Rapid Spanning Tree Protocol (RSTP)
Rapid Spanning Tree Protocol (RSTP) is the second STP type. RSTP is a fast converged version of STP as its name implies. RSTP bypass the Blocking State and Listenning State of STP, and provide Forwarding State in 15 Seconds. So, the convergence time is lower than STP.
In some cases, RSTP is similar to STP. But with some additional improvements, RSTP is more useful. Let’s check the details.
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RSTP Port Roles
RSTP has port roles like STP. These RSTP Port Roles are:
• Root Port
• Designated Port
• Alternate Port
• Backup Port
Root Port is the port on a switch that is the closest way (Lowest Cost) to the Root Bridge.
Designated Port is the port, that can send the best BPDU on its segment.
Alternate Port is a blocking port that receives better BPDU from another switch. It is the backup of Root Port.
Backup Port is a blocking port that receives better BPDU from the same switch. It is the backup of Designated Port.
STP Algorithm determines the role of a port based on BPDUs. STP BPDU was using two bits of BPDU part. RSTP uses all eight bits. RSTP (802.1w) BPDU is given below:
Here, there is an additional port definion called “Edge Port”. Edge Ports are the ports that connect to the Host devices like PCs, Servers, etc. So, Edge Ports are not participate in RSTP calculation. They do not receive BPDUs and they can go to the Forwarding State immediately.
STP use two type of BPDU, RSTP use single type BPDU. Contains additional paramaetrs to suppır the RSTP characteristics.
RSTP Port States
RSTP has three states. These RSTP States are:
• Discarding State
• Learning State
• Forwarding State
RSTP bypasses the two state of STP. It bypasses Blocking State, Listenning State of STP. So, after the Discarding State, RSTP go to Forwarding State immediately. You can check the below STP and RSTP State comparison table.
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RSTP Operation
RSTP Operation is similar to STP. Like STP, in RSTP, Root Bridge is selected firstly. Then port roles are determined. In RSTP there are two additional port roles.
Firtly Root or Designated Ports are selected. Then, if a port is not selected as Root or Designated Port, that port becomes;
• Alternate Port if it is connected to a port on different Switch
• Backup Port if it is connected to a port on the same Switch
As an example, think about the below RSTP Topology. In this topology, there are three Switches and a Hub. Let’s determine the port roles for this RSTP Topology.
Here, the port of the Root Bridge will be Designated and the lowest cost port to the Root Bridge on other switches will be Root Port.
In RSTP, two different blocking port is determined as we mentined above. Here the blocking port on other swicth than the designated port will be selected as Alternate Port. And the blocking port in the same switch or segment, will be selected as Backup Port.
RSTP and STP Interoperability
Lastly let’s mention, the interoparability of RSTP and STP. In different networks, these protocols can work together.
RSTP and STP are the similar two protocols. In your network, there can be switches that support both of these layer 2 protocol. But what if one switch in the network do not support RSTP? The answer is easy. The LAyer 2 topology works with STP only.
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