|
|
7.2 | |
Spanning-Tree Protocol | |
| 7.2.5 | |
Stages of spanning-tree port states |
|
Time is required for protocol information
to propagate throughout a switched network. Topology changes in one
part of a network are not instantly known in other parts of the
network. There is propagation delay. A switch should not change a port
state from inactive to active immediately, as this may cause data
loops.
Each port on a switch that is using the Spanning-Tree Protocol has one
of five states, as shown in Figure
In the blocking state, ports can only receive BPDUs. Data frames are discarded and no addresses can be learned. It may take up to 20 seconds to change from this state. Ports go from the blocked state to the listening state. In this state, switches determine if there are any other paths to the root bridge. The path that is not the least cost path to the root bridge goes back to the blocked state. The listening period is called the forward delay and lasts for 15 seconds. In the listening state, user data is not being forwarded and MAC addresses are not being learned. BPDUs are still processed. Ports transition from the listening to the learning state. In this state user data is not forwarded, but MAC addresses are learned from any traffic that is seen. The learning state lasts for 15 seconds and is also called the forward delay. BPDUs are still processed. A port goes from the learning state to the forwarding state. In this state user data is forwarded and MAC addresses continue to be learned. BPDUs are still processed. A port can be in a disabled state. This disabled state can occur when an administrator shuts down the port or the port fails. The time values given for each state are the default values. These values have been calculated on an assumption that there will be a maximum of seven switches in any branch of the spanning tree from the root bridge.
|
.
Web Links