5.2 STP Processes
5.2.5 Spanning-Tree port states
After the bridges have classified their ports as root, designated, or nondesignated, creating a loop-free topology is fairly straightforward: root and designated ports forward traffic, whereas nondesignated ports block traffic. Although forwarding and blocking are the only two states commonly seen in a stable network, Figure illustrates that there are actually five STP states.

This list can be examined as a hierarchy in that bridge ports start at the bottom (disabled or blocking) and work their way up to forwarding. The disabled state allows network administrators to manually shut down a port. It is not part of the normal, dynamic port processing. After initialization of the switch, ports start in the blocking state where they listen for BPDUs.

A variety of events (such as a bridge thinking it is the root bridge immediately after booting or an absence of BPDUs for a certain period of time) can cause the bridge to transition into the listening state. At this point, no user data is being passed. The port is sending and receiving BPDUs in an effort to determine the active topology. It is during the listening state that the three initial convergence steps discussed previously take place. Ports that lose the designated port election become nondesignated ports and drop back to the blocking state.

Ports that remain designated or root ports after 15 seconds (the default timer value) progress into the learning state, another 15-second period where the bridge is still not passing user data frames. Instead, the switch is building its bridging table and gathering other information such as source VLANs. As the bridge receives frames, it places the source MAC address and port into the bridging table. The learning state reduces the amount of flooding required when data forwarding begins.

If a port is still a designated or root port at the end of the learning state period, the port transitions into the forwarding state. At this stage, it finally starts sending and receiving user data frames. Figure illustrates the port states and possible transitions. Figure illustrates a sample network with the port classifications and states listed. Notice that all ports are forwarding except Cat-C Port-1/2.

Lab Activity
  In this lab activity, you will observe the default behavior of STP - Spanning-Tree Protocol.
Lab Activity
  In this lab activity, you will learn how to modify the default behavior of spanning-tree by using root bridge and port priorities.