After the topology becomes stable, the root bridge still sends configuration BPDUs at intervals
specified by the Hello timer. Each non-root bridge forwards the received configuration BPDUs
through its designated port. When a non-root bridge receives a superior configuration BPDU on
a port, the non-root bridge updates the configuration BPDU on the port based on the information
carried in the received configuration BPDU.
STP Topology Changes
Figure 6-9 shows the packet transmission process after an STP topology change.
Figure 6-9 Packet transmission after a topology change
Root Bridge
Root Bridge
T
2
nd
Step:The root advertises the TC
for max_age+ forward delay
A topology change is generated on
Point T. 1
st
Step: A TCN is going
up to the root.
1. After the network topology changes, a downstream device continuously sends TCN BPDUs
to the upstream device.
2. The upstream device processes only the TCN BPDUs received on the designated port and
drops TCN BPDUs on other ports.
3. The upstream device sets the TCA bit of the Flags field in the configuration BPDUs to 1
and returns the configuration BPDUs to instruct the downstream device to stop sending
TCN BPDUs.
4. The upstream device sends a copy of the TCN BPDUs toward the root bridge.
5. Steps 1, 2, 3 and 4 are repeated until the root bridge receives the TCN BPDUs.
6. The root bridge sets the TC bit of the Flags field in the configuration BPDUs to 1 to instruct
the downstream devices to delete MAC address entries.
Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
6 STP/RSTP Configuration
Issue 01 (2014-11-30) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
179
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