ZyXEL Communications 802.11b/g/n poe access point ユーザーズマニュアル
Chapter 10 IP Screen
NWA1100-N User’s Guide
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Path cost is the cost of transmitting a frame onto a LAN through that port. It is assigned according
to the speed of the link to which a port is attached. The slower the media, the higher the cost - see
the following table.
to the speed of the link to which a port is attached. The slower the media, the higher the cost - see
the following table.
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.
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.
among the bridges connected to the LAN.
10.5.2.3 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.
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.
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.
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.
10.5.2.4 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.
allowed to go directly from blocking state to forwarding state so as to eliminate transient loops.
Table 25
STP Path Costs
LINK SPEED
RECOMMENDED
VALUE
VALUE
RECOMMENDED
RANGE
RANGE
ALLOWED
RANGE
RANGE
Path Cost
4Mbps
250
100 to 1000
1 to 65535
Path Cost
10Mbps
100
50 to 600
1 to 65535
Path Cost
16Mbps
62
40 to 400
1 to 65535
Path Cost
100Mbps
19
10 to 60
1 to 65535
Path Cost
1Gbps
4
3 to 10
1 to 65535
Path Cost
10Gbps
2
1 to 5
1 to 65535
Table 26
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.
learning process but not forwarded.
Forwarding
All BPDUs are received and processed. All information frames are received and
forwarded.
forwarded.