Why does RSTP have a better convergence time than 802.1D?
A.
it is newer
B.
it has smaller timers
C.
it has less overhead
D.
it is not timer-based
Explanation:
RSTP identifies certain links as point to point. When a point-to-point link fails, the alternate link can
transition to the forwarding state.
Although STP provides basic loop prevention functionality, it does not provide fast network
convergence when there are topology changes. STP’s process to determine network state
transitions is slower than RSTP’s because it is timer-based. A device must reinitialize every time a
topology change occurs. The device must start in the listening state and transition to the learning
state and eventually to a forwarding or blocking state.
When default values are used for the maximum age (20 seconds) and forward delay (15 seconds),
it takes 50 seconds for the device to converge. RSTP converges faster because it uses a
handshake mechanism based on point-to-point links instead of the timer-based process used by
STP.
An RSTP domain running switch has the following components:A root port, which is the “best path” to the root device.
A designated port, indicating that the switch is the designated bridge for the other switch
connecting to this port.
An alternate port, which provides an alternate root port.
A backup port, which provides an alternate designated port.
Port assignments change through messages exchanged throughout the domain. An RSTP device
generates configuration messages once every hello time interval. If an RSTP device does not
receive a configuration message from its neighbor after an interval of three hello times, it
determines it has lost connection with that neighbor. When a root port or a designated port fails on
a device, the device generates a configuration message with the proposal bit set. Once its
neighbor device receives this message, it verifies that this configuration message is better than the
one saved for that port and then it starts a synchronizing operation to ensure that all of its ports
are in sync with the new information.
Similar waves of proposal agreement handshake messages propagate toward the leaves of the
network, restoring the connectivity very quickly after a topology change (in a well-designed
network that uses RSTP, network convergence can take as little as 0.5 seconds). If a device does
not receive an agreement to a proposal message it has sent, it returns to the original IEEE 802.D
convention. RSTP was originally defined in the IEEE 802.1w draft specification and later
incorporated into the IEEE 802.1D-2004 specification.