ZyXEL Communications ZyXEL Dimension ES-3124 - Manual (Page 100)

Dimension ES-3124 Ethernet Switch
10-2 Spanning Tree Protocol
10.1.2 How STP Works
After a bridge determines the lowest cost-spanning tree with STP, it enables the root port and the ports that are the
designated ports for connected LANs, and disables all other ports that participate in STP. Network packets are
therefore only forwarded between enabled ports, eliminating any possible network loops.
STP-aware switches exchange Bridge Protocol Data Units (BPDUs) periodically. When the bridged LAN
topology changes, a new spanning tree is constructed.
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 (Max
Age), the bridge assumes that the link to the root bridge is down. This bridge then initiates negotiations with other
bridges to reconfigure the network to re-establish a valid network topology.
10.1.3 STP Port States
STP assigns five port states (see next table) to eliminate packet looping. A bridge port is not allowed to go directly
from blocking state to forwarding state so as to eliminate transient loops.
Table 10-2 STP Port States
PORT STATE DESCRIPTION
Disabled STP is disabled (default).
Blocking Only configuration and management BPDUs are received and processed.
Listening All BPDUs are received and processed.
Learning All BPDUs are received and processed. Information frames are submitted to the learning
process but not forwarded.
Forwarding All BPDUs are received and processed. All information frames are received and forwarded.
10.2 STP Status
Click Advanced Application and then Spanning Tree Protocol in the navigation panel to display the STP status
as shown in the screen next.