Five STP States | Cisco Catalyst LAN Switching

Five STP States

After the bridges have classified their ports as Root, Designated, or non-Designated, creating a loop-free topology is straightforward: Root and Designated Ports forward traffic, whereas non-Designated Ports block traffic. Although Forwarding and Blocking are the only two states commonly seen in a stable network, Table 6-3 illustrates that there are actually five STP states.

Table 6-3. STP States

State

Purpose

Forwarding

Sending/receiving user data

Learning

Building bridging table

Listening

Building "active" topology

Blocking

Receives BPDUs only

Disabled

Administratively down

You can view this list 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, 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 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 in the previous section take place. Ports that lose the Designated Port election become non-Designated 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. This is another 15-second period where the bridge is still not passing user data frames. Instead, the bridge is quietly building its bridging table as discussed in Chapter 3. 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.

Note

In addition to storing source MAC address and port information, Catalysts learn information such as the source VLAN.

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 6-11 illustrates the port states and possible transitions.