bridge Information. Note that switches are also referred to as bridges. Root bridges are
related to the spanning tree.
RSTP Mode
: You must first enable the STP or RSTP mode, before configuring any related
parameters. The parameter settings required for both STP and RSTP are the same. Note that
802.1d refers to STP mode, while 802.1w refers to the faster RSTP mode.
Priority (0-61440)
: This value must be in multiples of 4096 and represents the bridge ID number.
This number is used to identify the root bridge. A root bridge continuously transmits network
topology information to other bridges (switches) using the spanning tree protocol and serves to
notify other bridges on the network when topology changes are required. It is located where the
paths that frames take through the network are assigned, and usually centrally located to provide
the shortest path to other links on the network. Like a hub, a root bridge will forward frames out to
all of its ports. Every network should have one root bridge and it is usually assigned the lowest
bridge ID. The bridge with the lowest bridge ID number has the highest priority, and is selected
as the root bridge. If you happen to change this value, you must reboot the switch to cause the
system to assign a new path priority number.
Note that you increase this switches priority by selecting a lower number. A device with a lower
number has a higher bridge priority and thus a greater chance of being established as the root
bridge of the spanning tree topology.
Max Age (6-40)
: Enter a value from 6 to 40 seconds here. This value represents the time that a
bridge will wait without receiving Spanning Tree Protocol configuration messages before
attempting a reconfiguration.
If this switch is not the root bridge, and it has not received a hello message from the root bridge
in an amount of time equal to Max Age, then this switch will reconfigure itself as a root bridge.
Then once two or more devices on the network are recognized as a root bridge, the devices will
renegotiate to set up a new spanning tree topology.
Hello Time (1-10)
: Enter a value from 1 to 10 seconds here. This is the periodic timer that drives
the switch to send out the BPDU (Bridge Protocol Data Unit) packet to check on the current STP
status.
The root bridge of the spanning tree topology periodically sends out a “hello” message to other
devices on the network to check if the topology is “healthy”. The “hello time” is the amount of time
the root waits between sending hello messages.
Forward Delay Time (4-30)
: Enter a value between 4 and 30 seconds. This value is the time
that a port waits before changing from the Spanning Tree Protocol learning and listening states
to the forwarding state.
The is the amount of time this switch will wait before checking to see if it should change to a
different state.
Once you have completed your configuration, click on
Apply
to apply your settings.
Note
: You must observe the following rule to configure the Hello Time, Forwarding Delay, and
Max Age parameters.
2 × (Forward Delay Time – 1 sec)
□
Max Age Time
□
2 × (Hello Time value + 1 sec)
Suggested Strategy:
1. Select a Hello Time value and compute the rightmost portion of the rule above to get the
lower limit of Max Age.
2. Select a Forward Delay time and compute the leftmost portion of the rule above to get
the upper limit of Max Age.
3. Select a Max Age from 6 to 40 seconds that is between the limits computed above.
