CS 6250 COMPUTER NETWORKS EXAM
2
What8are8the8main8components8of8a8router?8-8answer---Input/Output8ports
-The8switching8fabric
-The8routing8processor
What8do8the8input8ports8do8for8a8router?8-8answer---Line8termination
-Data8link8processing8unit8decapsulate8the8packets
-
Checks8with8the8forwarding8table8to8ensure8each8packet8is8forwarded8to8the8appropriate8
output8port
What8does8the8switching8fabric8do?8-8answer--
Moves8packets8from8input8to8output8ports.838Types:8memory,8bus,8and8crossbar
What8do8the8output8ports8do?8-8answer--
Receive8and8queue8the8packets8that8come8from8the8switching8fabric8and8send8them8over
8to8the8outgoing8link
Routers8control8plane8functions8-8answer--
implements8routing8protocols,8maintains8the8routing8tables,8computers8the8forwarding8ta
ble
Router8Lookup8(Task818of86)8-8answer--
When8a8packet8arrives,8router8looks8at8the8destination8IP8address8and8determines8the8ou
tput8link8by8looking8at8the8forwarding8table8or8FIB.
Router8Switching8(Task828of86)8-8answer--
After8lookup,8the8switching8system8takes8over8to8transfer8the8packet8from8the8input8link8to
8the8output8link
Router8Queueing8(Task838of86)8-8answer--
After8switching,8it8will8need8to8be8queued8if8the8link8is8congested.8The8queue8maybe8simp
le8as8FIFO8or8some8other8algorithm
, Router8Header8validation8and8checksum8(48of86)8-8answer--
The8router8checks8the8packet's8version8number,8decrements8the8time-to-
live8field,8and8recalculates8the8header8checksum
Router8Processing8(Task858of86)8-8answer--
The8routers8build8their8forwarding8tables8using8routing8protocols8such8as8RIP,8OSPF,8BG
P8-8they8are8implemented8in8the8routing8processors
Protocol8Processing8(Task868of86)8-8answer--
The8routers,8in8order8to8implement8their8functions,8need8to8implement8the8following8proto
cols:
-Simple8Network8Management8Protocol8which8provides8counters8for8remote8inspection
-TCP8and8UDP8for8remote8communication8with8the8router
-Internet8Control8Message8Protocol8for8sending8error8messages
Switching8via8memory8-8answer--
The8ports8operate8as8I/
O8devices8in8an8operating8system.8When8an8input8port8receives8a8packet,8it8sends8an8int
errupt8to8the8routing8processor8and8the8packet8is8coped8to8the8processors8memory.8Proc
ess8extracts8address8and8looks8to8table8to8find8output8port.8Then8its8copied8to8the8output8
port8buffer
Switching8via8bus8-8answer--
When8input8receives8a8packet,8it8puts8an8internal8header8that8designates8the8output8port,
8and8sends8it8to8a8shared8bus.8All8output8ports8will8receive8the8packet,8but8only8the8desig
nated8one8will8keep8it.8Speed8of8the8bus8limits8the8speed8of8the8router.
Switching8via8Interconnection8network8-8answer--
Interconnect8network8that8connects8N8input8ports8to8N8output8ports8using82N8buses.8Can8
carry8multiple8packets8at8a8time
Bandwidth8and8Internet8population8scaling8(problem818of828that8routers8face)8-8answer--
Caused8by:
1.8Increasing8number8of8devices
2.8increasing8volumes8of8network8traffic8due8to8new8applications
3.8new8technologies8such8as8optical8links8that8can8accommodate8higher8volumes8of8traffi
c
Services8at8high8speeds8(problem828of828that8routers8face)8-8answer--
New8applications8require8services8such8as8protection8against8delays8during8congestion,8
and8protection8during8attacks8or8failures.
