Configuring the Switch
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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 (Rapid Spanning Tree Protocol, IEEE 802.1w) is designed as a general
replacement for the slower, legacy Spanning Tree Protocol (STP, IEEE 802.1D).
RSTP achieves much faster reconfiguration (i.e., around 1 to 3 seconds, compared
to 30 seconds or more for 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.
Configuring RSTP
Use the RSTP Configuration page to specify global or port-specific parameters for
the Rapid Spanning Tree Protocol.
Field Attributes
RSTP System Configuration
• System Priority – Bridge priority is used in selecting the root device, root port, and
designated port. The device with the highest priority becomes the STA root device.
However, if all devices have the same priority, the device with the lowest MAC
address will then become the root device. (Note that lower numeric values indicate
higher priority.) (Default: 32768; Range: 0-61440, in steps of 4096)
• Hello Time – Interval (in seconds) at which the root device transmits a
configuration message.
- Default: 2
- Minimum: 1,
- Maximum: The lower of 10 or [(Max. Message Age / 2) - 1]
• 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.
- Default: 20
- Minimum: The higher of 6 or [2 x (Hello Time + 1)]
- Maximum: The lower of 40 or [2 x (Forward Delay - 1)]
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