RSTP (Rapid Spanning Tree Protocol) is like an intelligent helper for the network. It stops loops and works faster than the older STP. This protocol makes sure devices adjust quickly to changes in the LAN. It keeps things running well, making sure everything is available and works smoothly. That’s why many experts like using it. In this article, we’ll check out what RSTP is, its excellent features, and how it works in networking.
The Standard Spanning Tree Protocol prevents network loops on Cisco Switches (802.1D). If there’s a link or cable problem, it figures out the best route for the packet to reach its destination.
What Exactly is RSTP (Rapid Spanning Tree Protocol) in Networking, and How Does It Work?
The classical or Standard Spanning Tree Protocol is 50 seconds in total for the network to maintain its continuity. Classic STP Port States is different from the Rapid ST Protocol. In addition, Standard STP has Blocking, Listening, Learning, and Forwarding port states and has PortFast, UplinkFast, and BackboneFast features to ensure the continuity of the network.
When configuring the original STP protocol, we also enable the PortFast, UplinkFast, and BackboneFast features for fast Convergence processing. After enabling these features, the STP Convergence duration will be 30 seconds, which is the result of an indirect fault on the network.
In today’s computer networks, it is essential to carry high traffic, such as audio and video, on the network without interruption. RSTP has been developed instead since an extensive network will also be useless on the Original STP Protocol on Cisco Switches. The most significant advantage of this protocol is that it is not specific to a brand.
Rapid ST Protocol (802.1W) provides higher speed reconfiguration (Convergence) than the Original STP Protocol. While the standard STP (802.1D) protocol requires a maximum of 50 seconds for network recalculation, this protocol does this in 1 second.
The Port States of Rapid Spanning Tree
The Rapid Spanning Tree protocol covers all features, such as PortFast, UplinkFast, and BackboneFast. Also, older Switches that work on an existing network structure and use the standard STP protocol can provide compatibility with RSTP-supported Switches.
- Discarding Port State
The discarding port state of RSTP is a new port state according to STP, and it includes the blocks and listening ports in STP.
- Learning Port State
Learning port status works as in STP. So, it has an active role in topology and can be activated immediately. It also knows the MAC addresses on the network.
- Forwarding Port State
The forwarding port state is also active in the topology and learns the MAC addresses.
Other Port States of RSTP
In addition to the above RSTP Port states, two new port states have been added. The primary purpose of these ports is to speed up the STP Convergence process. These;
- Alternate Port State
When there is a link failure on a Cisco switch or there is no target route to the Root Bridge, the Switch immediately triggers the Alternate port without initiating the STP Convergence process to find an alternate route. In short, Switch evaluates the Alternate port state as plan B.
- Backup Port State
When a Designated port in the Rapid Spanning Tree is faulty, it immediately activates a Backup port as Designated. A Backup port status, such as the alternate port status, immediately switches on without STP Convergence operation.
When you examine the image on the network, you can better understand the Backup Port logic. If you remove the HUB device between IOU2 and IOU3, the IOU2 Ethernet 3/3 and Ethernet 0/2 interfaces will be set to Designated.
