NOTE
The proposal/agreement mechanism applies only to P2P full-duplex links between two switching devices.
When proposal/agreement fails, a designated port is elected after two Forward Delay intervals, the same
as designated port election in STP mode.
RSTP Topology Changes
RSTP considers that the network topology has changed when a non-edge port transitions to the
Forwarding state.
When detecting a topology change, RSTP devices react as follows:
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The local device starts a TC While timer on each non-edge designated port. The TC While
timer value is two times the Hello timer value.
Within the TC While time, the local device clears MAC address entries learned on ports
whose states have changed.
At the same time, these ports send out RST BPDUs with the TC bit set to 1. When the TC
While timer expires, the ports stop sending RST BPDUs.
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When other switching devices receive RST BPDUs, they clear MAC address entries learned
on all their ports except the ports that receive the RST BPDUs. These switching devices
also start a TC While timer on each non-edge designated port and repeat the preceding
process.
RST BPDUs are then flooded on the entire network.
Interoperability with STP
RSTP can interoperate with STP, but its advantages such as fast convergence are lost when it
interoperates with STP.
On a network has both STP-capable and RSTP-capable devices, STP-capable devices drop RST
BPDUs. If a port on an RSTP-capable device receives a configuration BPDU from an STP-
capable device, the port switches to the STP mode and starts to send configuration BPDUs after
two Hello intervals. In this manner, RSTP and STP are interoperable.
After STP-capable devices are removed, Huawei RSTP-capable devices can switch back to the
RSTP mode.
6.3 Application
This section describes the typical application of STP/RSTP.
STP Application
Loops often occur on a complex network, because multiple physical links are often deployed
between two devices to implement link redundancy (one as the primary link and the others as
backup links). Loops may cause broadcast storms and damage MAC address entries on network
devices.
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.
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