Routing

Question 1
Consider a computer network using the distance vector routing algorithm in its network layer. The partial topology of the network is as shown below.   The objective is to find the shortest-cost path from the router R to routers P and Q. Assume that R does not initially know the shortest routes to P and Q. Assume that R has three neighbouring routers denoted as X, Y and Z. During one iteration, R measures its distance to its neighbours X, Y and Z as 3, 2 and 5, respectively. Router R gets routing vectors from its neighbours that indicate that the distance to router P from routers X, Y and Z are 7, 6 and 5, respectively. The routing vector also indicates that the distanceto router Q from routers X, Y and Z are 4, 6 and 8, respectively. Which of the following statement(s) is/are correct with respect to the new routing table of R, after updation during this iteration
A
The next hop router for a packet from R to P is Y.
B
The distance from R to Q will be stored as 7.
C
The next hop router for a packet from R to Q is Z.
D
The distance from R to P will be stored as 10.
Question 1 Explanation: 

Given R gets the distance vector (3,2,5)

After the one iteration distance vector from X to P, Y to P,  and Z to P is (7, 6, 5) respectively

The distance vector from R to P via X Y Z is (3+7, 2+6, 5+5) =(10, 8, 10)

So Take minimum distance from R to P which is 8 via Y

After the iteration distance vector from X to Q, Y to Q, Z to Q is ( 4, 6, 8) respectively

The distance vector from R to Q via X Y Z is (3+4, 2+6, 5+8) = (7, 8 13)

So Take minimum distance from R to Q  which is 7 via X.



Question 2

For the network given in the figure below, the routing tables of the four nodes A, E, D and G are shown. Suppose that F has estimated its delay to its neighbors, A, E, D and G as 8, 10, 12 and 6 msecs respectively and updates its routing table using distance vector routing technique.


A
B
C
D
Question 2 Explanation: 
Distance from F to F is 0 which eliminates option (C).
Using distance vector routing protocol, F→D→B yeilds distance as 20 which eliminates option (B) and (D).
Question 3

A group of 15 routers are interconnected in a centralized complete binary tree with a router at each tree node. Router j communicates with router j by sending a message to the root of the tree. The root then sends the message back down to router j. The mean number of hops per message, assuming all possible router pairs are equally likely is

A
3
B
4.26
C
4.53
D
5.26
Question 3 Explanation: 
Similarly, average hops when message comes down
= (3×8) + (2×4) + (1×2) + (0×1)/15
= 2.267
So, Total hops = 2×2.267 = 4.53
Question 4

Two popular routing algorithms are Distance Vector(DV) and Link State (LS) routing. Which of the following are true?

    • (S1) Count to infinity is a problem only with DV and not LS routing
 
    • (S2) In LS, the shortest path algorithm is run only at one node
 
    • (S3) In DV, the shortest path algorithm is run only at one node
 
    (S4) DV requires lesser number of network messages than LS
A
S1, S2 and S4 only
B
S1, S3 and S4 only
C
S2 and S3 only
D
S1 and S4 only
Question 4 Explanation: 
→ Count to infinity problem only exist into the DVR.
→ In LSR shortest path is calculated at each every router. (B) is wrong.
→ In DVR also shortest path is calculated at each and every router. (C) is wrong.
→ Since DVR is based upon local knowledge whereas LSR is based upon global knowledge.
Question 5
Consider a network with three routers P, Q, R shown in the figure below. All the links have cost of unity.

The routers exchange distance vector routing information and have converged on the routing tables, after which the link Q−R fails. Assume that P and Q send out routing updates at random times, each at the same average rate. The probability of a routing loop formation (rounded off to one decimal place) between P and Q, leading to count-to-infinity problem, is___________.
A
1
Question 5 Explanation: 
If asked for R, Probablity = 1
If asked for R, from among P, Q, R
Probability = 1 /3
Question 6

Packets of the same session may be routed through different paths in:

A
TCP, but not UDP
B
TCP and UDP
C
UDP, but not TCP
D
Neither TCP nor UDP
Question 6 Explanation: 
Both TCP and UDP uses IP, which is a datagram service.
Question 7
Which of the following statements is/are INCORRECT about the OSPF (Open Shortest Path First) routing protocol used in the Internet?
A
OSPF implements Bellman-Ford algorithm to find shortest paths.
B
OSPF uses Dijkstra’s shortest path algorithm to implement least-cost path routing.
C
OSPF is used as an inter-domain routing protocol.
D
OSPF implements hierarchical routing.
Question 8

Consider the following statements about the routing protocols, Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) in an IPv4 network.

    • I: RIP uses distance vector routing
    • II: RIP packets are sent using UDP
    • III: OSPF packets are sent using TCP
    IV: OSPF operation is based on link-state routing

Which of the statements above are CORRECT?

A
I and IV only
B
I, II and III only
C
I, II and IV only
D
II, III and IV only
Question 8 Explanation: 
I: RIP uses distance vector routing. “TRUE”
RIP is one of the oldest DVR protocol which employ the hop count as a routing metric.
II: RIP packets are sent using UDP. “TRUE”
RIP uses the UDP as its transport protocol, and is assigned the reserved port no 520.
III: OSPF packets are sent using TCP. “FASLE”
OSPF encapsulates its data directly into IP Packets and does not use either TCP or UDP.
IV: OSPF operation is based on link state routing. “TRUE”
OSPF is a routing protocol which uses link state routing (LSR) and works within a single autonomous system.
Hence correct is answer “C”.
Question 9

Consider the following three statements about link state and distance vector routing protocols, for a large network with 500 network nodes and 4000 links.

    [S1] The computational overhead in link state protocols is higher than in distance vector protocols.
    [S2] A distance vector protocol (with split horizon) avoids persistent routing loops, but not a link state protocol.
    [S3] After a topology change, a link state protocol will converge faster than a distance vector protocol.

Which one of the following is correct about S1, S2, and S3 ?

A
S1, S2, and S3 are all true.
B
S1, S2, and S3 are all false.
C
S1 and S2 are true, but S3 is false.
D
S1 and S3 are true, but S2 is false.
Question 9 Explanation: 
S1: The computational overhead in link state protocols is higher than in distance vector protocols. Because LSR is based upon global knowledge whereas DVR is based upon Local info.(True)
S2: A distance vector protocol with split horizon avoid persistent routing loops is true, but not a link state protocol is false because link state protocols do not have count to infinity problem.
S3: As Distance vector protocol has count to infinity problem and converges slower. (True)
Question 10

Which one of the following is TRUE about the interior gateway routing protocols – Routing Information Protocol (RIP) and Open Shortest Path First (OSPF)?

A
RIP uses distance vector routing and OSPF uses link state routing
B
OSPF uses distance vector routing and RIP uses link state routing
C
Both RIP and OSPF use link state routing
D
Both RIP and OSPF use distance vector routing
Question 10 Explanation: 
RIP Uses Distance Vector Routing and OSPF uses Link State Routing.
Question 11

Consider a network with 6 routers R1 to R6 connected with links having weights as shown in the following diagram:

All the routers use the distance vector based routing algorithm to update their routing tables. Each router starts with its routing table initialized to contain an entry for each neighbour with the weight of the respective connecting link. After all the routing tables stabilize, how many links in the network will never be used for carrying any data?

A
4
B
3
C
2
D
1
Question 11 Explanation: 
Link R1- R2 will not be used because its cost is 6 and link R1-R3-R2 costs 5, which is lesser than R1-R2 link.
Similarly, link R4-R6 will not be used, instead this link we can use R4-R5-R6 link which costs only 5 unit.
There are 11 questions to complete.

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