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RSTP Questions

November 21st, 2019 in SWITCH 300-115 Go to comments

Question 1


There are five port roles in RSTP:

* 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

There is no “blocking” port role like STP. The “alternative” and “backup” roles are only in RSTP.

Question 2


RSTP is backward compatible with 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.

Question 3


When a Switch (Bridge) discovers topology change, it generates a TCN (Topology Change Notification) BPDU (Bridge Protocol Data Unit) and sends the TCN BPDU on its root port. The upstream Switch (Bridge) responds back the sender with TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit) and TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit)
The upstream Switch (Bridge) (bridge which received the TCN BPDU) generates another TCN BPDU and sends out via its Root Port. The process continues until the Root Switch (Bridge) receives the TCN BPDU.
When the Root Switch (Bridge) is aware that there is a topology change in the network, it starts to send out its Configuration BPDUs with the topology change (TC) bit set. Configuration BPDUs are received by every Switch (Bridge) in the network and all bridges become aware of the network topology change.

The switch never generates a TCN when a port configured for PortFast goes up or down -> it means no TC will be created for PortFast (or Edge Port) -> D is correct.

(Reference: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml)

Question 4


RSTP is backward compatible with PVST so when two interfaces on two switches are connected, the port that is running RSTP will revert back to PVST (or PVST+) with standard 802.1d timers.

Note: Answer D is not correct as we cannot say which switch is old or new. Moreover, RSTP should be used on new switch while PVST+ should be used on old one.

Question 5

Question 6


Switches transmit bridge protocol data units (BPDUs) throughout the STP instance. Each switch sends configuration BPDUs to communicate and compute the spanning tree topology -> Answer C is not correct.

  1. Libert
    May 25th, 2015

    ■ Alternate port: A port that has an alternative path to the root, different from the
    path the root port takes. This path is less desirable than that of the root port. (An
    example of this is an access layer switch with two uplink ports; one becomes the
    root port, and the other is an alternate port.)
    ■ Backup port: A port that provides a redundant (but less desirable) connection to
    a segment where another switch port already connects. If that common segment is
    lost, the switch might or might not have a path back to the root.

  2. lik
    August 15th, 2015

    An administrator recently configured all ports for rapid transition using PortFast. After testing, it has been determined that several ports are not transitioning as they should. What is the reason for this?

    A. RSTP has been enabled per interface and not globally.
    B. The STP root bridge selection is forcing key ports to remain in non-rapid transitioning mode.
    C. STP is unable to achieve rapid transition for trunk links.
    D. The switch does not have the processing power to ensure rapid transition for all ports.

    Answer: C

    RSTP can only achieve rapid transition to the forwarding state on edge ports and on point-to-point links, not on trunk links. The link type is automatically derived from the duplex mode of a port. A port that operates in full-duplex is assumed to be point-to-point, while a half-duplex port is considered as a shared port by default. This automatic link type setting can be overridden by explicit configuration. In switched networks today, most links operate in full-duplex mode and are treated as point-to-point links by RSTP. This makes them candidates for rapid transition to the forwarding state.

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  25. Archangel
    March 3rd, 2018

    Q3 Ans D is correct
    Topology Change Detection

    In RSTP, only non-edge ports that move to the forwarding state cause a topology change. This means that a loss of connectivity is not considered as a topology change any more, contrary to 802.1D (that is, a port that moves to blocking no longer generates a TC). When a RSTP bridge detects a topology change, these occur:

    It starts the TC While timer with a value equal to twice the hello-time for all its non-edge designated ports and its root port, if necessary.

    It flushes the MAC addresses associated with all these ports.

  26. Dany1
    September 20th, 2018

    Agree with Archangel. Theory presented at answer of Q3 is from legacy STP. In RSTP a TC is generated only when port move to forwarding.

  27. NoNymous
    February 15th, 2019


    One switch will continue using RSTP while the other remains PVST+. Only the interaction between the two switches changes. If the RSTP switch receives a BPDU from the PVST+ switch it will respond with a 802.1D BPDU. But it keeps sending 802.1W BPDUs between RSTP enabled switches.

  28. Anonymous
    February 15th, 2019

    Hi NoNymous,

    Im totally agree with you.

    Im fact, I got that wrong answer and i dont know why.

    Just the Port that it is connected to the PVST+ behave like the PVST+, RSTP wont change the whole system for it.

    According to what I read

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  34. Dmytro
    October 4th, 2019

    Q4 is wrong. Right answer is:
    D. Old switch will run RSTP and the new switch will run PVST+

    Because switch doesn’t need to change STP mode from RSTP to PVST+, it only need to send frames with PVST+ format to another switch who use PVST+. Please, correct the answer.

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