Redundant routes keep a network working

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Quick Questions

Shade in the A or B circle for each question.

1. A network has two different paths between a school and the internet. One cable is cut. What happens?

A All data stops until the cable is fixed

B Packets are sent along the other path and data keeps flowing

2. Which network is better at coping with one cable being cut?

A A network with two paths between devices

B A network with only one path between devices

3. What is a backup link?

A A spare connection that is used if the main link fails

B A link that makes the main cable faster

4. Why can packets use a different path when a router fails?

A Each packet is routed separately, so it can be sent another way

B All packets must follow the same fixed path

5. Why does adding redundant routes make a network cost more?

A It uses up all the packets

B It needs extra cables and routers

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1redundant route		ARedundancy makes a network cost more, because extra cables and routers must be bought. The benefit is that data keeps flowing when something fails.
2backup link		Bmain link DOWN -> use backup route
3If one cable is cut, packets are sent another way so data keeps flowing		Ccost = main_path + spare_path
4Redundancy costs more because a network needs extra cables and routers		DWith only one path there is no redundant route, so a cut cable stops the data. A second path is needed for data to keep flowing.
5Only one path: A -> Router -> B Cable cut. Data still arrives.		EAn extra cable or connection that stays ready to carry packets if the main link is cut
6Spare cables and routers make a network cheaper.		FA spare path between devices that packets can use if the main path fails

Definitions

redundant route

Fill in the Blanks

Complete each line of code by filling in the missing parts.

IF mainLink = "failed" ____ send(packet, backupLink) END IF

IF mainLink = "failed" THEN send(packet, __________) END IF

routes ← 2 IF routes _ 1 THEN PRINT "Data flows"

extraCables ← 3 extraRouters ← 1 cost ← extraCables _ extraRouters

IF cable1 = "cut" THEN use(route2) ____ use(route1)

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A mainLink ← up

B IF mainLink = up THEN

C SEND packet VIA backupLink

D END IF

Question 2

A cableCut ← true

B backup ← true

C IF cableCut OR backup THEN

D SEND packet VIA backup

E ELSE

F DISPLAY 'Data stops'

Question 3

A paths ← 2

B IF paths = 1 THEN

C redundant ← true

Question 4

A router1 ← failed

B router2 ← working

C SEND packet VIA router1

Question 5

A mainCost ← 100

B backupCost ← 40

C total ← mainCost + backupCost

D IF total < mainCost THEN

E DISPLAY 'Costs more'

Answer Key

Redundant routes keep a network working

Section A: Quick Questions

B — Packets are sent along the other path and data keeps flowing

A — A network with two paths between devices

A — A spare connection that is used if the main link fails

A — Each packet is routed separately, so it can be sent another way

B — It needs extra cables and routers

Section B: Match the Pairs

redundant route → A spare path between devices that packets can use if the main path fails

backup link → An extra cable or connection that stays ready to carry packets if the main link is cut

If one cable is cut, packets are sent another way so data keeps flowing → main link DOWN -> use backup route

Redundancy costs more because a network needs extra cables and routers → cost = main_path + spare_path

Only one path: A -> Router -> B Cable cut. Data still arrives. → With only one path there is no redundant route, so a cut cable stops the data. A second path is needed for data to keep flowing.

Spare cables and routers make a network cheaper. → Redundancy makes a network cost more, because extra cables and routers must be bought. The benefit is that data keeps flowing when something fails.

Section C: Definitions

redundant route — A spare path between devices. If one link or router fails, packets are sent another way so data keeps flowing. It costs more because extra cables and routers are needed.

Section D: Fill in the Blanks

IF mainLink = "failed" THEN
  send(packet, backupLink)
END IF

IF mainLink = "failed" THEN
  send(packet, backupLink)
END IF

routes ← 2
IF routes > 1 THEN PRINT "Data flows"

extraCables ← 3
extraRouters ← 1
cost ← extraCables + extraRouters

IF cable1 = "cut" THEN use(route2)
ELSE use(route1)

Section E: Spot the Error

B — The backup link should only be used when the main link is down. Using = up would switch to the backup when nothing has failed. It should be IF mainLink = down THEN.

C — Data can only be rerouted if the cable is cut AND a backup route exists. OR would try to reroute even when there is no backup. It should be IF cableCut AND backup THEN.

B — A network is redundant only when there is more than one path between devices. One path is not a spare. It should be IF paths > 1 THEN.

C — router1 has failed, so packets cannot go through it. They must be sent another way, via the working router2. It should be SEND packet VIA router2.

D — Redundancy costs more because the extra cables and routers are added to the main cost. The total should be greater than the main cost. It should be IF total > mainCost THEN.