Past Paper 2022

Past Paper 2022

by Yiren Cao -
Number of replies: 4

Hi there,

I have several questions regarding the past paper 2022 solution given.

Problem 1, question 3


Here it's discussing 3 cases by choosing different roots for the spanning tree. Whereas if referring back to the lecture slide attached below (as well as the similar one in 2020’s exam solution problem 1, 1(c)), wouldn't the root always be B1 ‘cause it’s the one with the smallest label? Also since we do not know the cost of the path, I suppose would answer that one of the 3 links (B1-B2, B2-B3, B1-B3) will be deactivated (based on the cost). Then, e.g. for the IPv4 packet, if B1-B2 or B1-B3 is deactivated, O3 will observe it; if B2-B3 is deactivated, O4, O5 will observe it. 

In Lecture slides [MAC] pages 29-31,



Problem 2, question 1 (d)
I'm just wondering where we could find that the injection of BGP next-hop into the forwarding table is enabled in the question specification, or do we assume that injection exists by default?


Problem 2, question 3 (a)
(highlighted in green below) at R2, would the cost both be 103 instead? To reach from R2 to A2, the cost is 1; from A2 to 2001:1:1::/48 the cost is 102, so in total 103; and for the other, would be cost of R2-R1 + cost of R1-A1 + cost of A1-2001:1:1::/48 = 101 + 1 + 1 = 103 as well I guess


Problem 4, question 1(c)

I'm quite confused about the (c) part. e.g. at the flight info server, why wouldn't the destination MAC address change to its next hop router’s MAC address? (Does it mean that for this packet, whenever it goes through a new router, only the MAC src address will be modified to the new router's MAC?)

Could you kindly point out where I could learn about the topic in the lecture slides please?



In addition, I have a less technical question about giving the justification as indicated on the front page of the exam sheet. I wonder if justification is required even for the multiple-choice questions (i.e. the ones asked to cross the box) and filling-up-table questions (e.g. the above-attached Problem 3, question 1 (b) part since in the solution sheet, it contains the justification "The addresses are the same ... IP address.")?

Thanks so much for taking the time to read this!

Best,
Yiren


In reply to Yiren Cao

Re: Past Paper 2022

by Ludovic Thomas -

Dear Yiren,

Exam 2022 Problem 1, Question 3.

You can perfectly say that B1 will be selected as root because it has the smallest label.
By this, you assume that the lexicographic comparison of the names B1<B2<B3 etc represent the comparison of the underlying real-life labels (in real-life, labels are 8-byte-long numbers. In the exams, we use more friendly, human-readable names).
This is the most straightforward assumption, the one used in the lecture.
Sometimes, lexicographic comparison is explicitly required in the problems (see for example item 5 of the description of Problem 2).

In this Problem 1, this was not the case, so a student could argue that we don't know the true real-life 8-byte-long labels and another root could also be selected. Such a student would also get the points as long as the assumption is clear and the conclusion is coherent with the assumption.
For this specific question, the solution PDF lists all possible solutions for completeness but you are only required to provide one possible solution.


This is the same idea for the costs. In the solution PDF, the equal-cost assumption is implicit (it's because it's the straightforward assumption since we have no information on the bit rates). But if you assume something more exotic, then you would have obtained the points, provided that your assumption is explicit and that your conclusion is coherent.

Exam 2022 Problem 2, Question 1d.

Yes, injection in the forwarding table is the default behavior. The goal of any routing protocol (OSPF, BGP) is to populate the forwarding table.

Exam 2022 Problem 2, Question 3a.

We will double-check this with the other TAs, we will get back to you asap on this question.

Exam 2022 Problem 4, Question 1c.

IP multicast packets are transported in Ethernet multicast frames.
In the same way that the multicast destination IP address is not the IP address of the destination, the mac destination address of a Ethernet multicast frame is not the MAC address of the next hop.  See page 13 of mcastsols.pdf


Yes, a small justification is required for all answers, including multiple choice and fill-in.

Ludovic
In reply to Ludovic Thomas

Re: Past Paper 2022

by Yiren Cao -
Thanks Ludovic! That is super helpful and all make much more sense now!