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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
• A designated switch is selected. This is the switch closest to the root switch through which
packets will be forwarded to the root.
• A port for each switch is selected. This is the port providing the best path from the switch to the
root switch.
• Ports included in the STP are selected.
Creating a Stable STP Topology
If all switches have STP enabled with default settings, the switch with the lowest MAC address in the
network will become the root switch. By increasing the priority (lowering the priority number) of the
best switch, STP can be forced to select the best switch as the root switch.
When STP is enabled using the default parameters, the path between source and destination stations
in a switched network might not be ideal. For instance, connecting higher-speed links to a port that has
a higher number than the current root port can cause a root-port change. The goal is to make the
fastest link the root port.
STP Port States
The BPDUs take some time to pass through a network. This propagation delay can result in topology
changes where a port that transitioned directly from a Blocking state to a Forwarding state could create
temporary data loops. Ports must wait for new network topology information to propagate throughout
the network before starting to forward packets. They must also wait for the packet lifetime to expire for
BPDU packets that were forwarded based on the old topology. The forward delay timer is used to allow
the network topology to stabilize after a topology change.
In addition, STP specifies a series of states a port must transition through to further ensure that a
stable network topology is created after a topology change.
Each port on a switch using STP exists is in one of the following five states:
• Blocking – the port is blocked from forwarding or receiving packets
• Listening – the port is waiting to receive BPDU packets that may tell the port to go back to the
blocking state
• Learning – the port is adding addresses to its forwarding database, but not yet forwarding
packets
• Forwarding – the port is forwarding packets
• Disabled – the port only responds to network management messages and must return to the
blocking state first
A port transitions from one state to another as follows:
• From initialization (switch boot) to blocking
• From blocking to listening or to disabled
• From listening to learning or to disabled
• From learning to forwarding or to disabled
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