H3C S7500 Series Ethernet Switches Operation Manual

Operation Manual – MSTP
H3C S7500 Series Ethernet Switches Chapter 1
MSTP Configuration
1-37
z 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.
z A upstream RSTP switch does not send agreement packets to the downstream
switch.
Figure 1-3 and Figure 1-4 illustrate the RSTP rapid transition mechanism and the
MSTP rapid transition mechanism respectively.
Upstream
device
Downstream
device
Root port
edge port
forwarding
Agreemen
R
D
Proposal for rapid transition
A
g
r
e
e
m
e
n
t
Designated port
changes to
forwarding state
blocks other non-
s, changes to
state and sends
t to upstream device
oot port
esignated port
Figure 1-3 The RSTP rapid transition mechanism
Root port bl
non-edge po
Upstream switch Downstream switch
Root port c
forwarding s
Agreemen
Designated port
changes to
forwarding state
Proposal for rapid transition
Agreement
A
g
r
e
e
m
e
n
t
Ro
De
ocks other
rts
hanges to
tate and sends
t to upstream switch
ot port
signated port
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.