STP vs RSTP
In a computer network, the interconnection of the computers results in having more than one possible path to reach a given destination. In order to choose the best path for the packets to travel across and to prevent looping, STP or the Spanning Tree Protocol was created. STP was created in 1985 and is a very old protocol. In 2001, a new protocol named RSTP or Rapid Spanning Tree Protocol was created as a successor to STP. RSTP is far superior to STP and should be used in any network.
The addition of the word rapid should already be a hint on how RSTP improves on STP. In STP, if the topology changes either because of a new link added or a link lost, it can take anywhere between 30 to 50 seconds in order to adapt to the new topology and provide the least cost path. With RSTP, that is reduced to three hello times, which has a default value of 2 seconds for a total of 6 seconds. The ability to adapt new topologies is very advantageous especially when dealing with high volume traffic in order to avoid lost packets.
BPDUs (Bridge Packet Data Units) are used to exchange information about network topology. RSTP is able to respond to BPDUs from the direction of the root while STP cannot. This enables an RSTP bridge to provide spanning tree information to its ports. If the receiving bridge recognizes that the information it receives is superior, it will then send an agreement to the sending bridge. Because the receiving bridge already has the information, it can then bypass the typical learning/listening states of STP and directly go to the forwarding state; thereby allowing the receiving bridge to function properly much faster than in STP.
As a replacement to STP, RSTP maintains backwards compatibility so that the transition from one to the other doesn’t necessarily have to mean replacing all your hardware at once. The downside to this is that the benefits of RSTP do not appear as long as you have old STP equipment connected. RSTP equipped devices are programmed to automatically revert to STP operation if a STP only device is connected to the network.
Summary:
RSTP is the successor to STP
RSTP responds much faster than STP
RSTP responds to BPDUs in both directions while STP does not
RSTP is backwards compatible with STP
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STP vs RSTP
in stable topology of stp only root bridge generates BPDUs where as in rstp all bridges generates BPDUs for every 2 secs(used as keep-alive msgs).
port state in stp:
1. Disable
2. Blocking
3. Listening
4. Learning
5. Forwarding
port states of rstp:
1. Discarding
2. Learning
3. Forwarding
port roles of stp :
1. Root port
2. Designated port
3. Nondesignated port
rstp port role:
1. Root port
2. Designated port
3. Alternate port
4. Backup port
stp convergence time:
30 secs – direct link
50 secs – indirect link
rstp convergence :
<=6secs by default i.e., 3*hello interval time
flag bits used by stp:
bit 0: TCN
bit 7: TCA
flag bits used in rstp:
0: TCN
1: proposal
2,3: port role
4: learning
5: forwarding
6: agreement
7: TCN
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