Chapter 13 Spanning Tree Protocol
GS2200-24 User’s Guide
119
For each LAN segment, a designated bridge is selected. This bridge has the lowest
cost to the root among the bridges connected to the LAN.
13.3.2 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.
13.3.3 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.
13.3.4 Multiple RSTP
MRSTP (Multiple RSTP) is ZyXEL’s proprietary feature that is compatible with RSTP
and STP. With MRSTP, you can have more than one spanning tree on your Switch
and assign port(s) to each tree. Each spanning tree operates independently with
its own bridge information.
Table 25 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.
Note: The listening state does not exist in RSTP.
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.
