INS_CWGE24MS2_REV–
10/05/16 PAGE 174
INSTALLATION AND OPERATION MANUAL
CWGE24MS2
TECH SUPPORT: 1.888.678.9427
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.
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
