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RSTP Questions

May 20th, 2017 Go to comments

Note: If you are not sure about Rapid Spanning Tree Protocol, please read our Rapid Spanning Tree Protocol RSTP Tutorial.

Question 1

Explanation

There are only three port states left in RSTP that correspond to the three possible operational states. The 802.1D 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

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.

Question 2

Explanation

The question says “no other configuration changes have been made” so we can understand these switches have the same bridge priority. Switch C has lowest MAC address so it will become root bridge and 2 of its ports (Fa0/1 & Fa0/2) will be designated ports -> E is incorrect.

Because SwitchC is the root bridge so the 2 ports nearest SwitchC on SwitchA (Fa0/1) and SwitchD (Gi0/2) will be root ports -> B and F are correct.

Now we come to the most difficult part of this question: SwitchB must have a root port so which port will it choose? To answer this question we need to know about STP cost and port cost.

In general, “cost” is calculated based on bandwidth of the link. The higher the bandwidth on a link, the lower the value of its cost. Below are the cost values you should memorize:

Link speed Cost
10Mbps 100
100Mbps 19
1 Gbps 4

SwitchB will choose the interface with lower cost to the root bridge as the root port so we must calculate the cost on interface Gi0/1 & Gi0/2 of SwitchB to the root bridge. This can be calculated from the “cost to the root bridge” of each switch because a switch always advertises its cost to the root bridge in its BPDU. The receiving switch will add its local port cost value to the cost in the BPDU.

In the exhibit you also we FastEthernet port is connecting to GigabitEthernet port. In this case GigabitEthernet port will operate as a FastEthernet port so the link can be considered as FastEthernet to FastEthernet connection.

One more thing to notice is that a root bridge always advertises the cost to the root bridge (itself) with an initial value of 0.

Now let’s have a look at the topology again

RSPT_port_states_explanation.jpg

SwitchC advertises its cost to the root bridge with a value of 0. Switch D adds 19 (the cost value of 100Mbps link although the port on Switch D is GigabitEthernet port) and advertises this value (19) to SwitchB. SwitchB adds 4 (the cost value of 1Gbps link) and learns that it can reach SwitchC via Gi0/1 port with a total cost of 23. The same process happens for SwitchA and SwitchB learns that it can reach SwitchC via Gi0/2 with a total cost of 38 -> Switch B chooses Gi0/1 as its root port -> D is not correct.

Now our last task is to identify the port roles of the ports between SwitchA & SwitchB. It is rather easy as the MAC address of SwitchA is lower than that of SwitchB so Fa0/2 of SwitchA will be designated port while Gi0/2 of SwitchB will be alternative port -> A is correct but C is not correct.

Below summaries all the port roles of these switches:

RSPT_port_roles.jpg

+ DP: Designated Port (forwarding state)
+ RP: Root Port (forwarding state)
+ AP: Alternative Port (blocking state)

Question 3

Explanation

IEEE 802.1w is Rapid Spanning Tree Protocol (RSTP). 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).

Question 4

Explanation

RSTP only has 3 port states that are discarding, learning and forwarding. When RSTP has converged there are only 2 port states left: discarding and forwarding.

Question 5

Question 6

Question 7

Question 8

Explanation

In this question, we only care about the Access Layer switches (Switch3 & 4). Switch 3 has a lower bridge ID than Switch 4 (because the MAC of Switch3 is smaller than that of Switch4) so both ports of Switch3 will be in forwarding state. The alternative port will surely belong to Switch4.

Switch4 will need to block one of its ports to avoid a bridging loop between the two switches. But how does Switch4 select its blocked port? Well, the answer is based on the BPDUs it receives from Switch3. A BPDU is superior than another if it has:

1. A lower Root Bridge ID
2. A lower path cost to the Root
3. A lower Sending Bridge ID
4. A lower Sending Port ID

These four parameters are examined in order. In this specific case, all the BPDUs sent by Switch3 have the same Root Bridge ID, the same path cost to the Root and the same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). In this case the port priorities are equal because they use the default value, so Switch4 will compare port index values, which are unique to each port on the switch, and because Fa0/12 is inferior to Fa0/1, Switch4 will select the port connected with Fa0/1 (of Switch3) as its root port and block the other port -> Port fa0/11 of Switch4 will be blocked (discarding role).

