When it comes to port definition, RSTP has changed some aspects from the STP
protocol. While in STP there were blocking, disabled, designated and root ports, in
RSTP they are defined as alternate, backup, designated and root ports.
Figure below exemplifies a topology after the RSTP protocol has identified all of the
loops.
Figure 25: RSTP port status in a loop topology
Besides the difference in port states and definition, there are also two port definitions
that are not used in the STP protocol and are used in the RSTP, which are called Edge
and Link.
Edge ports are connected to end nodes, e.g., IEDs or computers. Care must be taken
when using an IED that is RSTP-aware, as it behaves as a bridge, it ports should not
be treated as Edge ports.
Trunk ports are ports connected between switches with RSTP. The figure below
shows these port types.
Figure 26: RSTP edge and truck ports
These definitions are used to increase the RSTP performance, as edge ports do not
send or receive BPDU packets, they go from disabled to forwarding state without
passing through other states. If an edge port starts receiving BPDU packets, it moves
to trunk state and starts being a part of the RSTP protocol.
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