ZyXEL Communications GS1350 Series User Manual page 145

higher the cost.
Table 59 STP Path Costs
L INK SPEED
Path Cost 4Mbps
Path Cost 10Mbps
Path Cost 16Mbps
Path Cost 100Mbps
Path Cost 1Gbps
Path Cost 10Gbps
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 switch 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.
Ho w ST P Wo rks
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.
ST P Po rt Sta te s
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 60 STP Port States
PO RT ST AT E DESC RIPT IO N
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.
Chapter 13 Spanning Tree Protocol
REC O MMENDED VAL UE
250
100
62
19
4
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REC O MMENDED RANG E
100 to 1000
50 to 600
40 to 400
10 to 60
3 to 10
1 to 5
AL L O WED RANG E
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535