If you are still not sure about this question, please read my RSTP tutorial.

Question 9

Explanation

For PVST and PVST+, any change in the STP topology will result in a Topology Change Notification (TCN) BPDU. The TCN tells the switches that a change in the topology table has occurred, and they must therefore flush their Content-Addressable Memory (CAM) tables. Switches will set their CAM tables to age out after ForwardDelay seconds, which is 15 seconds by default. In other words, if a host doesn’t send traffic within 15 seconds to update the CAM table, the switch will have to begin flooding traffic to that host. This can lead to excessive amounts of flooded traffic.

For more information please read: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html

Comments (16) Comments
  1. Ttepi
    February 16th, 2017

    Q2. Although the answer is correct, the diagram solution is not. The links between A and B and between C and D will auto-negotiate to FastEthernet (cost 19). Only the link between B and D is a true Gigabit link (cost 4).

  2. okc_dsm
    February 17th, 2017

    Why are the questions not showing?

  3. takpan
    March 4th, 2017

    Q2: I think that the decision of port roles between SwitchA & SwitchB should not be based on their MAC addresses. The first priority is the lowest path cost to the root. In this case SwitchA advertises a cost of 19 while SwitchB advertises a cost of 23, so Fa0/2 of SwitchA is a designated port and Gi0/2 of SwitchB is an alternate port. MAC addresses are considered when there is a path cost to root tie.

  4. Budbakhastra
    March 15th, 2017

    Takpan, you are correct.

  5. banay
    March 20th, 2017

    @takpan
    are the answers correct

  6. Anymous777
    May 3rd, 2017

    @Ttepi – to clarify what Ttepi had said, the written explanation given is correct, but the diagram that shows “+4” on Gi0/2 on switch D and Switch B, should both actually be “+19”, since both ports are negotiating as fast Ethernet. And the written explanation actually says this much.

  7. Anymous777
    May 3rd, 2017

    @takpan & @banay are both making the same mistake. The stated explanation is correct, it is the mac address that would set the port roles between switch A and B. @takpan statement that “MAC addresses are considered when there is a path cost to root tie” is correct when you are searching for the Root Port, but that is not what we are searching for when deciding the port roles between Switch A & B. We know that the Root Port for Switch A is Fa0/1 – this is based on cost (as stated by @takpan). And we know that Port Gi0/1 is the Root Port for switch B. So we are no longer looking for the root ports on Switch A & B. What we are now looking for are the Designated & Alternative Ports on the link between A & B. And to find this we use a different metric – First: Bridge Priority (but as stated in the question, the Bridge Priorities are all the same, because they are set to the default value), So next we look at the MAC address to break the tie.

  8. Anonymous
    May 26th, 2017

    i am not mean

  9. Anonymous
    June 1st, 2017

    Why the question are not showing?

  10. Anonymous
    June 1st, 2017

    Why the questions aren’t showing?

  11. wassa
    June 10th, 2017

    Which option describes how a switch in rapid PVST+ mode responds to a topology change?

    A. It immediately deletes dynamic MAC addresses that were learned by all ports on the switch.
    B. It sets a timer to delete all MAC addresses that were learned dynamically by ports in the same STP instance.
    C. It sets a timer to delete dynamic MAC addresses that were learned by all ports on the switch.
    D. It immediately deletes all MAC addresses that were learned dynamically by ports in the same STP instance.

    “I dont understand why you said B, its Rapid PVST, as I know, RPVST delete dynamic Mac immediately !! “

  12. Cristopher
    June 13th, 2017

    Confirming the 747q dumps are valid.

  13. Burnek
    June 17th, 2017

    @9tut… In your rstp question 3 your answers are discarding and learning… but on your quiz the answers are discarding and listening.. which are the valid answer? thanks!

  14. wiper
    June 22nd, 2017

    @Christopher how can I get 747q?

  15. Roberta
    July 7th, 2017

    Q2.
    Is g0/2 of switchB the designated port for SwitchA-SwitchB segment because SwitchB have the lower path cost to the root bridge

  16. Souza
    July 13th, 2017

    I no longer know the right answer. Could you please clarify this once and for all ??