3Com 4200G 12-Port Configuration Manual page 157

Figure 40 The MSTP rapid transition mechanism
Limitation on the combination of RSTP and MSTP exists to implement rapid transition.
For example, when the upstream switch adopts RSTP, the downstream switch adopts
MSTP and does not support RSTP mode, the root port 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 operates as the upstream switch of an S4200G 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 an S4200G
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 S4200G 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 41, an S4200G 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.
Upstream sw itch Upstream sw itch
Send proposal packets Send proposal packets
to request rapid transition to request rapid transition
Send agreement packets Send agreement packets
Send agreement packets Send agreement packets
Designated port Designated port
change to change to
Forw arding state Forw arding state
Rapid Transition Configuration 143
Dow nstream sw itch Dow nstream sw itch
Root port blocks Root port blocks
other non-edge ports other non-edge ports - -
Root port changes to Root port changes to
Forw arding state and Forw arding state and
sends agreement packets sends agreement packets
to upstream switch to upstream switch
Root port Root port
Designated port Designated port