Chapter 13 Spanning Tree Protocol
GS1915 Series User’s Guide
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higher the cost.
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.
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.
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.
Table 61 STP Path Costs
LINK SPEED
RECOMMENDED VALUE
RECOMMENDED RANGE
ALLOWED RANGE
Path Cost 4 Mbps
250
100 to 1000
1 to 65535
Path Cost 10 Mbps
100
50 to 600
1 to 65535
Path Cost 16 Mbps
62
40 to 400
1 to 65535
Path Cost 100 Mbps
19
10 to 60
1 to 65535
Path Cost 1 Gbps
4
3 to 10
1 to 65535
Path Cost 10 Gbps
2
1 to 5
1 to 65535
Table 62 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.
