ZyXEL Communications NWA3550 - Manual (Page 97)

Chapter 7 Wireless Configuration
NWA3550 User’s Guide
97
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.
7.4.3 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 bridges 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.
7.4.4 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.
7.5 DFS
When you choose 802.11a in Access Point, Bridge/Repeater or AP + Bridge mode, the
ZyXEL Device uses DFS (Dynamic Frequency Selection) to give you a wider choice of
wireless channels.
DFS allows you to use channels in the frequency range normally reserved for radar systems.
Radar uses radio signals to detect the location of objects for military, meteorological or air
traffic control purposes. As long as your ZyXEL Device detects no radar activity on the
channel you select, you can use the channel to communicate. However, a wireless LAN
operating on the same frequency as an active radar system could disrupt the radar system.
Therefore, if the ZyXEL Device detects radar activity on the channel you select, it
automatically instructs the wireless clients to move to another channel, then resumes
communications on the new channel.
Table 21 STP Port States
PORT STATES DESCRIPTIONS
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.