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RSTP Questions

February 6th, 2017 Go to comments

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

Comments (8) 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. T
    April 11th, 2017

    Used this dumps https://twitter.com/premiumdumps1/status/851507107157467137 , covered all the question on the exam.
    good luck all!

  7. 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.

  8. 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.