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S12700 V200R010C00 Configuration Guide - Ethernet Switching

This document describes the configuration of Ethernet services, including configuring link aggregation, VLANs, Voice VLAN, VLAN mapping, QinQ, GVRP, MAC table, STP/RSTP/MSTP, SEP, and so on.
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RSTP Technology Details

RSTP Technology Details

The Proposal/Agreement mechanism enables a designated port to enter the Forwarding state quickly. In Figure 14-14, root bridge S1 establishes a link with S2. On S2, p2 is an alternate port, p3 is a designated port in Forwarding state, and p4 is an edge port.

Figure 14-14  Proposal/Agreement negotiation process

The Proposal/Agreement mechanism works as follows:
  1. p0 and p1 become designated ports and send RST BPDUs to each other.
  2. The RST BPDU sent from p0 is superior to that of p1, so p1 becomes a root port and stops sending RST BPDUs.
  3. p0 enters the Discarding state and sets the Proposal and Agreement fields in its RST BPDU to 1.
  4. After S2 receives an RST BPDU with the Proposal field set to 1, it sets the sync variable to 1 for all its ports.
  5. As p2 has been blocked, its state remains unchanged. p4 is an edge port and does not participate in calculation, so only the non-edge designated port p3 needs to be blocked.
  6. After the synced variable of each port is set to 1, p2 and p3 enter the Discarding state, and p1 enters the Forwarding state and returns an RST BPDU with the Agreement field being set to 1 to S1.
  7. After S1 receives this RST BPDU, it identifies that the RST BPDU is a response to the proposal that it has sent. Then p0 immediately enters the Forwarding state.

The proposal/agreement process can proceed to downstream devices.

STP can select designated ports quickly; however, to prevent loops, all ports must wait at least one Forward Delay interval before initiating data forwarding. RSTP blocks non-root ports to prevent loops and uses the proposal/agreement mechanism to shorten the time that an upstream port waits before transitioning to the Forwarding state.


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, 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:
  • The local device starts a TC While timer on each non-edge designated port and root port. The TC While timer value is twice the Hello Time value.

    Within the TC While time, the local device clears MAC address entries learned on all ports.

    At the same time, the non-edge designated ports and root ports send out RST BPDUs with the TC bit set to 1. When the TC While timer expires, the ports stop sending RST BPDUs.

  • 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 root port and repeat the preceding process.

RST BPDUs are then flooded on the entire network.

Interoperability with STP

RSTP can interoperate with STP, but doing so causes RSTP to lose its advantages, such as fast convergence.

On a network with both STP-capable and RSTP-capable devices, STP-capable devices discard 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 timer intervals.

After STP-capable devices are removed, Huawei RSTP-capable devices can be switched back to the RSTP mode.

Updated: 2019-08-21

Document ID: EDOC1000142081

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