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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.