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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 19 Configuring Standard-Compliant IEEE MST
Understanding RSTP
Figure 19-4 Proposal and Agreement Handshaking for Rapid Convergence
Synchronization of Port Roles
When the router receives a proposal message on one of its ports and that port is selected as the new root
port, the RSTP forces all other ports to synchronize with the new root information.
The router is synchronized with superior root information received on the root port if all other ports are
synchronized. An individual port on the router is synchronized if:
• That port is in the blocking state.
• It is an edge port (a port configured to be at the edge of the network).
If a designated port is in the forwarding state and is not configured as an edge port, it transitions to the
blocking state when the RSTP forces it to synchronize with new root information. In general, when the
RSTP forces a port to synchronize with root information and the port does not satisfy any of the above
conditions, its port state is set to blocking.
After ensuring that all of the ports are synchronized, the router sends an agreement message to the
designated router corresponding to its root port. When the routers connected by a point-to-point link are
in agreement about their port roles, the RSTP immediately transitions the port states to forwarding. The
sequence of events is shown in Figure 19-5.
Proposal
Switch A Switch B
FF
RPDP
FF
RPDP
FF
RPDP
FF
RPDP
Switch C
88760
Agreement
Root
Designated
switch
Root
Designated
switch
Proposal
Root
Designated
switch
Agreement
DP = designated port
RP = root port
F = forwarding
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