2 how stp works, 3 stp port states – ZyXEL Communications ZyXEL Dimension ES-3024 User Manual
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Dimension ES-3024 Ethernet Switch
General, Switch and IP Setup
5-3
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 next table.
Table 5-2 STP Path Costs
LINK SPEED
RECOMMENDED VALUE
RECOMMENDED RANGE
ALLOWED 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
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.
5.5.2
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.
5.5.3
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 5-3 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.