19
FAST TRANSITION
Introduction The designated port fast transition mechanism of RSTP and MSTP uses two types
of protocol packets:
■ proposal packet: Requests for fast transition.
■ agreement packet: Permits the opposite end to perform fast state transition.
RSTP and MSTP request that a designated port of the upstream switch can
perform fast transition after receiving the agreement packet from the downstream
switch. RSTP and MSTP are different in the following:
■ For MSTP, the upstream switch sends the agreement packet to the downstream
switch first. After receiving the agreement packet, the downstream switch
sends the agreement packet to the upstream switch.
■ For RSTP, the upstream switch does not send the agreement packet to the
downstream switch.
Figure 33 and Figure 34 show the designated port fast transition mechanisms of
RSTP and MSTP.
Figure 33 Designated port fast transition mechanism of RSTP
Designated port
Root port
Upstream switch
Downstream switch
Send the proposal message to
request for fast transition
Send the agreement message
The root port congests other
non-edge ports and enters the
forwarding state, and then
Forwards the agreement
message to the upstream switch
The designated
port enters the
forwarding state
Designated port
Root port
Upstream switch
Downstream switch
Send the proposal message to
request for fast transition
Send the agreement message
The root port congests other
non-edge ports and enters the
forwarding state, and then
Forwards the agreement
message to the upstream switch
The designated
port enters the
forwarding state
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