IOU1#show running-config | inc spanning-tree
spanning-tree mode rapid-pvst
spanning-tree extend system-id
IOU1#
IOU2#show running-config | inc spanning-tree
spanning-tree mode rapid-pvst
spanning-tree extend system-id
IOU2#
IOU3#show running-config | inc spanning-tree
spanning-tree mode rapid-pvst
spanning-tree extend system-id
IOU3#When we apply the show spanning-tree command on IOU2 and IOU3 Switches in the topology above, you can check the following output …
IOU2#show spanning-tree
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address aabb.cc00.0100
Cost 100
Port 2 (Ethernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address aabb.cc00.0200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Desg FWD 100 128.1 Shr
Et0/1 Root FWD 100 128.2 Shr
Et0/2 Desg FWD 100 128.3 Shr
Et0/3 Desg FWD 100 128.4 Shr
Et1/0 Desg FWD 100 128.5 Shr
Et1/1 Desg FWD 100 128.6 Shr
Et1/2 Desg FWD 100 128.7 Shr
Et1/3 Desg FWD 100 128.8 Shr
Et2/0 Desg FWD 100 128.9 Shr
Et2/1 Desg FWD 100 128.10 Shr
Et2/2 Desg FWD 100 128.11 Shr
Et2/3 Desg FWD 100 128.12 Shr
Et3/0 Desg FWD 100 128.13 Shr
Et3/1 Desg FWD 100 128.14 Shr
Et3/2 Desg FWD 100 128.15 Shr
Et3/3 Back BLK 100 128.16 Shr
IOU2#IOU3 Switch shows spanning-tree command output;
IOU3#show spanning-tree
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address aabb.cc00.0100
Cost 100
Port 11 (Ethernet2/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address aabb.cc00.0300
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Desg FWD 100 128.1 Shr
Et0/1 Desg FWD 100 128.2 Shr
Et0/2 Desg FWD 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Desg FWD 100 128.5 Shr
Et1/1 Desg FWD 100 128.6 Shr
Et1/2 Desg FWD 100 128.7 Shr
Et1/3 Desg FWD 100 128.8 Shr
Et2/0 Desg FWD 100 128.9 Shr
Et2/1 Desg FWD 100 128.10 Shr
Et2/2 Root FWD 100 128.11 Shr
Et2/3 Desg FWD 100 128.12 Shr
Et3/0 Desg FWD 100 128.13 Shr
Et3/1 Desg FWD 100 128.14 Shr
Et3/2 Altn BLK 100 128.15 Shr
Et3/3 Desg FWD 100 128.16 Shr
IOU3#
Comparison
Rapid STP is a faster protocol than the old version. The purpose of both protocols is to maintain the continuity of the network by preventing loops. The slowness of the STP protocol is due to the Max-Age Timer (20 Seconds) and Forward Delay Timer (15 Seconds) periods, and you can change these values if you wish.
After configuring BackboneFast on Cisco Switches, we can save 20 seconds by skipping the Max-Age Timer. The classic STP network recomputation reduces from 50 seconds to 30 seconds.
Even if we change some settings to improve STP performance, the Rapid Spanning Tree Protocol will perform the recalculation process in less than 1 second.
How Does the RSTP Protocol Work?
In the original STP (802.1D) protocol, BPDU packets are sent only by the Root Bridge Switch. If the RSTP BPDU is in a packet exchange, each Switch will send its BPDU packets in one second (Hello Time). You can think of it as the RSTP protocol, such as OSPF or EIGRP Routing Protocol. RSTP uses a mechanism that is very different from the original STP.
In short, it uses a keepalive mechanism to keep the network topology alive.
In a network using Rapid Spanning Tree Protocol, a topology change is not considered as a result of a link error. This is regarded as a topology change in the Classic Spanning Tree Protocol. Also, it sends Multicast Frames to update the MAC addresses of all Switches.
The RSTP working logic is as follows:
- When a topology change occurs on RSTP-enabled Switches, a Topology Change Timer with twice Hello Time will start. TC Timer will make for all non-edge Designated and Root ports.
- All of the MAC addresses learned through these ports will be cleared.
- As soon as the TC Timer activates, the Topology Change bit value will be set in the BPDU packet to send from the non-Edge and Root ports.
- A neighboring switch that receives a TC-valued BPDU packet will clear the MAC addresses of all other interfaces except for the interface that it received.
- Then, the neighboring Switch itself will initiate a Topology Change and set the TC Bit value. Finally, these packets will be sent to all the Designated and Root ports.
Conclusion
RSTP is active by default on most Switches used today, and they run smoothly without the need for any configuration. Thanks for following us!
TolgaBagci
Hi, I'm Tolga, a computer expert with 20 years of experience. I help fix computer issues with things like hardware, systems, networks, virtualization, servers, and operating systems. Check out my website for helpful info, and feel free to ask me anything. Keep yourself in the loop about the newest technologies!
