Port States
In a multisegmented network connected through network bridges, each port assumes one of four states. The following table lists the states a port can assume.
| Port state | Description |
|---|---|
| Blocking | Inbound traffic is monitored only for STA packets. The bridge transmits only STA packets, no frames are forwarded to other ports. |
| Listening | Inbound traffic is monitored only for STA packets. This state is transitory and is only entered during topology changes. |
| Learning | Inbound traffic is examined and the forwarding table is updated according to the source MAC address of the frames. The bridge transmits only STA packets, no frames are forwarded to other ports. This state is transitory and is only entered during topology changes. |
| Forwarding | Inbound traffic is examined and the forwarding table is updated according to the source MAC address of the frames. The bridge transmits inbound frames through other ports as appropriate. Frames from other ports are transmitted through this specific port. |
The listening and the learning states both function as intermediate states. When a port is in the process of transitioning from the blocking state to the forwarding state, it is first placed in the listening state, and a timer is started. After the timer has expired, the port is moved to the learning state, and the timer is restarted. After the second timer has expired, the port is moved into the forwarding state.
The intermediate states ensure that ports are not immediately transitioned into an active forwarding state when information causing forwarding topology changes is still in the process of filtering through the network. It provides a grace period, within which ports can receive STA packets that might cause them to revert to the blocking state without having introduced topology loops. The intermediate states prevent the bridge from entering erroneous information into its forwarding table while changes are still being made to the forwarding topology.
See Also
Network Bridging Spanning Tree Algorithm
Last updated on Friday, April 09, 2004
© 1992-2003 Microsoft Corporation. All rights reserved.