33596497 Routing Protocols

8/10/2019 33596497 Routing Protocols

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Routing Protocols- Dynamic Routing

Packet routing in the Internet is divided into two general groups

Interior and Exterior Routing.

Interior routing with (IGPs) Interior Gateway protocols happens inside independent network

systems also known as autonomous systems. ithin an autonomous system (!")# routingin$ormation is exchanged using an interior routing protocols RIP# RIPv%# IGRP# EIGRP#&"P'# I"I".

he Exterior routing protocols (EGPs) External Gateway Protocols are used *etween theautonomous systems such as +oarder Gateway protocol# +GP.

here are two main types o$ algorithms $or IP routing, Distance Vector and Link StateRouting

ithin each autonomous system routing is done separately $rom other autonomous systems#Routing *etween autonomous systems is per$ormed *y running +GP (+order GatewayProtocol).

IGP (Interior Gateway Protocol) is a protocol $or routing within a single autonomous systemhese protocols de$ine how to route to networks within the !"# and can also distri*ute routingin$ormation $or networks outside the !" (that in$ormation will come# naturally# $rom the routersat the edges o$ the !"# which also run the +GP protocol).

Distance Vector Routing and Link State Routing.

-istance ector protocols determine *est path on how $ar the destination is./ink "tate protocols are more sophisticated taking into consideration link varia*les# such as*andwidth# delay# relia*ility and load.

-istance ector protocols 0udge *est path on how $ar it is.-istance can *e hops or a com*ination o$ metrics calculated to represent a distance value.he -istance ector routing protocols are,Routing In$ormation Protocol (RIP v1 and v%) and (IGRP) Interior Gateway Routing Protocol.

-istancevector routing protocols are simple and e$$icient in small networks# and re2uire little#i$ any management. 3owever# they do not scale well# and have poor convergence properties#which has led to the development o$ more complex *ut more scala*le linkstate routingprotocols $or use in large networks.

! /inkstate routing is a concept used in routing o$ packetswitched networks in computercommunications. /inkstate routing works *y having the routers tell every router on the


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network a*out its closest neigh*ors. he entire routing ta*le is not distri*uted $rom any router#only the part o$ the ta*le containing its neigh*ors.

/inkstate routing protocols are the &"P'# I"I"# EIGRP and 4ovell5s 4/"P (4etare /ink"tate Protocol) which only supports IP6.his type o$ routing protocol re2uires each router to maintain at least a partial map o$ thenetwork. hen a network link changes state (up to down# or vice versa)# a noti$ication# calleda link state advertisement (/"!) is $looded throughout the network. !ll the routers note the

change# and recompute their routes accordingly.

/ink "tate Routing protocols provide greater $lexi*ility and sophistication than the -istanceector routing protocols. hey reduce overall *roadcast tra$$ic and make *etter decisionsa*out routing *y taking characteristics such as *andwidth# delay# relia*ility# and load intoconsideration# instead o$ 0ust distance or hop count.

Exploring the update behavior o distance-vector and link-state protocols rom theperspective o a single router R.

'or the $ollowing scenarios# consider whether R will 7always7# 7never7# or 7sometimes7transmit routing data as a result o$ a certain event in the network


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!nterior "ate#ay Routing Protocol $!"RP%

IGRP is a 8isco proprietary distance vector protocol. his means to use IGRP all routers must*e 8isco. IGRP has a max hop count o$ %99 with the de$ault *eing 1::. IGRP uses *andwithand delay as the de$ault metric this is called a composite metric. Relia*ility# load and ;

!. he IGRP route

+. he static route8. he RIP route-. !ll three will load *alance.

'ns#er (

Explanationo decide which route to use# I&" uses a concept called !dministrative -istance.!dministrative distance is a num*er that denotes how *elieva*le an entire routing protocol ison a single router. he lower the num*er# the *etter# or more *elieva*le the routing protocol.+y de$ault# static routes have an administrative distance o$ 1.

