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Rapid STP

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  • 1.  Rapid STP

    Posted 06-12-2020 10:15
    I understand from Juniper public documents that rapid stp is the default available one. What I have understood of it is - it has faster re building time. Am I correct ? Is there any device which has stp as default l2 loop prevention protocol. And is there any other difference why I should go for rapid stp. I am not sure how different is this other than better speed. Since I am good with stp so thought of understanding if I should go with default behaviour or stick to my expertise.
    Any answers or suggestions will help. Thanks.


  • 2.  RE: Rapid STP

    Posted 06-12-2020 10:25

    Hi Drazzel, 

     

    To answer your questions, firstly, 

    • The Juniper Devices use Rapid Spanning Tree Protocol (RSTP) by default and this is to provide quicker convergence time than the base Spanning Tree Protocol (STP) does. RSTP identifies certain links as point to point. When a point-to-point link fails, the alternate link can transition to the forwarding state, which speeds up convergence.
    • Although STP provides basic loop prevention functionality, it does not provide fast network convergence when there are topology changes. The STP process to determine network state transitions is slower than the RSTP process because it is timer-based.
    • Further, when you explicitly want to configure STP, the Juniper switches use the IEEE 802.1D 2004 specification, force version 0. This configuration runs a version of RSTP that is compatible with the classic, basic STP.
    • RSTP evolved from the original STP IEEE 802.1D protocol to provide faster spanning-tree reconvergence after a switch port, switch, or LAN failure. Where STP took up to 50 seconds to respond to topology changes, RSTP responds to changes within the timeframe of three hello BPDUs (bridge protocol data units), or 6 seconds. This is the primary reason that RSTP is the default spanning-tree configuration.

     

    Link for more details: https://www.juniper.net/documentation/en_US/junos/topics/topic-map/spanning-tree-configuring-rstp.html#id-example-faster-convergence-and-improved-network-stability-with-rstp-on-ex-series-switches

     

    Hope this helps. Smiley Happy

     

    Please mark "Accept as solution" if this answers your query. 

     

    Kudos are appreciated too! 

     

    Regards, 

    Sharat Ainapur

     



  • 3.  RE: Rapid STP

    Posted 06-12-2020 10:26

    Hi drazzel,beryn,

     

    The main difference between Rapid Spanning Tree Protocol (RSTP IEEE 802.1W) and Spanning Tree Protocol (STP IEEE 802.1D) is that Rapid Spanning Tree Protocol (RSTP IEEE 802.1W) assumes the three Spanning Tree Protocol (STP) ports states Listening, Blocking, and Disabled are same (these states do not forward Ethernet frames and they do not learn MAC addresses). Hence Rapid Spanning Tree Protocol (RSTP IEEE 802.1W) places them all into a new called Discarding state. Learning and forwarding ports remain more or less the same.

     

     In Spanning Tree Protocol (STP IEEE 802.1D), bridges would only send out a BPDU when they received one on their Root Port. They only forward BPDUs that are generated by the Root Switch (Root Bridge). Rapid Spanning Tree Protocol (RSTP IEEE 802.1W) enabled switches to send out BPDUs every hello time, containing current information.

     

    Spanning Tree Protocol (STP IEEE 802.1D) includes two port types; STP Root Port and Designated Port. Rapid Spanning Tree Protocol (RSTP IEEE 802.1W) includes two additional port types called as alternate ports and backup ports.

     

    An alternate port is a port that has an alternative path or paths to the Root Switch (Root Bridge) but is currently in a discarding state (can be considered as an additional unused Root Port).  A backup port is a port on a network segment that could be used to reach the root switch, but there is already an active STP Designated Port for the segment (can be considered as an additional unused designated port).

     

    Hope this helps 🙂

     

    Please mark "Accepted Solution" if this helps you solve your query.

    Kudos are always appreciated!



  • 4.  RE: Rapid STP

    Posted 06-12-2020 10:31
    Ah ! Seems to take some time to understand through the links. If I am right, the difference in convergence time is close to 40 seconds. I will get back to forum once I face trouble in understanding. Thanks for replies.


  • 5.  RE: Rapid STP

    Posted 06-12-2020 10:38

    Hi drazzel.beryn,

     

    Just to add more, you can read below for a quick understanding on the major additions/differences that you can see in RSTP.
     

    RSTP needs backward compatibility with STP switches. Thus Discarding state merges Disabled, Blocking, Listening into one. Ideally if running a complete RSTP topology then discarding becomes practically unneeded due to the explanation below.

     

    In STP BPDU will only be sent from root bridge > down, therefore non-root bridges would only forward on BPDUs that are received from the root-bridge via their root port. Therefore topology change will mean a longer convergence time as the TCN has to be propagated to the root bridge first then from the root bridge out to all remaining bridges in the switching domain.

     

    For STP Hello(2 secs), max age timer (10 x hello time, by default 20s), then 15s each for Listening and Learning. If no timers are touched for STP and you haven't enabled any add-on features like backbonefast or uplinkfast then you can be looking at ~50s for convergence time.

     

    In RSTP all switches can send BPDUs every hello time period, not just the root bridge, the BPDUs carry up to date information. This enables faster convergence as the switch that determines the failure can advise the rest of the switching domain(bridge) of the TCN(Topology Change Notification) immediately. As the failure detection time becomes 3 x hello timer. Therefore if the hello timers are left at the default of 2 secs, detection of failure can happen within ~6 secs.

     

    RSTP also has two additional port states:( these are not present in STP )

    Alternate port: Technically in discarding state but is allocated as an alternate best path to the root-bridge. If the current root port fails the alternate port will take over.

    Backup port: This port is allocated as a redundant designated port. If there is already a designated port forwarding for that segment then this port will remain backup until it is needed. Technically this port is in discard state until needed.

     

    One more point RSTP marks edge ports as ports connected to end devices (No BPDUs expected on this port). This enables STP 'portfast' features to ensure the host port comes up as quickly as possible to avoid dropping DHCP packets etc.

     

    These are the main differences.

     

    Hope this helps 🙂

     

    Please mark "Accepted Solution" so that it can help others too 

    Kudos are always appreciated!



  • 6.  RE: Rapid STP
    Best Answer

    Posted 06-15-2020 19:05

    Take a look at this video:

    https://youtu.be/5xMcvfn61-E

     

    It should clear up some of the fundamentals for you. After that, the rest should make more sense.