Link type provides a categorization for each port participating in RSTP. The
link type can predetermine the active role that the port plays as it stands by
for immediate transition to a forwarding state, if certain parameters are met.
These parameters are different for edge ports and non-edge ports. Non-edge
ports are categorized into two link types. Link type is automatically
determined but can be overwritten with an explicit port configuration.
– 
Edge ports, the equivalent of PortFast enabled ports, and Point-to-Point
Links are candidates for rapid transition to a forwarding state. Before the
link type parameter can be considered for the purpose of expedient port
transition, RSTP must determine the port role.
Root ports do not use the link type parameter. Root ports are able to
make a rapid transition to the forwarding state as soon as the port is in
"sync."
Alternate and backup ports do not use the link
type parameter in most cases.
Designated ports are those that
make the most use of the link type parameter. Rapid transition to the
forwarding state for the designated port occurs only if the link type parameter
indicates a point-to-point link.