3Com 7757 Configuration Manual page 271

on the downstream switch receives no agreement packet from the upstream
switch and thus sends no agreement packets to the upstream switch. As a result,
the designated port of the upstream switch fails to transit rapidly and can only
change to the Forwarding state after a period twice the Forward Delay.
Some partners' switches adopt proprietary spanning tree protocols that are similar
to RSTP in the way to implement rapid transition on designated ports. When a
switch of this kind operating as the upstream switch connects with the 3Com
series switch running MSTP, the upstream designated port fails to change their
states rapidly.
The rapid transition feature is developed to resolve this problem. When a 3Com
series switch running MSTP is connected in the upstream direction to a partner's
switch running proprietary spanning tree protocol, you can enable the rapid
transition feature on the ports of the 3Com series switch operating as the
downstream switch. Among these ports, those operating as the root ports will
then send agreement packets to their upstream ports after they receive proposal
packets from the upstream designated ports, instead of waiting for agreement
packets from the upstream switch. This enables designated ports of the upstream
switch to change their states rapidly.
Rapid Transition Prerequisites
Configuration
As shown in Figure 64, a 3Com series switch is connected to a partner's switch.
The former operates as the downstream switch, and the latter operates as the
upstream switch. The network operates normally.
The upstream switch is running a proprietary spanning tree protocol that is similar
to RSTP in the way to implement rapid transition on designated ports. Port 1 is a
designated port.
The downstream switch is running MSTP. Port 2 is the root port.
Figure 64 Network diagram for rapid transition configuration
Third - party switch
Port 1
Port 2
3Com switch
Rapid Transition Configuration 271