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Chapter 4: Web-Based Management
86 GE-DS-82 and 82-PoE Ethernet Managed Switch User Manual
Figure 58: STP Port State Transitions
You can modify each port state by using management software. When you enable
STP, every port on every switch in the network goes through the blocking state and
then transitions through the states of listening and learning at power up. If properly
configured, each port stabilizes to the forwarding or blocking state. No packets
(except BPDUs) are forwarded from, or received by, STP enabled ports until the
forwarding state is enabled for that port.
Illustration of STP
A simple illustration of three switches connected in a loop is depicted in the following
figure. In this example, you can anticipate some major network problems if the STP
assistance is not applied.
If switch A broadcasts a packet to switch B, switch B will broadcast it to switch C, and
switch C will broadcast it to back to switch A and so on. The broadcast packet will be
passed indefinitely in a loop, potentially causing a network failure. In this example,
STP breaks the loop by blocking the connection between switch B and C. The decision
to block a particular connection is based on the STP calculation of the most current
Bridge and Port settings.
Now, if switch A broadcasts a packet to switch C, then switch C will drop the packet
at port 2 and the broadcast will end there. Setting-up STP using values other than the
defaults, can be complex. Therefore, you are advised to keep the default factory
settings and STP will automatically assign root bridges/ports and block loop
connections. Influencing STP to choose a particular switch as the root bridge using