Alternate title: Monday morning musing on the RSTP backup port role.
Arguably the most misunderstood role in RSTP is the so called “backup” port role. The mantra of this role is typically that this type of port should not been seen on a modern network where full-duplex is prolific. The reasoning here is that this role is found when our RSTP bridge is connected to a half-duplex, shared media aka hub like so:
Indeed this topology will cause Fa0/2 to move into the backup port role since we are connected to a hub. However, to be clear here, the reason we are moving into backup is not merely due to the fact that it is a hub but rather due the propagation of BPDU’s.
The second reason we would see this is when there is a self-looped port:
Observe. Notice that this is not half-duplex media. The point is that we have the backup port role and we are talking full-duplex which proves disproves the notion that backup port is found only on half-duplex media.
The third time we see this backup port role is when we are connected to a switch that does not support STP and the BPDU’s are flooded as unknown multicast. (MAC: 0180-c200-00xx)
Again, notice here we are not half-duplex or self-looped and the interface Fa0/24 is in backup port role:
EDIT: My port numbering in the IOS output does not match my last topology diagram, but you get the idea…