Tripp Lite NGI-S04C2 Owner's Manual page 43

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 ages 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.
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
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