STP Vs. RSTP – Question 3
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!
- Earlier questions in the STP vs. RSTP series
- Other STP Puzzles
- Link to the ICND2 book (to learn more STP)
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.
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.
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.)
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!
- RSTP, SW1, G0/2, Designated Port
- RSTP, SW2, G0/1, Root Port
- RSTP, SW3, G0/2, Designated Port
- STP, SW1, G0/1, Alternate Port
- STP, SW2, G0/2, Backup Port
- STP, SW3, G0/1, Alternate Port
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?