Content

| 3.2 |  | STP Processes |
| 3.2.9 |  | Topology changes and STP |

Topology Changes and STP
STP implements a series of timers to prevent bridging loops from occurring within the network. It can take from 30 to 50 seconds for a network to converge to a new topology when there is a change in an otherwise stable STP process.

While the network is converging, physical addresses that can no longer be reached are still listed in the switch table. Because these addresses are in the table, the switch attempts to forward frames to devices it cannot reach. Fortunately, the STP change process requires the switch to clear the table faster in order to get rid of unreachable physical addresses.

Figure illustrates a link failure between Switches D and E, which in turn triggers a topology change condition. This topology change condition triggers a Topology Change Notification BPDU to be generated and sent toward the Root Bridge. In order for this BPDU to reach the Root Bridge, each switch forwards the update out the Root Port (RP) and to the Designated Port (DP) of each Designated Bridge along the path to the Root Bridge.

The topology change in this Spanning Tree network triggers the following steps:

Step 1 | Switch D notices that a change has occurred to a link.
Step 2 | Switch D sends a Topology change Notification (TCN) BPDU out the Root Port destined ultimately for the Root Bridge. The TCN BPDU is indicated by the value 0x80 in the BPDUs 1-byte Type field. The bridge sends out the TCN BPDU until the Designated Bridge for that segment responds with a Topology Change Acknowledgment (TCA) Configuration BPDU, indicated by the high-order bit in the 1-byte Flag field.
Step 3 | The Designated Bridge (Switch B) for that segment sends out a TCA configuration BPDU, to the originating bridge (Switch D). Switch B also sends a TCN BPDU out the Root Port destined for the Root Bridge.
Step 4 | When the Root Bridge receives the (upstream) topology-change message (TCN BPDU), it sends out Configuration BPDUs to indicate that a topology change is occurring (using the low-order bit in the Flag field). The Root Bridge sets the topology change in the configuration for a period of time equal to the sum of the Forward Delay and Max Age parameters.
Step 5 | A bridge receiving a (downstream) topology change configuration message from the Root Bridge will use the Forward Delay timer (15 seconds) to age out entries in the address table. This allows the device to age out entries faster than the normal 5-minute default so that stations no longer available are aged out faster. The bridge continues this process until it no longer receives topology change configuration messages from the Root Bridge.