Synchronization Of Port Roles - Cisco Catalyst 3550 series Software Configuration Manual

Understanding RSTP
Figure 17-2 Proposal and Agreement Handshaking for Rapid Convergence
Switch A
DP = designated port
RP = root port
F = forwarding

Synchronization of Port Roles

When the switch 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 switch is synchronized with superior root information received on the root port if all other ports are
synchronized. An individual port on the switch is synchronized if
•
•
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 all of the ports are synchronized, the switch sends an agreement message to the designated
switch corresponding to its root port. When the switches 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
Catalyst 3550 Multilayer Switch Software Configuration Guide
17-8
Proposal
Root
Agreement
F
DP RP
Root
F
DP RP
Root
F
DP RP
That port is in the blocking state
It is an edge port (a port configured to be at the edge of the network)
Figure 17-3.
Switch B
Designated
switch
F
Designated
switch
Proposal
F
Designated
switch
Agreement
F F
DP
Chapter 17
Switch C
F
RP
Configuring MSTP
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