ZyXEL Communications ES-2108 User Manual

ES-2108 Series User’s Guide

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Chapter 11 Spanning Tree Protocol

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.

STP assigns five port states 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.  

Click Advanced Application, Spanning Tree Protocol in the navigation panel to display the 
status screen as shown next.  

Table 21   STP Port States

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.