Synchronization Of Port Roles - Cisco ONS 15454 SDH Configuration Manual

Synchronization of Port Roles

Figure 7-4
Switch A
Switch A
Switch A
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.
If a designated port is in the forwarding state, 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
Cisco ONS 15454 and Cisco ONS 15454 SDH Ethernet Card Software Feature and Configuration Guide, R8.0
7-12
Proposal and Agreement Handshaking for Rapid Convergence
Proposal
Root
Agreement
F
DP RP
Root
F
DP RP
Root
F
DP RP
Figure 7-5.
Switch B
F
Switch B
Proposal
F
Switch B
Agreement
F F
DP
Switch B
Chapter 7 Configuring STP and RSTP
Switch B
F
RP
Switch B