STP Vs. RSTP – Question 1

certskills
By certskills June 20, 2014 13:05

STP has been around forever it seems, and its newer and faster cousin, RSTP, has been around for over a decade now as well. For those first learning STP and RSTP, some of the biggest challenges come in not getting confused about what functions they both do with the same results and mechanisms, with the same results but different underlying mechanisms, and what’s simply different between the two.

Today’s post begins a series of sample exam questions that give us a chance to work through some of those details.

Related posts:

 

RSTP on the ICND2/CCNA Exams

Before getting into today’s question…

I added some additional RSTP material to the ICND2 200-101 Cert Guide after the book was published. To do that, I added the material to Appendix B, “Exam Updates”, which is intended as a place where I can add more content after publication.

For those of you who happen to purchase printing 6 (or later) of the book, you’ll get the RSTP material. But the material is free for anyone – just go download the updated Appendix B from the “Updates” tab at the book’s web page.

Note that I cannot say more about the why/wherefore about adding the materials (so no need to ask), other than to summarize the reasons listed in the book. The books include an “Exam Updates” Appendix (Appendix B) as a place to add more content after publication. I wanted to add more coverage of RSTP, particularly in comparison to STP. That’s it!

For that same reason, I added some more questions to the question database that comes with the book. And while I’m at it, I thought I’d do a few STP and RSTP questions here in the blog. Enjoy!

Scenario

This question uses a simple 3-switch design. You can imagine that hosts connect to each switch as well, but the interesting parts of the discussion occur on the links between the switches. Figure 1 shows the design, with switch names and port numbers.

Figure 1

For this scenario, and the other similar STP/RSTP Comparison questions, make the following assumptions:

  • Any switches are configured as layer 2 switches only, not as layer 3 (aka multilayer) switches.
  • Any hubs do not support any form of Spanning Tree.
  • All routers are configured to act as routers, not as layer 2 bridges that support Spanning Tree. (In case you didn’t know, because it’s outside the scope of CCNA: Routers can be configured to also enabled transparent bridging, which then allows the routers to bridge traffic – the equivalent of switching – and to also run STP.)

The Question

IEEE 802.1D STP and 802.1w RSTP have some similarities and some differences. This question focuses specifically on STP and RSTP port roles.

Each answer lists a protocol (either 802.1D STP or 802.1w RSTP), a switch, a port, and a port role. Select the answers that show a possible combination, that if using that type of spanning tree (STP or RSTP) on all switches in the design, given the assumption of switch SW1 as the root switch.

Multiple answers may be correct.

If You Post an Answer, Convince Me!

If you post a suggested correct answer – convince me! Tell me the circumstances that make that answer true. For instance, would it happen with all default settings, with the assumptions in the question? Would it happen with a particular configuration setting made on a particular switch? Convince me!

Answers

  1. STP, SW1, G0/1, Root Port
  2. STP, SW1, G0/2, Backup Port
  3. STP, SW2, G0/2, Designated Port
  4. STP, SW2, G0/1, Alternate Port
  5. STP, SW3, G0/1, Root Port
  6. STP, SW3, G0/2, Backup Port

 

Further Instructions

If you are a little unsure about what the question asks, here are some further instructions. For each answer, begin with the protocol (STP or RSTP). Then assume that all switches in Figure 1 use only that protocol. Then think about the combination of switch/port/role, and think about whether that port, on that switch, could end up in that port role.

For instance, if an answer listed:

STP, SW1, G0/1, Root Port

  • You would first see STP, and consider the case in which all three switches use STP.
  • Then, can you think of a combination of settings that would make SW1’s G0/1 port be its root port?

It may be helpful to write down some combinations, for instance: if you make SW2’s Bridge ID (BID) lower/better than SW3’s, does that then change the rules enough so that SW1’s G0/1 could be it’s root port?

STP Vs. RSTP - Answer 1
Answer to the #CCNA STP Question
certskills
By certskills June 20, 2014 13:05
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6 Comments

  1. ruissalo June 21, 06:58

    let’s say all the bridges use STP so:

    1- Incorrect: The root switch does not have root ports.
    2-Incorrect: The root switch does not have backups ports.(they are all designated ports and STP does not have a backup role)
    3- Incorrect: With all default settings and costs, Sw2 G0/2 would be the root port for SW2.
    4-Incorrect: STP does not have an alternate port role.
    5-Correct answer, with default settings, SW3 G0/1 would be the root port for SW3.
    5-Incorrect: STP does not have a backup port role and even if using RSTP, it only have one port sharing that segment so it would be (in another scenario) a designated port or an alternate port..

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  2. Marcos Cotomacio June 24, 21:08

    Answer: 5. STP, SW3, G0/1, Root Port > > Assuming same BW value on these links (e.g 100mbps = 19), Gi0/1 has a better cost to the root, which is considered a Root Port;

    Why not:
    1) STP, SW1, G0/1, Root Port > All ports on the root bridge are Designated;
    2) STP, SW1, G0/2, Backup Port > All ports on the root bridge are Designated;
    3) STP, SW2, G0/2, Designated Port > Assuming same BW value on these links (e.g 100mbps = 19), this port would have a better cost to the root, in this case would be a Root Port;
    4) STP, SW2, G0/1, Alternate Port > Same cost to the root in this segmento between SW2-SW3. Without knowing local bridge ID is dificult to say which one is going to be Forwarding/Blocking;
    6) STP, SW3, G0/2, Backup Port > Backup is a RSTP port role

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