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TRENDnet User's Guide
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On each bridge, the bridge communicates with the root through the root
port. The root port 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.
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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.
Forward Time (Forward Delay):
This is the maximum time (in seconds) the Switch will wait before changing
states. This delay is required because every switch must receive information
about topology changes before it starts to forward frames. In addition, each port
needs time to listen for conflicting information that would make it return to a
blocking state; otherwise, temporary data loops might result. The allowed range
is 4 to 30 seconds.
Max Age:
This is the maximum time (in seconds) the Switch can wait without receiving a
BPDU before attempting to reconfigure. All Switch ports (except for designated
ports) should receive BPDUs at regular intervals. Any port that age out STP
information (provided in the last BPDU) becomes the designated port for the
attached LAN. If it is a root port, a new root port is selected from among the
Switch ports attached to the network. The allowed range is 6 to 40 seconds.
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Hello Time:
This is the time interval in seconds between BPDU (Bridge Protocol Data Units)
configuration message generations by the root switch. The allowed range is 1 to
10 seconds.
PathCost:
Path cost is the cost of transmitting a frame on to a LAN through that port. It is
recommended to assign this value according to the speed of the bridge, the
slower the media, the higher the cost.
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.
802.1D STP
The Spanning Tree Protocol (STP) is a link layer network protocol that ensures a loop-
free topology for any bridged LAN. It is based on an algorithm invented by Radia
Perlman while working for Digital Equipment Corporation. In the OSI model for
computer networking, STP falls under the OSI layer-2. Spanning tree allows a network
design to include spare (redundant) links to provide automatic backup paths if an active
link fails, without the danger of bridge loops, or the need for manual enabling/disabling
of these backup links. Bridge loops must be avoided because they result in flooding the
network.
Managed Industrial L2 Switch
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