STP Vs. RSTP – Question 3

certskills
By certskills July 9, 2014 09:05

This post continues the #CCNA exam practice questions comparing STP and RSTP. Can you look at a switch design, and with confidence, describe what RSTP will do differently than STP? The questions in this series give you some practice with that kind of analysis. Join in the fun, defend your answers, and practice your RSTP and STP at the same time!

Related posts:

Aside: Where to Read More about STP vs RSTP

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. Just go download the updated Appendix B from the “Updates” tab at the book’s web page.

Scenario

This question uses the same scenario as the others in this series. The design uses three switches in a triangle. 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. RSTP, SW1, G0/2, Designated Port
  2. RSTP, SW2, G0/1, Root Port
  3. RSTP, SW3, G0/2, Designated Port
  4. STP, SW1, G0/1, Alternate Port
  5. STP, SW2, G0/2, Backup Port
  6. STP, SW3, G0/1, Alternate 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 2
STP Vs. RSTP – Answer 3
certskills
By certskills July 9, 2014 09:05
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2 Comments

  1. Punya Atma December 29, 02:42

    Answers:-

    1). Correct:
    As SW1 is the root switch, all of its other ports are designated ports.

    2). Wrong:
    RSTP, SW2, G0/2 is a root port because it is the shortest and direct link to the root switch, SW1. Therefore the other port G0/1 of SW2 is a designated port, but not a root port.

    3). Correct:
    RSTP, SW3, G0/1 is a root port as it is the shortest path to the root switch SW1. Therefore SW3’s other port G0/2 is a designated port.

    4). wrong:
    As SW1 is the root switch, its all other ports are designated ports. Therefore SW1, G0/1 is a designated port. Alternate ports are selected on links neither ends are not root ports, but on the link SW1 – SW2, G0/2 of SW2 is a root port.

    5). Wrong:
    STP, SW2, G0/2 is a root port, on link SW1 – SW2. A backup port is selected on link neither end is a root port. So this answer is knocked down.

    6). Wrong:
    SW1 is the root switch, and so SW3, G0/1 is a root port. An alternate port is selected on a link neither end is a root port. This answer knocked down.

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