Specifications

CHAPTER 3
Spanning Tree
RSTP Port Roles
RSTP also defines different Spanning Tree roles for ports:
n Root port: The best path to the root (same as STP)
n Designated port: Same role as with STP
n Alternate port: A backup to the root port
n Backup port: A backup to the designated port
n Disabled port: Not used in the Spanning Tree
n Edge port: Connected only to an end user
BPDU Differences in RSTP
In regular STP, BPDUs are originated by the root and relayed by each switch. In RSTP, each switch originates BPDUs,
whether or not it receives a BPDU on its root port. All eight bits of the BPDU type field are used by RSTP. The TC and
TC Ack bits are still used. The other six bits specify the port’s role and its RSTP state and are used in the port handshake.
The RSTP BPDU is set to Type 2, Version 2. PVST is done by Rapid PVST+ on Catalyst switches.
RSTP Fast Convergence
The Rapid Spanning Tree process understands and incorporates topology changes much quicker than the previous version:
n RSTP uses a mechanism similar to BackboneFast: When an inferior BPDU is received, the switch accepts it. If
the switch has another path to the root, it uses that and informs its downstream switch of the alternative path.
n Edge ports work the same as Portfast ports: They automatically transition directly to forwarding.
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CCNP SWITCH 642-813 Quick Reference by Denise Donohue