Routing

Question 1

What is a router?

Answer 1

A router relays packets among interconnected networks. It has links to 2 or more networks at the same time.

Question 2

In which layers does a router operate.

Answer 2

Network, Datalink and Physical

Question 3

Explain least cost routing?

Answer 3

A value is assigned to each link called a cost.
The cost of a route is the sum of the link costs.
The best route is the one with the lowest cost.

Question 4

What two categories of routing algorithms exist?

Answer 4

Non-adaptive (Virtual circuit) Routing decisions pre-determined.

Adaptive (Datagram) Traffic load can change route.

Question 5

Explain distance-vector routing algorithm.

Answer 5

each router exchanges information about the entire network with
neighbouring routers at regular intervals.

Question 6

Explain link state.

Answer 6

Each router exchanges information about its neighbourhood with all
routers in the network when there is a change.

Question 7

Advantage of link state.

Answer 7

Converges faster.

Question 8

What is convergence?

Answer 8

Process of determining optimal routes.

Question 9

Explain distributed Bellman-Ford algorithm.

Answer 9

-Add 1 to cost of each incoming route.
-New destination learned, add to routing table
-Existing destination,
* if Next Hop field is the same, replace existing entry with the new
information always
* if Next Hop field is not the same, only replace existing entry with the new information if the cost is lower

Question 10

What do the following stand for?
ARPANET
RIP
BGP
IS-IS

Answer 10

Advanced Research Projects Agency Network
Routing Information Protocol
Border Gateway Protocol
Intermediate System to Intermediate System

Question 11

What are the problems with distance-vector routing?

Answer 11

Count-to-infinity problem/slow convergence
Slow to react to router failure

Question 12

Explain Dijkstra’s algorithm.

Answer 12

Starting at root, we get from link-state database, the cumulative cost of each arc out of the root.
We select lowest cost arc, and make that node the new root.

Question 13

Explain transparent bridge.

Answer 13

Essentially bridge floods network, sending frame to every router in particular section, and each router can then check if it was the destination.

Question 14

Distinguish between Transparent bridge and source routing bridge.

Answer 14

TB: Bridges co-operate to perform function.
SR: The route to be followed is inserted in the frame.
TB: Suboptimal flooding.
SR: Optimum route selected.
TB: Will not use full bandwidth.
SR: Will use full bandwidth.

Question 15

What is a repeater?

Answer 15

Operates in physical layer is an electronic device which regenerates incoming signals. Allows the physical reach of a network to be extended.

Question 16

In which layer do bridges operate?

Answer 16

Datalink, physical

Question 17

In which layer are link-state routing or distance-vector routing used.

Answer 17

Network, datalink, physical

Question 18

Distinguish between broadcast and point to point.

Answer 18

Broadcast: star - single communication channel shared by all network nodes.
Point to point: mesh - many possible connection paths between a pair of nodes.

Question 19

What happens if a frame arrives at a bridge with a destination 37 and the forwarding database says destination 37 is port 2?

Answer 19

If frame arrived on any port except port 2 we forward it to port 2 otherwise we drop the frame.

Question 20

If a frame arrives at a bridge with a destination number not in the forwarding database what happens?

Answer 20

The bridge forwards the frame to every port apart from the one it arrived on.

Question 21

T or F. If the control field in a HDLC frame starts with 11 then this is a U-Frame.

Answer 21

True

Question 22

T or F. Only one station in a token ring network needs to be able to manage the ring.

Answer 22

False.

Question 23

T or F. Optimising the total throughput in a network will not give a fair usage to all flows.

Answer 23

True.

Question 24

T or F. Distance Vector shares their view of the whole network with it’s neighbouring routers.

Answer 24

True.

Question 25

T or F. A router operates only at the Network layer.

Answer 25

False.

Question 26

T or F. A neighbouring router is where both routers share a network.

Answer 26

True.

Question 27

T or F. A router is a device that is connected to a single network.

Answer 27

False.

Question 28

T or F. The cost of a route is the sum of all the link costs.

Answer 28

False.

Question 29

T or F. Transparent bridges form a spanning tree for frame delivery.

Answer 29

True.

Question 30

T or F. Source Routing bridge finds all possible routes.

Answer 30

True.

Question 31

T or F. A ring network with N nodes has N links.

Answer 31

True.

Question 32

T or F. Link State routing suffers from slow convergence called “count to infinity”.

Answer 32

False.

Question 33

T or F. Link state routing sends information about its own neighbours to all other routers.

Answer 33

True.

Question 34

T or F. Distance Vector routing replaces information only when it is lower when the next hop field is the same.

Answer 34

False.

Question 35

T or F. A bridge operates in the physical layer.

Answer 35

True.

Question 36

T or F. Source routing bridges can use all available links.

Answer 36

True.

Question 37

T or F. A transparent bridge has to be configured in advance of any frames being sent.

Answer 37

False.