D-Link DES-3226 User Manual - Stp Port States

DES-3226 NWay Standalone Fast Ethernet Switch User’s Guide 

•  The port identifier of the transmitting port 

The Switch sends BPDUs to communicate and construct the spanning-tree topology. All switches 

connected to the LAN on which the packet is transmitted will receive the BPDU. BPDUs are not directly 

forwarded by the switch, rather the receiving switch uses the information in the frame to calculate a 

BPDU, and, if the topology changes, initiates a BPDU transmission. 

The communication between switches via BPDUs results in the following: 

•  One switch is elected as the root switch 

•  The shortest distance to the root switch is calculated for each switch 

•  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 

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