System information

2-32
Configuring the Switch
2
connected to designated bridging devices are assigned as designated ports.
After determining the lowest cost spanning tree, it enables all root ports and
designated ports, and disables all other ports. Network packets are therefore only
forwarded between root ports and designated ports, eliminating any possible
network loops.
Once a stable network topology has been established, all bridges listen for Hello
BPDUs (Bridge Protocol Data Units) transmitted from the Root Bridge. If a bridge
does not get a Hello BPDU after a predefined interval (Maximum Age), the bridge
assumes that the link to the Root Bridge is down. This bridge will then initiate
negotiations with other bridges to reconfigure the network to reestablish a valid
network topology.
RSTP is designed as a general replacement for the slower, legacy STP. RSTP
achieves must faster reconfiguration (i.e., around one tenth of the time required by
STP) by reducing the number of state changes before active ports start learning,
predefining an alternate route that can be used when a node or port fails, and
retaining the forwarding database for ports insensitive to changes in the tree
structure when reconfiguration occurs.
Displaying Global Settings
Command Attributes
• Spanning Tree State – Shows if the switch is enabled to participate in an
STA-compliant network.
• Bridge ID – A unique identifier for this bridge, consisting of the bridge priority and
MAC address (where the address is taken from the switch system).
• Max Age – The maximum time (in seconds) a device can wait without receiving a
configuration message before attempting to reconfigure. All device ports (except
for designated ports) should receive configuration messages at regular intervals.
Any port that ages out STA information (provided in the last configuration
message) becomes the designated port for the attached LAN. If it is a root port, a
new root port is selected from among the device ports attached to the network.
(References to “ports” in this section mean “interfaces,” which includes both ports
and trunks.)
• Hello Time – Interval (in seconds) at which the root device transmits a
configuration message.
• Forward Delay – The maximum time (in seconds) the root device will wait before
changing states (i.e., discarding to learning to forwarding). This delay is required
because every device must receive information about topology changes before it
starts to forward frames. In addition, each port needs time to listen for conflicting
information that would make it return to a discarding state; otherwise, temporary
data loops might result.