Synchronization Of Port Roles - Cisco IE-4000 Software Configuration Manual

Configuring MSTP
Information About Configuring MSTP
Figure 42 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

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 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
page 10.
Proposal
Root
Agreement
F
DP RP
Root
F
DP RP
Root
F
DP RP
Switch B
Designated
switch
F
Designated
switch
Proposal
F
Designated
switch
Agreement
F F
DP
341
Switch C
F
RP
Figure 5 on