For example, the downtime caused by using the defaults would be the following:
2 * 15 + 20 = 50 seconds
Now that you have learned about the timers and how BPDUs operate in the network, let’s take a closer look at
how ports transition through different states before forwarding data.
STP Port States
Each port participating in STP transitions through four port states, or modes, in a designated order before the
port can forward frames it receives. These states are blocking, listening, learning, and forwarding. A fifth
state—the disabled state—can be manually configured by the switch.
Let’s look at the different port states and when each is used (see Figure 10.6):
Figure 10.6: The convergence process of the port states in Spanning Tree Protocol.
Blocking—The port will not forward frames. It merely accepts BPDUs the port receives and processes
them. All ports are in the blocking state by default when the switch is powered up. The port stays in a
blocked state if STP determines that a lower−cost path exists to the root bridge. The port does not put
any of the information it hears into the address table.
•
Listening—The port continues to process BPDUs to make sure no loops occur on the network before
it passes data frames. In this state the port is not forwarding frames or learning new addresses.
•
Learning—The port is not forwarding frames but is learning addresses and putting them in the
address table. The learning state is similar to the listening state, except the port can now add
information it has learned to the address table. The port is still not allowed to send or receive frames.
•
Forwarding—The port now begins to learn from the BPDUs and starts to build a filter table. A port is
not placed in a forwarding state until there are no redundant links or the port determines the lowest
cost path to the root bridge or switch.
•
Disabled—The port has been manually shut down by the network administrator or by the system due
to a hardware problem.
•
Let’s take a step−by−step look at what happens to a port when the switch is powered up:
After the switch’s initialization or startup, all the ports immediately go to a blocking state.1. After the configured MaxAge has been reached, the switch transitions from the blocking state to the
learning state.
2.
After the configured FwdDelay time has been reached, the port enters the learning state.3. After the configured FwdDelay has been reached in the learning state, the port either transitions into
forwarding mode or back to blocking mode. If STP has decided the port will be a forwarding port, the
port is placed in forwarding mode; but if the port is a higher−cost redundant link, the port is placed in
blocking mode again.
4.
Each port state can be manually modified using the Cisco IOS. If properly configured, the ports should create
a stable network, and the ports of each switch should transition to either a forwarding or blocking state.
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