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• For MSTP, the downstream device’s root port sends an agreement packet only after it receives an
agreement packet from the upstream device.
• For RSTP, the downstream device sends an agreement packet regardless of whether an agreement
packet from the upstream device is received.
Figure 24 sho
ws the rapid state transition mechanism on MSTP designated ports.
Figure 24 Rapid state transition of an MSTP designated port
Root port Designated port
Upstream device Downstream device
(1) Proposal for rapid transition
(3) Agreement
The root port blocks non-edge
ports.
The root port changes to the
forwarding state and sends an
Agreement to the upstream
device.
(2) Agreement
The designated port
changes to the
forwarding state.
Figure 25 shows rapid state transition of an RSTP designated port.
Figure 25 Rapid state transition of an RSTP designated port
If the upstream device is a third-party device, the rapid state transition implementation may be limited. For
example, when the upstream device uses a rapid transition mechanism similar to that of RSTP, and the
downstream device adopts MSTP and does not work in RSTP mode, the root port on the downstream
device receives no agreement packet from the upstream device and sends no agreement packets to the
upstream device. As a result, the designated port of the upstream device fails to transit rapidly and can
only change to the forwarding state after a period twice the forward delay.
You can enable the no agreement check feature on the downstream device’s port to enable the
designated port of the upstream device to transit its state rapidly.
Configuration prerequisites
Before you configure the no agreement check function, complete the following tasks:
• Connect a device is to a third-party upstream device supporting MSTP via a point-to-point link.
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