Longest8prefix8matching8(Router8bottlenecks818of84)8-8answer--
Due8to8increasing8number8of8internet8hosts8and8networks,8routers8need8to8group8destinat
ions8into8prefixes,8but8then8run8into8the8problem8of8more8complex8algorithms8for8longest8
prefix8matching
2
What8are8the8main8components8of8a8router?8-8answer---Input/Output8ports
-The8switching8fabric
-The8routing8processor
What8do8the8input8ports8do8for8a8router?8-8answer---Line8termination
-Data8link8processing8unit8decapsulate8the8packets
-
Checks8with8the8forwarding8table8to8ensure8each8packet8is8forwarded8to8the8appropriate8
output8port
What8does8the8switching8fabric8do?8-8answer--
Moves8packets8from8input8to8output8ports.838Types:8memory,8bus,8and8crossbar
What8do8the8output8ports8do?8-8answer--
Receive8and8queue8the8packets8that8come8from8the8switching8fabric8and8send8them8over
8to8the8outgoing8link
Routers8control8plane8functions8-8answer--
implements8routing8protocols,8maintains8the8routing8tables,8computers8the8forwarding8ta
ble
Router8Lookup8(Task818of86)8-8answer--
When8a8packet8arrives,8router8looks8at8the8destination8IP8address8and8determines8the8ou
tput8link8by8looking8at8the8forwarding8table8or8FIB.
Router8Switching8(Task828of86)8-8answer--
After8lookup,8the8switching8system8takes8over8to8transfer8the8packet8from8the8input8link8to
8the8output8link
Router8Queueing8(Task838of86)8-8answer--
After8switching,8it8will8need8to8be8queued8if8the8link8is8congested.8The8queue8maybe8simp
le8as8FIFO8or8some8other8algorithm
, Router8Header8validation8and8checksum8(48of86)8-8answer--
The8router8checks8the8packet's8version8number,8decrements8the8time-to-
live8field,8and8recalculates8the8header8checksum
Router8Processing8(Task858of86)8-8answer--
The8routers8build8their8forwarding8tables8using8routing8protocols8such8as8RIP,8OSPF,8BG
P8-8they8are8implemented8in8the8routing8processors
Protocol8Processing8(Task868of86)8-8answer--
The8routers,8in8order8to8implement8their8functions,8need8to8implement8the8following8proto
cols:
-Simple8Network8Management8Protocol8which8provides8counters8for8remote8inspection
-TCP8and8UDP8for8remote8communication8with8the8router
-Internet8Control8Message8Protocol8for8sending8error8messages
Switching8via8memory8-8answer--
The8ports8operate8as8I/
O8devices8in8an8operating8system.8When8an8input8port8receives8a8packet,8it8sends8an8int
errupt8to8the8routing8processor8and8the8packet8is8coped8to8the8processors8memory.8Proc
ess8extracts8address8and8looks8to8table8to8find8output8port.8Then8its8copied8to8the8output8
port8buffer
Switching8via8bus8-8answer--
When8input8receives8a8packet,8it8puts8an8internal8header8that8designates8the8output8port,
8and8sends8it8to8a8shared8bus.8All8output8ports8will8receive8the8packet,8but8only8the8desig
nated8one8will8keep8it.8Speed8of8the8bus8limits8the8speed8of8the8router.
Switching8via8Interconnection8network8-8answer--
Interconnect8network8that8connects8N8input8ports8to8N8output8ports8using82N8buses.8Can8
carry8multiple8packets8at8a8time
Bandwidth8and8Internet8population8scaling8(problem818of828that8routers8face)8-8answer--
Caused8by:
1.8Increasing8number8of8devices
2.8increasing8volumes8of8network8traffic8due8to8new8applications
3.8new8technologies8such8as8optical8links8that8can8accommodate8higher8volumes8of8traffi
c
Services8at8high8speeds8(problem828of828that8routers8face)8-8answer--
New8applications8require8services8such8as8protection8against8delays8during8congestion,8
and8protection8during8attacks8or8failures.
Longest8prefix8matching8(Router8bottlenecks818of84)8-8answer--
Due8to8increasing8number8of8internet8hosts8and8networks,8routers8need8to8group8destinat
ions8into8prefixes,8but8then8run8into8the8problem8of8more8complex8algorithms8for8longest8
prefix8matching
