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Routing

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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.

Computer-NetworksRoutingGATE 2021 CS-Set-2Video-Explanation

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

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___________.

Computer-NetworksRoutingGATE 2022Video-Explanation

Question 2 Explanation:

If asked for R, Probablity = 1
If asked for R, from among P, Q, R
Probability = 1 /3

Question 3

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

Computer-NetworksRoutingGATE 2007-IT

Question 3 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 4

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

Computer-NetworksRoutingGATE 2007-IT

Question 4 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 5

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

(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

Computer-NetworksRoutingGATE 2008-IT

Question 5 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 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

Computer-NetworksRoutingGATE 2005

Question 6 Explanation:

Both TCP and UDP uses IP, which is a datagram service.

Question 7

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

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

Computer-NetworksRoutingGATE 2017 [Set-2]Video-Explanation

Question 7 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 8

Consider a network path P--Q--R between nodes P and R via router Q. Node P sends a file of size 10⁶ bytes to R via this path by splitting the file into chunks of 10³ bytes each. Node P sends these chunks one after the other without any wait time between the successive chunk transmissions. Assume that the size of extra headers added to these chunks is negligible, and that the chunk size is less than the MTU Each of the links P--Q and Q--R has a bandwidth of 10⁶ bits/sec, and negligible propagation latency. Router Q immediately transmits every packet it receives from P to R, with negligible processing and queueing delays. Router Q can simultaneously receive on link P--Q and transmit on link Q--R . Assume P starts transmitting the chunks at time t=0. Which one of the following options gives the time (in seconds, rounded off to 3 decimal places) at which R receives all the chunks of the file?

A	8.000
B	8.008
C	15.992
D	16.000

computer-networksRoutingGATE-2024-CS1(Forenoon)

Question 9

Consider the entries shown below in the forwarding table of an IP router. Each entry consists of an IP prefix and the corresponding next hop router for packets whose destination IP address matches the prefix. The notation “/N” in a prefix indicates a subnet mask with the most significant N bits set to 1.

This router forwards 20 packets each to 5 hosts. The IP addresses of the hosts are 10.1.1.16, 10.1.1.72, 10.1.1.132, 10.1.1.191, and 10.1.1.205 . The number of packets forwarded via the next hop router R2 is _______

A	40
B
C
D
E

computer-networksRoutingGATE-2024-CS1(Forenoon)

Question 10

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.

Computer-NetworksRoutingGATE 2023Video-Explanation

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

Computer-NetworksRoutingGATE 2010

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.

Question 12

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

Suppose the weights of all unused links in the previous question are changed to 2 and the distance vector algorithm is used again until all routing tables stabilize. How many links will now remain unused?

A	0
B	1
C	2
D	4

Computer-NetworksRoutingGATE 2010

Question 12 Explanation:

Now Graph will look like

And only link that will be removed is R5-R6 link.

There are 12 questions to complete.

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