15-8
Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
78-17058-01
Chapter15 Configuring MSTP
Understanding RSTP
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.
Figure15-2 Proposal and Agreement Handshaking for Rapid Convergence
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 a ll ot her ports a re
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 (NNI on the Cisco ME switch) is in the forwarding state a nd 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 ro ot 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 co nnecte d by a poin t-to -poi nt li nk
are in agreement about their port roles, the RSTP immedi ate l y tran sitions the port states to forwarding.
The sequence of events is shown in Figure 15-3.
Proposal
Switch A Switch B
FF
RPDP
FF
RPDP
FF
RPDP
FF
RPDP
Switch C
88760
Agreement
Root Designated
switch
Root Designated
switch Proposal
Root Designated
switch Agreement
DP = designated port
RP = root port
F = forwarding