3com 8807 User Guide

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