Today’s Cisco Certified Network Associate (CCNA) exam question of the day is on the topic of Rapid Spanning Tree Protocol. As if Spanning Tree Protocol was not confusing enough now on the new 200-120 exam we have to learn another one? That is what helps keep Cisco as one of the premier networking certifications out there. They continue to refine the exam and keep it relevant as new technologies come on to the market place. So hopefully you got our last exam scenario question correct on STP so this one will not be too much of stretch for you to learn the new RSTP concepts.
So let’s take a look at the network diagram below with four Cisco switches. Each of these four switches has been configured with a hostname, as well as being configured to run RSTP. No other configuration changes have been made. Which three of these show the correct RSTP port roles for the indicated switches and interfaces? (Choose three)
CCNA & CCENT RSTP Exam Question
A. Switch DETROIT, Fa1/1, designated
B.Switch MIAMI, Fa1/2, root
C. Switch NEW YORK, Gi1/1, root
D. Switch AUSTIN, Gi1/2, root
E. Switch MIAMI, Fa1/1, alternate
F. Switch NEW YORK, Gi1/2, alternate
Before we give you the answer, let’s take a look at the theory behind it and go over them one by one. The RSTP standard and has a faster convergence than STP (standard 802.1D). Rapid PVST+ includes Cisco-proprietary extensions such as BackboneFast, UplinkFast, and PortFast. Just like in STP, there is an election for the root bridge. In this scenario, switch NEW YORK has been elected the root bridge since it has a lower MAC address and therefore a lower BID.
a) Answer A is correct since there must be a designated port on each link.
b) B is correct since the best path to reach the root bridge, is through AUSTIN, therefore the port fa1/2 is a root port.
c) No port on switch NEW YORK can be a root port or blocked since this is the root bridge all its ports are designated. Therefore, answer C and F in this case are wrong.
d) Port GI 1/2 on switch AUSTIN is not the best path to the root bridge, therefore, it cannot be a root port since there is port GI 1/1, which is the root port.
e) Switch MIAMI fa1/1 is the alternate port in this scenario and is blocked and thus E is correct.
So our correct answers are A, B and E. Now if you really want to see how STP and RSTP work in the real world, a Cisco CCENT or CCNA lab is really the way to go. It is very abstract to understand the concepts of STP and RSTP for the CCENT and CCNA exams by just reading about it. What really helps it sink into your head is having the above topology and simply disconnecting a cable from Detroit going to New York and then plugging it into Austin and seeing how that changes everything. So if you want to check out some of our labs, you can see them here!
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