1. Check ALL statements that are true regarding statistical multiplexing.
a. It allows more users to access a resource such as bandwidth than if each user
were given dedicated access to the resource.
b. It needs all users to access the resource at the same time.
c. It needs N resources to serve for N users.
d. It needs users who do not always access the resource.
2. “HTTP, TCP, IP, 802.11” is an example of….
a. A protocol stack
b. A layer
c. A reference model
d. A protocol
3. A message M is encapsulated by the UDP, IP and Ethernet protocols in that order
as it travels down a protocol stack. What does the message look like when observed
“on the wire” of the Ethernet link?
Use U to represent the UDP header, similarly for I and E, and M to represent the
message. Write the parts in the order that they are transmitted “on the wire”, i.e., the
first part sent is leftmost. We suggest you draw your own picture of the
encapsulation.
a. I U M
b. E I U M
c. M U I E
d. U I E M
4. This question explores the backward learning algorithm of switches. Consider a
switch with hosts A and B connected to it (and the other ports empty). The switch
has just started operation. A sends a frame to B and, then B replies by sending a
frame to A.
Check ALL statements that CORRECTLY describe what happens when the second
frame (send from B back to A) is processed by the switch.
a. The switch does not learn anything new.
b. The switch forwards the frame directly to A.
c. The switch broadcasts the frame to reach A.
d. The switch learns the port for B.
5. This question explores the spanning tree algorithm for switches. Consider four
switches numbered 1, 2, 3, and 4 that are joined in that order in a circle (i.e., switch
4 is connected back to switch 1).
Check ALL statements that are TRUE concerning the spanning tree that is computed.
We suggest that you draw the figure and work out the spanning tree first yourself.
a. The link between switch 4 and switch 3 is turned off to break loops by the
spanning tree algorithm.
b. The path from switch 3 to the root goes via switch 4.
c. The link between switch 3 and switch 3 is turned off to break loops by the
spanning tree algorithm.
d. The root of the spanning tree is switch 1.
6. Assume that a router connects a link L1 to link L2. Assume that the MTU of L2 is
slightly more than half that of the MTU of L1 and that all maximum-sized packets on
L1 are fragmented into two packets on L2. Also assume that 20% of packets sent on
L2 are corrupted or lost. What is the probability that all of the fragments of a
maximum-sized packet sent across L1 are transmitted successfully across L2? Do
not consider retransmissions.
a. 0.2
b. 0.6
c. 0.64
d. 0.4
7. Which of the following statements regarding a NAT is NOT true?
a. Different NAT table entries could contain different internal IP addresses.
b. Outgoing data packets that do not contain a matching internal IP:Port pair in the
NAT table result in the creation of a new NAT table entry.
c. There could be multiple NAT table entries for a single internal IP:Port pair
d. Incoming data packets that do not contain a matching external IP:Port pair would
be dropped.
e. There could be multiple NAT table entries for a single external IP address
f. There could be multiple NAT table entries for a single internal IP address
g. Different NAT table entries could contain different external IP addresses.
7. Dijkstra’s algorithm finds shortest paths to all destinations in the network from a
given source. In what order are shortest paths found as the algorithm runs?
a. The destinations that have the lowest cost paths for all possible pairs of sources
and destinations are found first.
b. The destinations that are the lowest distance from the source are found first.
c. The destinations that are the fewest hops from the source are found first.
d. The destinations that are the greatest distance from the source are found first.
8. Consider a node N in a network in which a message is distributed by flooding; the
message originates at some other node. How many copies of the message can N
receive in the worst case? Select the single best answer?
a. Only one copy, which is sent to N along the shortest path from the origin to N.
b. As many copies as there are links between N and other nodes.
c. As many copies as there are equal-cost shortest paths between the origin and N
d. As many copies as there are links between the origin node and other nodes.
9. This question tests your understanding of the operation of distance vector routing.
Suppose a router A is connected to a router B by a link of cost 1, and connected to a
router C by a link of cost 3. A receives a distance vector from B that lists the costs to
reach destinations A, B, C and D in that order as (1, 0, 1, 5). This means that, for
example, B can reach D with cost 5. A also receives a distance vector from C that is
(3, 1, 0, 1). What is the next distance vector that A computes and sends to its
neighbors?
a. (0, 1, 3, 1)
b. (0, 1, 2, 4)
c. (2, 1, 2, 4)
d. (0, 1, 2, 1)
10. Which answer below best describes the information that is present in a linkstate packet that is flood by a node A?
a. The identifiers of the neighboring routers to which A is directly linked and the
cost of those links.
b. The identifiers of all routers in the network and the estimated shortest cost to
reach those routers.
c. The identifier of the neighboring routers to which A is linked.
d. The cost of all of the links from A to its neighboring routers.
11. In the network below, and assuming the cost of each link is 1, what are the ECMP
routes (i.e., equally good shortest paths) from A to C?
a. Only A->B->C
b. A->B->C, A->E->C, and A->F->C
c. A->B->F->E->C, A->E->F->B->C and A->E->F->D->C
d. A->B->C, and A->E->C
12. What IP prefix do you obtain when you aggregate 18.31.128.0/17 and
18.31.0.0/17 into a single prefix that contains only the same addresses?
a. 18.31.0.0/17
b. 18.31.0.0/18
c. 18.31.128.0/16
d. 18.31.0.0/16
13. Your task is to divide the IP prefix for your company, 12.24.0.0/17 into four
equal-size subnets. What are the prefixes of the subnets?
a. 12.24.0.0/18, 12.24.32.0/18, 12.24.64.0/18 and 12.24.96.0/18
b. 12.24.0.0/18, 12.24.64.0/18, 12.24.128.0/18 and 12.24.192.0/18
c. 12.24.0.0/19, 12.24.64.0/19, 12.24.128.0/19 and 12.24.192.0/19
d. 12.24.0.0/19, 12.24.32.0.19, 12.24.64.0/19 and 12.24.96.0/19
14. Suppose a router in ISP A receives a BGP route announcement that says “To
prefix P, path ISP C, ISP B, ISP A, at nexthop N”. What will happen when the router
processes the announcement? Select the single best answer.
a. The route announcement will be dropped because it contains a loop.
b. The policy at ISP A will be used to decide if this route is selected.
c. The path will be shortened to cut out loops.
d. The path will be propagated with ISP A prepended to the start of the path.