About RSTP (Rapid Spanning Tree Protocol)

Rapid Spanning Tree Protocol starts here. This is how it works:

• Blocks ports to prevent loops.
• Same rules for root bridge selection as STP
• Same rules for root port selection as STP
• Same rules for designated port selection as STP
• Takes less than 30 seconds to move a port from blocking to forwarding state; thus, the name “Rapid”.

1st difference of RSTP from STP is RSTP root cost:

STP vs RSTP Path Cost by Link Speed

• 10 Mbps
• STP cost: 100
• RSTP cost: 2,000,000
• 100 Mbps
• STP cost: 19
• RSTP cost: 200,000
• 1 Gbps
• STP cost: 4
• RSTP cost: 20,000
• 10 Gbps
• STP cost: 2
• RSTP cost: 2,000
• 100 Gbps
• STP cost: Not defined
• RSTP cost: 200
• 1 Tbps
• STP cost: Not defined
• RSTP cost: 20
  
  

2nd difference will be in port states:

STP Port States (Discarding / Learning / Forwarding)

• Discarding (Stable)
• BPDUs: Send No / Receive Yes
• MAC learning: No
• Forward traffic: No
• Learning (Transitional)
• BPDUs: Send Yes / Receive Yes
• MAC learning: Yes
• Forward traffic: No
• Forwarding (Stable)
• BPDUs: Send Yes / Receive Yes
• MAC learning: Yes
• Forward traffic: Yes

• For RSTP, root and designated port roles remains the same as STP.
• The non-designated port role is divided into two roles. One is alternating and another is blocking.
• Alternating port works as a backup of root port. If a root port fails, the switch immediately makes it’s alternating port to a root port. There is no listening or learning state. No 30 second delay.
• Backup port in a switch is a port that receives a superior BPDU (lower port ID) from another one of the same switch’s interfaces. It usually happens when two interfaces from same switch is connected to a hub. Now, in between these two interfaces, the interface with the lowest port ID becomes the designated port and another one becomes the backup port. Backup port works as a backup for the designated port. If the designated port fails, the backup port immediately becomes a designated port. There is no listening or learning state. No 30 second delay.
• Unlike STP, in RSTP BPDU is sent by all switches. In STP, only the root bridge sends the BDPU.
• Some optional features of STP are built-in to RSTP like UplinkFast, BackboneFast, PortFast.

Link types in RSTP:

• Edge is a link type. It is the same thing as PortFast. Edge port is a port connected to end host.
• Point to point another link-type. It is a direct connection between two switches.
• Shared is another link-type. Which is not a direct connection between two switches, there is a hub in between. Works in half-duplex mode only.