Huawei AR550 Series

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Figure 6-12 Proposal/Agreement negotiation process

1 Proposal

2 sync 2 sync 2 sync

3 Agreement

(Leaves the port

state unchanged)

(Blocks the

port)

(leaves the port

state unchanged)

Designated port

Alternate port

Edge port

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 field 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. Therefore, only the non-edge designated port p3 needs to be

blocked.

6. After p2, p3, and p4 enter the Discarding state, their synced variable is set to 1. The synced

variable of the root port p1 is also set to 1, and p1 sends an RST BPDU with the Agreement

field set to S1. This RST BPDU carries the same information as the one sent from the root

bridge S1, except that the Agreement field is set to 1 and the Proposal field is set to 0.

7. After S1 receives this RST BPDU, it identifies that the RST BPDU is sent in 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 starting 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.

Huawei AR530&AR550 Series Industrial Switch Routers

Configuration Guide - Ethernet Switching

6 STP/RSTP Configuration

Issue 01 (2014-11-30) Huawei Proprietary and Confidential

186

Huawei AR550 Series - Edge Port