STP Vs. RSTP – Answer 4

certskills
By certskills July 19, 2014 09:05

Today’s question once again gets specific about RSTP logic instead of STP. Look back to the original question first before looking at this answer. As usual, the answer post gives the answers, and the reasons why each answer is either wrong or right.

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General Advice

Before listing the answers, let me summarize a few key points that can let you quickly rule out some of the answers as incorrect.

Ruling Out Port Roles that do NOT Apply to STP

One key difference between RSTP and STP relates to two port roles added by RSTP: the alternate port and backup port role. STP simply does not include these port roles. So, knowing this fact, any of the answers that list “STP” as the protocol, and either alternate port or backup port as the port role, cannot possibly be true.

Root switches Cannot have Root Ports

Both STP and RSTP use the same logic of choosing one switch to be the root switch. Then, both STP and RSTP have each non-root switch determine its port that’s part of the best path back towards the root switch. That port is that switch’s root port.

As a side effect of the above rules, one switch – the root switch – does not have a root port. So, any question that identifies the root switch cannot then claim a root port exists on that switch!

Correct and Incorrect Answers

Here come the answers!

(Note: All three STP answers happen to mention port roles that RSTP has in common with STP. All the logic shown with each answer applies to both STP and RSTP.)

 

STP, SW1, G0/2, Designated Port – Correct. SW1 is the root switch, so it will advertise cost 0 Hellos onto each of its links. The root switch will therefore always win the election to be the designated port on every one of the root switch’s ports in that spanning tree.

STP, SW2, G0/1, Root Port – Correct. The obvious possibility for SW2’s root port is SW2’s G0/2 port, which connects directly to the root switch (SW1). However, SW2 may pick its G0/1 port as its root port. For example, if all links are running at gigabit speeds, with all defaults, SW1’s root cost over G0/2 will be 4, and its root cost out G0/1 will be 8, so SW2 picks G0/2 as its root port. A configuration change to make SW2’s G0/2 port have a cost of 9 makes that path have a greater root cost, so SW2 picks its G0/1 port.

STP, SW3, G0/2, Designated Port – Correct. SW1 becomes the root switch, so SW3 (G0/2) will compete with SW2 (G0/1) to become the designated port. With all defaults, they must use tiebreakers, but any configuration change that gives SW3 a better root cost than SW2 will result in SW3’s G0/2 becoming a designated port.

RSTP, SW1, G0/1, Alternate Port – Incorrect. The RSTP Alternate port role gives a switch an alternative root port. Because root switches do not have root ports, the alternate port role does not apply.

RSTP, SW2, G0/2, Backup Port – Incorrect. The Backup port requires one switch to have two ports connected to the same collision domain, which is not the case with this topology.

RSTP, SW3, G0/1, Alternate Port – Correct. The RSTP alternate port role serves as a non-root switch’s alternative port to use in the root port role if the current root port fails or otherwise ceases to be the root port. In this case, if SW3 chose its G0/2 port as its root port, then SW3’s G0/1 would become its alternate port, by virtue of being neither a root port or designated port, and also receiving Hellos advertising the current root.

 

Figure 1

 

STP Vs. RSTP – Question 4
Five Impressions for the new CCNP R&S
certskills
By certskills July 19, 2014 09:05
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1 Comment

  1. Jeffrey July 8, 22:32

    Hello Wendell,
    I must say that I do disagree with your logic and thus the answers you have posted for answers # 2 & 6. For answer # 2, where it is called into question whether Switch 2’s Gi 0/1 could be a root port…Based on what the example provides, we cannot tell the actual root-cost values for any of the links/ports. Thus even if we assume default values, it appears that Gi 0/2 would be the obvious RP. This would designate Switch 2’s Gi 0/1 as the de facto designated port (DP) for that switch. The same holds true for Switch 3. Its RP would be Gi 0/1, and its DP would be Gi 0/2. I do understand that a link cannot have DP’s on both ends, therefore one end would be a DP and the other would be blocked. Based on the information provided, we simply cannot determine which would be blocked, but that in no way rules out either’s ability to assume the role of DP. I feel your answer grasps-at and inserts values from sources unknown. I don’t know, is it just me, or what? Please elaborate.
    Now regarding answer # 6…
    Seeing how it appears that Switch 3’s Gi 0/1 should be the RP for the switch, I question how implementing RSTP would enable it to be designated as an “alternate Port” if it is already the RP. I understand your answer, but there is nothing within the information provided that would indicate that it is not already used in this capacity.

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