Download Mid-term Exam

Mid-term Exam
Selected Topics in Computer Networking
FESB, University of Split
With Solutions
May 26, 2006
Note: Each correctly answered question yields the number of points as indicated
inline in the text. Total number of points is 12. An incorrect answer or no answer
yield 0 points.
Problem 1
Answer whether the following statements are true or false.
1. Bridges are said to be “multiprotocol” because they work independently of the
upper layer protocol (e.g. IPv4 or IPv6).
[1/2 point]
Solution
False. Instead, routers are said to be multiprotocol if they speak more than one
network-layer protocol.
2. Assume a host A sends an IP packet to a host B via a bridge X, and assume all
these three systems are on the same bridge network. Then the source MAC
address of the IP packet received by host B is the MAC address of X. [1/2
point]
Solution
False. Bridges are transparent. The source MAC of the IP packet received by
host B is the MAC address of an interface of the host A.
3. When an IP router X, between two Ethernet segments, forwards an IP packet
(received from some other system), it modifies the source IP address to the IP
address of the outgoing router interface of router X. [1/2 point]
Solution
1
False. The source IP address of the IP packet is the IP address of the host that
generated the packet at first place.
4. An autonomous system must be assigned an autonomous system number in
order to be an autonomous system. [1/2 point]
Solution
True.
5. An IP router cannot have more than one IP address. [1/2 point]
Solution
False.
6. A link state routing can use Bellman-Ford algorithm instead Dijkstra’s
algorithm. [1/2 point]
Solution
True.
2
Problem 2
0
5
5
7
1
1
1
1
6
5
1
1
1
1
5
2
5
3
4
5
5
Figure 1: Network scenario for Problem 1.
1. Compute all shortest paths from node 0 for the network shown in Figure 1 by
using Bellman-Ford algorithm. Assume initial value p0(i) = 0 for each node i.
Write down the intermediate steps in obtaining the solution. [3 points]
Solution
k/i
0
1
2
3
0
0
0
0
0
1
0
1
2
2
2
0
1
2
3
3
0
1
1
1
4
0
1
1
1
5
0
1
1
1
6
0
1
2
3
7
0
1
2
2
i
pred(i)
0
0
1
3
2
1
3
0
4
0
5
0
6
7
7
5
0
5
5
7
1
1
6
5
1
1
1
1
1
5
1
3
4
5
3
5
2
5
Problem 3
2
2
4
1
4
0
2
5
1
5
3
Figure 2: Network scenario for Problem 2.
1. Compute all shortest paths from node 0 for the network shown in Figure 2 by
using Dijkstra’s algorithm. Write down the intermediate steps in obtaining the
solution. [1 point]
Solution
init M = {0}
step 1:
step 2:
step 3:
step 4:
i0 = 0, j0 = 4, m(4) = 1, M = {0,4}
i0 = 0, j0 = 3, m(3) = 2, M = {0,4,3}
i0 = 4, j0 = 2, m(2) = 3, M = {0,4,3,2}
i0 = 4, j0 = 1, m(1) = 6, M = {0,4,3,2,1}
2
2
4
1
4
0
2
5
1
5
3
2. For what cost values of the edge (2,0) is the spanning tree different than that in
item 1? [1 point]
Solution
4
For cost(2,0) < 3, the spanning tree is different than in item 1. (For cost(2,0) =
3, there are two distinct shortest-paths spanning trees and one is different than
in item 1.)
3. What is the spanning tree in item 2? [1 point]
2
3
2
1
4
0
2
5
5
1
3
Problem 4
AS1
R3
R1
R4
R2
AS2
R5
AS3
E-BGP
I-BGP
Figure 3: Network scenario for Problem 4.
Consider the network scenario in Figure 3 and answer the following questions.
1. Assume that the link from AS1 to AS2 is more expensive than the link from
AS1 to AS3. By what BGP policy can AS1 prefer routes over AS2 than over
AS3? [1 point]
Solution
By setting LOCAL-PREF at R3 larger than at R4.
5
2. Suppose that the link between R3 and R5 fails. Can BGP updates be
forwarded from the router R3 via router R4 to router R5? Explain your
answer. [1 point]
Solution
No. Routes learned from a neighbour in the I-BGP mesh are not repeated
inside the mesh.
3. Assume R1 announces the route AS2 AS4 AS5 157.58/16 to R3 and R2
announces the route AS3 AS6 AS7 157.58/16 to R4. Assume that AS1 does
not use LOCAL-PREF. Which routes to 157.58/16 are chosen by routers R3
and R4 as best routes? [1 point]
Solution
Router R3 will choose the route AS2 AS4 AS5 157 58/16 and router R4 will
choose the route AS3 AS6 AS7 157.58/16. Each router prefers routes learned
from E-BGP over those learned from I-BGP.
6