RSTP, as defined in the IEEE 802.1D-2004 standards, transitions to the forwarding state by a handshaking mechanism (rapid transition), without any waiting times. If handshaking fails (such as on shared links, as follows), the system falls back to the timer-based mechanism defined in the original STP (802.1D-1998) standard.
A shared link is a link with more than two Ethernet bridges (for example, a shared 10/100BaseT segment). The port-type command is used to configure a link as point-to-point or shared (see section SAP link type).
For timer-based transitions, the 802.1D-2004 standard defines an internal variable forward-delay, which is used in calculating the default number of seconds that a SAP spends in the discarding and learning states when transitioning to the forwarding state. The value of the forward-delay variable depends on the STP operating mode of the VPLS instance:
In rstp mode, but only when the SAP has not fallen back to legacy STP operation, the value configured by the hello-time command is used.
In all other situations, the value configured by the forward-delay command is used.
config>service>vpls service-id# stp
forward-delay secondsRange: 4 to 30 seconds
Default: 15 seconds
Restore Default: no forward-delay