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Understanding RSTP
Figure 15-2
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 (NNI on the Cisco ME switch) 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
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
15-8
The switch learns the link type from the port duplex mode: a full-duplex port is considered to have
a point-to-point connection; a half-duplex port is considered to have a shared connection. You can
override the default setting that is controlled by the duplex setting by using the spanning-tree
link-type interface configuration command.
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
15-3.
Chapter 15
Switch C
F
RP
Configuring MSTP
78-17058-01
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