STP Vs. RSTP – Answer 3

Today’s question and answers actually exercise more STP logic than RSTP, with the RSTP materials basically reviewing terms and features. 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.

Related posts:

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!

(Note: All three RSTP 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.)

RSTP, 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.

RSTP, 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.

RSTP, 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.

STP, SW1, G0/1, Alternate Port – Incorrect. The Alternate port concept is an RSTP concept, and does not apply to STP.

STP, SW2, G0/2, Backup Port – Incorrect. The Backup port concept is an RSTP concept, and does not apply to STP.

STP, SW3, G0/1, Alternate Port – Incorrect. The Alternate port concept is an RSTP concept, and does not apply to STP.

Figure 1

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1. Marcelo August 31, 01:20

Hello Wendell, I have the same answers that you in questions 1, 4, 5 and 6. I used another assumptions in questions 2 and 3, so, I obtained differents results than yours and I wanna know if I’m correct. In both questions, I assumed all default STP costs, without configuring new ones.

2) Incorrect. With the default costs, it is not possible that G0/1 is RP because it is not the least cost path to the root, unless G0/2 (the RP) fails.

3) Incorrect. Given that SW2 G0/2 is RP and therefore SW2 G0/1 is DP (if SW2’S BID< SW3's BID, because there's a tie in root cost=19), the SW3's G0/2 can't be DP too, so in RSTP, it must be ALT. If SW3 was STP, G0/2 would be in BLK state.

Thanks in advance and thanks for this way to teach us, giving always your best.

• Marcelo August 31, 15:03

There’s a mistake in my answer, in the 2nd item the root cost that ties is 4, not 19.

• Carlos August 16, 03:22

For the 2nd item, I think this sentence should be related to SW2 instead of SW1: “with all defaults, SW1’s root cost over G0/2 will be 4, and its root cost out G0/1 will be 8”, am I right?. Thank you.

• Ben Tauler (@bentauler) January 19, 07:36

Made same 2 mistakes than you Marcelo, and unsure why we’re wrong… 2) I still don’t get how G0/1 becomes RP… , and 3) Though that would be in bloking state as well.

• certskills Author January 24, 17:18

Hey Ben,
Turns out the early comments in this thread are years old – impossible to tell, but ignoring your #1 in your list of 3 for that reason…
Your #2 – Per the explanation above, configuring SW2’s G0/2 port with a cost of 9 or greater, with default costs on other ports in the topology, gives SW2 a lower root cost using its G0/1 port… making G0/1 its root port.
And #3 – not getting your point on that one. Feel free to follow up…
Wendell

2. Taylor March 7, 23:13

Hey Wendell, just a quick question on rstp behavior.

When RSTP converges, it selects root port, then designated THEN alternate and backup roles if additional ports beyond the 2 root and designated receive bpdus for the same spanning tree instance?

It goes in order because alternate ports end up discarding and that would break RSTP if it selected alternates first, in this case if everything was default, Switch 2 g0/1 could be an alternate port.

• certskills Author March 15, 11:22

Taylor,
Yes.
🙂
I think I followed you, but yes, a non-root switch begins by determining which switch is too, then by selecting a root port to reach that switch, before doing anything else, including choosing an alternate to the root port. I think was the crux of your argument, but I must admit, I didn’t quite follow it all. But if that connects and confirms the dots for you, great!
Wendell

• Taylor March 15, 12:15

It was more so, how does STP decide what to make other ports (that are not root ports). does it choose designated ports first, before thinking about any alternate ports? Or does it choose alternate ports first then designated? But I got it now! Designated is necessary, so it is selected first, because it is forwarding. Anything else (alternate or backup) would be discarding and that could break STP. To choose alternate ports first, could result in no designated/forwarding ports, and only a root and alternate port, so only leftover ports (after root and designated) are considered for alternate and backup roles.

Sometimes I have a hard time wording my questions 🙂 Thanks!

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