H3C S7500 Series Operation Manual page 281

Operation Manual – MSTP
H3C S7500 Series Ethernet Switches
An upstream MSTP switch sends an agreement packet to the downstream switch;
and an MSTP downstream switch sends an agreement packet to the upstream
switch only after it receives an agreement packet from the upstream switch.
A upstream RSTP switch does not send agreement packets to the downstream
switch.
Figure 1-3
MSTP rapid transition mechanism respectively.
Designated port
changes to
forwarding state
Figure 1-3 The RSTP rapid transition mechanism
Designated port
changes to
forwarding state
Figure 1-4 The MSTP rapid transition mechanism
The combination of the RSTP rapid transition mechanism and the MSTP rapid
transition mechanism is limited. For example, when the upstream switch adopts RSTP,
the downstream switch adopts MSTP and 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
change to the Forwarding state after a period twice the Forward Delay.
and Figure 1-4 illustrate the RSTP rapid transition mechanism and the
Upstream
device
Proposal for rapid transition
Upstream switch
Proposal for rapid transition
Downstream
device
Downstream switch
Agreement
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Chapter 1 MSTP Configuration
Root port blocks other non-
edge port s, changes to
forwarding state and sends
Agreemen t to upstream device
R oot port
D esignated port
Root port bl ocks other
non-edge po rts
Root port c hanges to
forwarding s tate and sends
Agreemen t to upstream switch
Ro ot port
De signated port