RSTP implementation covers three aspects: P/A mechanism, RSTP topology change operation, and interoperability between RSTP and STP.
To allow a Huawei device to communicate with a non-Huawei device, a proper rapid transition mechanism needs to be configured on the Huawei device based on the Proposal/Agreement (P/A) mechanism on the non-Huawei device.
The P/A mechanism helps a designated port to enter the Forwarding state as soon as possible. As shown in Figure 1, the P/A negotiation is performed based on the following port variables:
This P/A negotiation process finishes, and Device B continues to perform the P/A negotiation with its downstream device.
Theoretically, STP can quickly select a designated port. To prevent loops, STP has to wait for a period of time long enough to determine the status of all ports on the network. All ports can enter the Forwarding state at least one forward delay later. RSTP is developed to eliminate this bottleneck by blocking non-root ports to prevent loops. By using the P/A mechanism, the upstream port can rapidly enter the Forwarding state.
In RSTP, if a non-edge port changes to the Forwarding state, the topology changes.
Start a TC While Timer for every non-edge port. The TC While Timer value doubles the Hello Timer value.
All MAC addresses learned by the ports whose status changes are cleared before the timer expires.
These ports send RST BPDUs with the TC field being 1. Once the TC While Timer expires, they stop sending the RST BPDUs.
After another device receives the RST BPDU, it clears the MAC addresses learned by all ports excluding the one that receives the RST BPDU and the edge. The device then starts a TC While Timer for all non-edge ports and the root port, the same as the preceding process.
In this manner, RST BPDUs flood the network.
To use the P/A mechanism, ensure that the link between the two devicesis a point to point (P2P) link in full-duplex mode. Once the P/A negotiation fails, a designated port can forward traffic only after the forwarding delay timer expires twice. This delay time is the same as that in STP.
When RSTP switches to STP, RSTP loses its advantages such as fast convergence.
On a network where both STP-enabled and RSTP-enabled devices are deployed, STP-enabled devices ignore RST BPDUs; if a port on an RSTP-enabled device receives a configuration BPDU from an STP-enabled device, the port switches to the STP mode after two Hello intervals and starts to send configuration BPDUs. In this manner, RSTP and STP are interoperable.
After STP-enabled devices are removed, Huawei RSTP-enabled datacom devices can switch back to the RSTP mode from the STP mode by running a command.