How Stp Works; Stp Port States; Stp Status; Table 21 Stp Port States - ZyXEL Communications ES-2108 User Manual

ES-2108 Series User's Guide

11.1.2 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.

11.1.3 STP Port States

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.

Table 21 STP Port States

PORT STATE DESCRIPTION
Disabled
Blocking
Listening
Learning
Forwarding

11.2 STP Status

Click Advanced Application, Spanning Tree Protocol in the navigation panel to display the
status screen as shown next.
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STP is disabled (default).
Only configuration and management BPDUs are received and processed.
All BPDUs are received and processed.
All BPDUs are received and processed. Information frames are submitted to the
learning process but not forwarded.
All BPDUs are received and processed. All information frames are received and
forwarded.
Chapter 11 Spanning Tree Protocol