Explanation/Reference:
Explanation:
Convergence
Cisco enhanced the original 802.1D specification with features such as Uplink Fast, Backbone Fast, and
Port Fast to speed up the convergence time of a bridged network. The drawback is that these mechanisms
are proprietary and need additional configuration.
Alternate and Backup Port Roles
These two port roles correspond to the blocking state of 802.1D. A blocked port is defined as not being the
designated or root port. A blocked port receives a more useful BPDU than the one it sends out on its
segment.
Remember that a port absolutely needs to receive BPDUs in order to stay blocked. RSTP introduces these
two roles for this purpose.
Rapid Transition to Forwarding State
Rapid transition is the most important feature introduced by 802.1w. The legacy STA passively waited for
the network to converge before it turned a port into the forwarding state. The achievement of faster
convergence was a matter of tuning the conservative default parameters (forward delay and max_age
timers) and often put the stability of the network at stake. The new rapid STP is able to actively confirm that
a port can safely transition to the forwarding state without having to rely on any timer configuration. There
is now a real feedback mechanism that takes place between RSTP-compliant bridges. In order to achieve
fast convergence on a port, the protocol relies upon two new variables: edge ports and link type.
Reference:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml