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Rapid Spanning Tree Protocol RSTP Tutorial

June 5th, 2011 Go to comments

Note: Before reading this article you should understand how STP works. So if you are not sure about STP, please read my article about Spanning Tree Protocol tutorial first.

Rapid Spanning Tree Protocol (RSTP)

One big disadvantage of STP is the low convergence which is very important in switched network. To overcome this problem, in 2001, the IEEE with document 802.1w introduced an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP), which significantly reduces the convergence time after a topology change occurs in the network. While STP can take 30 to 50 seconds to transit from a blocking state to a forwarding state, RSTP is typically able to respond less than 10 seconds of a physical link failure.

RSTP works by adding an alternative port and a backup port compared to STP. These ports are allowed to immediately enter the forwarding state rather than passively wait for the network to converge.

RSTP bridge port roles:

* Root port – A forwarding port that is the closest to the root bridge in terms of path cost
* Designated port – A forwarding port for every LAN segment
* Alternate port – A best alternate path to the root bridge. This path is different than using the root port. The alternative port moves to the forwarding state if there is a failure on the designated port for the segment.
* Backup port – A backup/redundant path to a segment where another bridge port already connects. The backup port applies only when a single switch has two links to the same segment (collision domain). To have two links to the same collision domain, the switch must be attached to a hub.
* Disabled port – Not strictly part of STP, a network administrator can manually disable a port

Now let’s see an example of three switches below:

RSTP_port_roles_initial.jpg

Suppose all the switches have the same bridge priority so the switch with lowest MAC address will become root bridge -> Sw1 is the root bridge and therefore all of its ports will be Designated ports (forwarding).

Two ports fa0/0 on Sw2 & Sw3 are closest to the root bridge (in terms of path cost) so they will become root ports.

On the segment between Sw2 and Sw3, because Sw2 has lower MAC than Sw3 so it will advertise better BPDU on this segment -> fa0/1 of Sw2 will be Designated port and fa0/1 of Sw3 will be Alternative port.

RSTP_port_roles_middle.jpg

Now for the two ports connecting to the hub, we know that there will have only one Designated port for each segment (notice that the two ports fa0/2 & fa0/3 of Sw2 are on the same segment as they are connected to a hub). The other port will be Backup port according to the definition of Backup port above. But how does Sw2 select its Designated and Backup port? The decision process involves the following parameters inside the BPDU:

* Lowest path cost to the Root
* Lowest Sender Bridge ID (BID)
* Lowest Port ID

Well, both fa0/2 & fa0/3 of Sw2 has the same “path cost to the root” and “sender bridge ID” so the third parameter “lowest port ID” will be used. Because fa0/2 is inferior to fa0/3, Sw2 will select fa0/2 as its Designated port.

 

RSTP_port_roles.jpg

Note: Alternative Port and Backup Port are in discarding state.

RSTP Port States:

There are only three port states left in RSTP that correspond to the three possible operational states. The 802.1D disabled, blocking, and listening states are merged into the 802.1w discarding state.

* Discarding – the port does not forward frames, process received frames, or learn MAC addresses – but it does listen for BPDUs (like the STP blocking state)
* Learning – receives and transmits BPDUs and learns MAC addresses but does not yet forward frames (same as STP).
* Forwarding – receives and sends data, normal operation, learns MAC address, receives and transmits BPDUs (same as STP).

STP State (802.1d) RSTP State (802.1w)
Blocking Discarding
Listening Discarding
Learning Learning
Forwarding Forwarding
Disabled Discarding

Although the learning state is also used in RSTP but it only takes place for a short time as compared to STP. RSTP converges with all ports either in forwarding state or discarding state.

RSTP Quick Summary:

RSTP provides faster convergence than 802.1D STP when topology changes occur.
* RSTP defines three port states: discarding, learning, and forwarding.
* RSTP defines five port roles: root, designated, alternate, backup, and disabled.

Note: RSTP is backward compatible with legacy STP 802.1D. If a RSTP enabled port receives a (legacy) 802.1d BPDU, it will automatically configure itself to behave like a legacy port. It sends and receives 802.1d BPDUs only.

Comments (44) Comments
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  1. KOWO
    August 31st, 2012

    Please help
    Designated ports are selected based on the lowest path cost to the root bridge for a segment. Since the root bridge will have a path cost of “0,” any ports on it that are connected to segments will become designated ports. For the other switches, the path cost is compared for a given segment. If one port is determined to have a lower path cost, it becomes the designated port for that segment. If two or more ports have the same path cost, then the switch with the lowest BID is chosen.

    or

    lowest root bridge id
    lowest root path cost
    lowest sender bridge id
    lowest sender port id

  2. KOWO
    August 31st, 2012

    So designated port are choose by lower Mac Adress or Path cost???
    On scenario u choosed (designatet port SW2 (FA01) but CISCO says that
    switches, the path cost is compared for a given segment. If one port is determined to have a lower path cost, it becomes the designated port for that segment. If two or more ports have the same path cost, then the switch with the lowest BID is chosen.
    So please help me with this
    KOWO

  3. Ben
    October 24th, 2012

    “Well, both fa0/2 & fa0/3 of Sw2 has the same “path cost to the root” and “sender bridge ID” so the third parameter “lowest port ID” will be used. Because fa0/2 is inferior to fa0/3, Sw2 will select fa0/2 as its Designated port.”

    Small fix but:
    Shouldn’t this be, “Because fa0/2 is SUPERIOR to fa0/3, Sw2 will select fa0/2 as its Designated Port”

  4. Asif
    December 13th, 2012

    Hi 9tut,
    Thanks for your excellent updates !!!!
    I have one doubt, Can you please confirm me whether root bridge is selected on basis of high bridge priority or low bridge priority.
    Once more thanks for your study materials!!!! :)

  5. Mohan
    December 26th, 2012

    Root bridge is selected based on high bridge priority(lowest value)

  6. Ozgun
    December 28th, 2012

    is there any config. command for this mode?

  7. jrrivers
    February 13th, 2013

    FYI… each interface on a bridge has a unique MAC address… see section 7.12.2 of IEEE 802.1D-2004.

  8. ALI
    March 27th, 2013

    hi KOWO cisco said right if you read the tutorial carefully again you will get you answer, 9tut say the same meaning

  9. oceandographer
    April 29th, 2013

    This is an excellent article. This cleared up several questions that I had on this topic. This is the kind of thing that needs to be included in the Cisco Network Academy–Lan Switching & Wireless portion. Thanks 9tut!

  10. Rahul Singh
    July 3rd, 2013

    Thanks for your excellent updates

  11. Sagar
    July 15th, 2013

    Very easy to understand the concepts!!! btw, the statement “Because fa0/2 is inferior to fa0/3, Sw2 will select fa0/2 as its Designated port”, does it signifies that the number which is low becomes the DP ????

  12. India
    August 7th, 2013

    when we are choosing a root bridge, what do we check first? lower priority or lower mac address?

  13. Juls
    August 8th, 2013

    @India
    1st – priority

  14. Anonymous
    September 6th, 2013

    first check priority regardless mac addressee value only use mac address if Priority tie

  15. piush
    April 10th, 2014

    wonderfully described

  16. Ravindra
    June 27th, 2014

    awesome explanation…

    Grate help … in learning

  17. dhanesh
    September 2nd, 2014

    Really he;pful…

  18. Zameerul Haque
    September 19th, 2014

    EK NUMBER

  19. amandi
    October 11th, 2014

    awesome explanation !!!!!!!!!! best site I ever see!!!!!!!!once again thanks

  20. Guru
    October 30th, 2014

    i love people taking part in this ” challenging blog”

  21. adeel
    November 26th, 2014

    in above case if sw 2 has lower cost path to bridge then port from sw 3 to sw 1 selected as alternate

  22. Mohamed
    January 2nd, 2015

    hi!!!!
    if we connect one more link between sw2 to sw3(fa0/2 to fa0/2) what will be the status of sw3 port fa0/2? backup or alternate?if administrator put port as disable manually….what is the use of that port?

  23. yadav
    January 11th, 2015

    gr8 explaination

  24. Awesome stuff….this is really helpful!!!!
    February 3rd, 2015

    Katrina

  25. Maran
    February 4th, 2015

    which two states are the port states when RSTP has converged ?

    1. discarding
    2. listening
    3. learning
    4. forwarding
    5. disabled

    please anyone explain ????

  26. Ryan
    February 20th, 2015

    “which two states are the port states when RSTP has converged ?
    1. discarding
    2. listening
    3. learning
    4. forwarding
    5. disabled
    please anyone explain ????”

    Maran,

    Forwarding and Blocking are the two primary port states when RSTP has converged.

  27. Anil
    March 15th, 2015

    Simple and straight forward explanation :)

  28. rahul
    March 18th, 2015

    what is edge port and non-edge port in rstp ?

  29. rahul
    March 18th, 2015

    in between two switch any possibility of edge port ?

  30. matt
    March 28th, 2015

    @Ryan.
    Not totally wrong. . but when RSTP has converged the states are forwarding and ‘discarding’. Blocking is a term for ieee 802.1D common spanning tree and PVST. Blocking and discarding are almost truly synonymous, however if looking from an exam perspective, blocking would be considered a wrong answer. Also, if troubleshooting and only given partial show command that does not state RSTP as the spanning-tree mode, alternate, discarding, and blocking ports are a dead giveaway that you aren’t running 802.1D flavors of spanning tree.

  31. matt
    March 28th, 2015

    to the comment above: alternate, discarding, and *****backup***** ports are a dead giveaway that you aren’t running 802.1D flavors of spanning tree.

  32. Anonymous
    May 20th, 2015

    Simple and easy to understand explanation, good work, 9tut. Thanks!

  33. Anonymous
    August 19th, 2015

    very quick, very concise. ty again!

  34. JackRabbit
    November 17th, 2015

    A nice little flash tutorial about STP, made by Cisco:
    http://www.cisco.com/image/gif/paws/10556/spanning_tree1.swf

  35. Alex
    January 30th, 2016

    Guys please read this
    it is a great place to lean by comparison
    https://cciethebeginning.wordpress.com/2008/11/20/differences-between-stp-and-rstp/

    Admins please add to content. Lots of confusion between “ROLES” and “STATES” of STP and RSTP

  36. Emma
    February 22nd, 2016

    i like the way u explained it. short and neet..

  37. chinna
    April 13th, 2016

    nice to see this rstp…thank you for sharing :)

  38. Anonymous
    May 1st, 2016

    Hello ,
    Please help.

    Can someone please let me know that RSTP and PVST+ are interoperable in a network.

    in my network some switches are running on RSTP and some on PVST+ are we are facing
    STP loop.

  39. Ralf
    May 5th, 2016

    Great..

  40. kokki
    May 11th, 2016

    i have my ccna exam in 2 weeks what are the suggested protocol to study

  41. Dusan
    June 22nd, 2016

    @kokki
    All , especially this and FHRPs

  42. Anonymous
    June 24th, 2016

    guys need your inputs..
    There is STP issue in our network , We have voice gateway which is connected to a root switch for the VLAN 20 , from the root switch we are connecting two ports to the voice gateway .
    both the ports are in forwarding states and the issue is we are not able to ping the voice gateway IP..

    Tried to shutdown the one of the interface in switch and able to ping the IP..but when both are enable not able to ping..kindly suggest what can be done to resolve the issue.

  43. hamburgWhich two spanning-tree port states does RSTP combine
    October 6th, 2016

    Which two spanning-tree port states does RSTP combine to allow faster convergence? (Choose two.)

    answers: Blocking & Listening

    Why?

  44. raju
    November 11th, 2016

    I had read multiple documents to understand the RSTP concepts but still I have multiple questions to get clarity. If you answer the below questions for me, which I can develop good confidence on RSTP.

    1.Let’s say I have only one switch.I enabled RSTP in my switch. by default, all ports are on edge ports.

    a) Will edge port forward BPDUs ?
    b) If yes”, will they continuously forward BPDUs? How long did they forward ?

    2. Let’s say Switch A and Switch B is connected. Switch A is Root Bridge.

    Case:1 Switch A (D/F)————————(R/F) Switch B
    |
    (A/D)

    Case:2 Switch A (D/F)————————(R/F) Switch B
    |
    (D/F)

    Case:3 Switch A (D/F)———–HUB————–(R/F)Switch B
    |
    (A/D)

    When the connection between S-A and S-B fails, then S-B thins ks itself as a Root Bridge and move its non -edge ports to (Designated /Discarding) and send proposal bit set in BPDU.In this process, if S-B receives better BPDU with proposal bit set, S-B sends an Agreement and move its port (Alternate/Discarding) to (Root / forwarding).

    a) How much time did it take to (A/D) Alternate / Discarding port to move to (Root / Forwarding) ?
    b) How much time did it take to (D/F) Designated / Forwarding port to move to (Root / Forwarding) ?
    c) Connection fails between S-A and HUB.The link between HUB and Switch -B is up.How much time did it take to (A/D) Alternate / Discarding port to
    move to (Root / Forwarding) ?

    3. In STP only Root bridge will generate but In RSTP, Is all the Switches will generate BPDU periodically (2sec) what is the benefit?

    4. In RSTP, link failure is considered as a topology change Only non-edge interfaces (Point to Point links Switch to Switches) that move to the forwarding state are considered as a topology change. Link failure at Edge port is not considered as topology change.

    I want to understand the below points. Once a switch detects a topology change,

    a) It will start a “topology change while timer” (4sec). what is the benefit of this?
    b) If Switch receives BPDUs with TC bit set, It clears the MAC addresses learned on all its ports, except the one that receives the topology change.why?

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