OBJECTIVE
The purpose of this post is to present an introduction to the Details About RSTP.
P/A Mechanism
The Proposal/Agreement (P/A) mechanism helps a designated port to enter the Forwarding state as soon as possible. As shown in Figure 1, a new link is established between the root bridges S1 and S2. On S2, p2 is an alternate port; p3 is a designated port in the Forwarding state; p4 is an edge port.
Figure 1 - Schematic diagram for the P/A negotiation
The P/A mechanism works in the following process:
p0 and p1 become designated ports and send RST BPDUs.
After receiving an RST BPDU with a higher priority, p1 determines that it will become a root port but not a designated port. p1 then stops sending RST BPDUs.
p0 enters the Discarding state, and sends RST BPDUs with the Proposal field being 1.
After receiving an RST BPDU with the Proposal field being 1, S2 sets the sync variable to 1 for all its ports.
As p2 has been blocked, its status keeps unchanged; p4 is an edge port, and does not participate in calculation. Therefore, only the non-edge designated port p3 needs to be blocked.
After p2, p3, and p4 enter the Discarding state, their synced variables are set to 1. The synced variable of the root port p1 is then set to 1, and p1 sends an RST BPDU with the Agreement field being 1 to S1. Except for the Agreement field, which is set to 1, and the Proposal field, which is set to 0, the RST BPDU is the same as that was received.
After receiving this RST BPDU, S1 identifies it as a reply to the proposal that it just sent, and p0 immediately enters the Forwarding state.
This P/A negotiation process finishes, and S2 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 stat
To use the P/A mechanism, ensure that the link between the two devices is a P2P link infull-duplex mode. Once the P/A negotiation fails, a designated port can be selected by performing the STP negotiation after the forwarding delay timer expires twice.
RSTP Topology Change
In RSTP, if a non-edge port changes to the Forwarding state, the topology changes.
After a switching device detects the topology change (TC), it performs the following procedures:
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 switching device receives the RST BPDU, it clears the MAC addresses learned by all ports excluding the one that receives the RST BPDU. 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.
Interoperability Between RSTP and STP
When RSTP switches to STP, RSTP loses its advantages such as fast convergence.
On a network where both STP-capable and RSTP-capable devices are deployed, STP-capable devices ignore 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 after two Hello intervals and starts to send configuration BPDUs. In this manner, RSTP and STP are interoperable.
After STP-capable devices are removed, Huawei RSTP-capable datacom devices can switch back to the RSTP mode.
--- End
