
20-35
[AC A-GigabitEthernet0/0/1] quit
[AC A] stp config-digest-snooping
2) Enable Digest Snooping on AC B (the same as above, omitted)
Configuring No Agreement Check
Two types of messages are used for rapid state transition on designated RSTP and MSTP ports:
z Proposal: sent by designated ports to request rapid transition
z Agreement: used to acknowledge rapid transition requests
Both RSTP and MSTP switches can perform rapid transition operation on a designated port only when
the port receives an agreement packet from the downstream switch. The differences between RSTP
and MSTP switches are:
z For MSTP, the downstream device’s root port sends an agreement packet only after it receives an
agreement packet from the upstream device.
z For RSTP, the down stream device sends an agreement packet regardless of whether an
agreement packet from the upstream device is received.
Figure 20-7 and Figure 20-8 show the rapid state transition mechanism on MSTP and RSTP designated
ports.
Figure 20-7 Rapid state transition of a designated port in MSTP
A
g
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e
e
m
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n
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Figure 20-8 Rapid state transition of a designated port in RSTP
Root port
Designated port
Root port blocks other
non-edge ports , changes
to forwarding state and
sends Agreement to
upstream switch
Downstream switch
Upstream Switch
Proposal for rapid transition
Designated port
changes to
forwarding state
Ag
r
ee
m
e
nt
If the upstream device comes from another vendor, the rapid state transition implementation may be
limited. For example, when the upstream device adopts RSTP, the downstream device adopts MSTP
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