1-40
z Proposal packets: Packets sent by designated ports to request rapid transition
z Agreement packets: Packets used to acknowledge rapid transition requests
Both RSTP and MSTP specify that the upstream switch can perform rapid transition operation on the
designated port only when the port receives an agreement packet from the downstream switch. The
difference between RSTP and MSTP are:
z For MSTP, the upstream switch sends agreement packets to the downstream switch; and the
downstream switch sends agreement packets to the upstream switch only after it receives
agreement packets from the upstream switch.
z For RSTP, the upstream switch does not send agreement packets to the downstream switch.
Figure 1-6 and Figure 1-7 illustrate the rapid transition mechanisms on designated ports in RSTP and
MSTP.
Figure 1-6 The RSTP rapid transition mechanism
Root port blocks other non-
edge ports, changes to
forwarding state and sends
Agreement to upstream device
Downstream switch
Upstream switch
Proposal for rapid transition
A
g
r
e
e
m
e
n
t
Designated port
changes to
forwarding state
Root port
Designated port
Figure 1-7 The MSTP rapid transition mechanism
Downstream switch
Root port blocks other
non- edge ports
Root port changes to
forwarding state and
sends Agreement to
upstream switch
Designated port
changes to
forwarding state
Upstream switch
Proposal for rapid transition
Agreement
Root port
Designated port
The cooperation between MSTP and RSTP is limited in the process of rapid transition. For example,
when the upstream switch adopts RSTP, the downstream switch adopts MSTP and the downstream
switch does not support RSTP-compatible 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 turn to
the forwarding state after a period twice the forward delay.
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