Today I’ll wrap up the discussions of the various discussions of the last few weeks, in which we’ve looked at the layer 2 and 3 addresses, and layer 4 port numbers, that could be included on the exams used for the CCENT and CCNA certifications. I’ll wrap up today’s entry with some thoughts about why this kind of question should be viewed as an opportunity to go faster on the exam, and remove some of the time pressure.
First, the answer to Monday’s question is… “C”.
To find the answer, you have to apply a few key points from Wednesday’s post. First, Frame Relay headers do not include MAC addresses, making answers E and F incorrect. Next, R1 will discard the incoming frame’s Ethernet header and trailer, encapsulating the packet inside a Frame Relay header and trailer. The Frame Relay header holds a single DLCI value – the local DLCI that identifies the PVC from R1 to R2 – the DLCI used on R1’s local access link. The specific DLCI can be found from the show commands, with the pertinent parts shown in the following figure:
To find the right info, start by thinking about where R1 will forward the packet. The question stated that the packet was going to PC2, which is in subnet 172.30.2.0/24, from the diagram in Monday’s question. R1’s routing table lists an outgoing interface of S0/0/0.2 for subnet 172.30.2.0/24. The show frame-relay map command output lists enough information to answer the question – that S0/0/0.2 is a point-to-point subinterface, with DLCI 103 associated with it. Because it’s a point-to-point subinterface, only 1 DLCI can be associated with the subinterface – so the only DLCI R1 uses when forwarding packets out S0/0/0.2 is DLCI 103.
The question included the show frame-relay pvc command, which listed the 3 DLCIs known to R1, but this command was mainly included as noise to help make the question more difficult.
Now, consider why this question could be an opportunity to go faster. First, look at the survey listed back on Monday’s question. It lists a similar distribution of answer times compared to other questions in this blog, with around 1/3 taking more than the 70 second per question. However, there’s a higher percentage of folks who got the answer within 30 seconds. I think this number helps support the idea that with questions like this one, that ask about the addresses used at various layers, you have an opportunity to make up some time on the exam.
In my opinion, with a small amount of practice, you can do these kinds of questions very fast. The underlying concepts aren’t all that difficult – however, if you’ve only read the theory a time or two, and never applied it, these questions can make you a bit tentative, particularly if you have to link theory that you only read about once with the output of show commands. So, what can you do? Run traces on your own LAN with free tools like Wireshark (https://www.wireshark.org/). To this end, I put some more text that shows examples of these addresses into the latest editions of my CCNA books. I also added a few more exam questions like the last few in this blog into the CD in the back of the books, again to help you take advantage and go faster. In short, it’s a chance to speed up, and a lot of
