Synchronization Of Port Roles - Cisco 3845 - Security Bundle Router Software Manual

Chapter 14 Configuring MSTP
Figure 14-4
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 STP 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
OL-23400-01
Proposal and Agreement Handshaking for Rapid Convergence
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).
Switch B
Designated
switch
F
Designated
switch
Proposal
F
Designated
switch
Agreement
F F
DP
Figure 14-5.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
Understanding RSTP
Switch C
F
RP
14-11