ZyXEL Communications Zyair B-1000 v.2, Руководство пользователя

Страница: 56 / 237

ZyAIR Access Point Series User’s Guide
5-6 Wireless Configuration and Roaming
On each bridge, the root port is the port through which this bridge communicates with the root. It is the port
on this switch with the lowest path cost to the root (the root path cost). If there is no root port, then this
bridge has been accepted as the root bridge of the spanning tree network.
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.
5.3.3 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.
5.3.4 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.
Table 5-2 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.
5.4 Configuring Wireless
Click the WIRELESS link under ADVANCED to display the Wireless screen.
The screen varies depending upon the operating mode you select.