&uestion=ou want to con$igure a router $or load *alancing across ? une2ual cost paths on yournetwork. hich o$ the $ollowing routing protocols can you use> (8hoose two)

!. RIP v1+. RIP v%8. &"P'-. IGRPE. EIGRP'. /";'ns#er D ) E

Routing (asics

Routing is the process o$ directing packets $rom a source node to a destination node on adi$$erent network. Getting packets to their next hop re2uires a router to per$orm two *asicactivities,path determinationandpacket switching.

Path determinationInvolves reviewing all paths to a destination network and choosing the optimal route.o determine the optimal route# in$ormation is put in a route ta*le# which includesin$ormation such as destination network# the next hop# and an associated metric.

Packet s#itchingInvolves changing a packet5s physical destination address to that o$ the next hop (thepacket5s logical destination and source addresses will stay the same).

!normation a Router needs to kno# to route a packet

-estination address.

4eigh*or routers.

Possi*le routes to all remote networks.

he *est route to each network.

3ow to maintain and veri$y routing in$ormation.

here are two main types o$ routing# static and dynamic# the third type o$ routing is called

3y*rid. "tatic routing involves the cum*ersome process o$ manually con$iguring andmaintaining route ta*les *y an administrator. -ynamic routing ena*les routers to 7talk7 toeach other and automatically update their routing ta*les. his process occurs through the useo$ *roadcasts.

Static Routing

!dvantages


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4o over head on 8P Every network known *y router ! in with a hop count higher than :# has *eenlearned $rom router +. 8ommon sense suggests that $or router ! to *roadcast the networks ithas learned $rom router + *ack to router + is a waste o$ resources. &*viously# + alreadyknows a*out those networks.

! route pointing *ack to the router $rom which packets were received is called a reverseroute. Split horizonis a techni2ue $or preventing reverse routes *etween two routers.

"o the routing protocol knows which inter$ace a network route was learned on and will notadvertise the route *ack out to same inter$ace.

4ow a packet with a destination address o$ 1:.1.9.C arrives at router +. + consults its route

ta*le and $orwards the packet to !. ! consults its route ta*le and $orwards the packet to +# +$orwards it *ack to !# ad infinitum. ! routing loop has occurred.

Implementing split horion prevents the possi*ility o$ such a routing loop. here are twocategories o$ split horion, simple split horion and split horion with poisoned reverse.

Simple split horion does not advertise routes back to the neighbours rom #hich theroutes #ere learned.

he routers in implement simple split horion. Router + sends an update to router ! $ornetworks 1:.1.1.:# 1:.1.%.:# and 1:.1.C.:. 4etworks 1:.1.?.: and 1:.1.9.: are not included*ecause they were learned $rom router !. /ikewise# updates to router 8 include 1:.1.?.: and

1:.1.9.: with no mention o$ 1:.1.1.:# 1:.1.%.:# and 1:.1.C.:.

1:.1.1.: 1:.1.%.: 1:.1.C.: 1:.1.?.: 1:.1.9.:

10.1.4.0 10.1.1.0

10.1.5.0 10.1.2.0

10.1.3.0

"imple split horion works *y suppressing in$ormation. "plit horion with poisoned reverse isa modi$ication that provides more in$ormation.


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Split ,orion #ith poisoned reverse

10.1.1.0 10.1.2.0 10.1.3.0 10.1.4.0 10.1.5.0

hops hops 1:.1.1.: in$ 1:.1.1.: % 1:.1.%.: in$ 1:.1.%.: 1 1:.1.C.: in$ 1:.1.C.: : 1:.1.?.: : 1:.1.?.: in$ 1:.1.9.: 1 1:.1.9.: in$

In the scenario router + would in $act advertise 1:.1.?.: and 1:.1.9.: to router !# *ut thenetwork would *e marked as unreacha*le. !*ove shows what the route ta*les $rom + to 8and ! would look like. 4otice that a route is marked as unreacha*le *y setting the metric toin$inityD

Split horion #ith poisoned reverse advertises reverse routes but #ith an unreachable$ininite% metric.

"plit horion with poisoned reverse is considered sa$er and stronger than simple split horionBa sort o$ 7*ad news is *etter than no news at all7 approach. 'or example# suppose thatrouter 8 in receives corrupted in$ormation causing it to *elieve that su*net 1:.1.1.: isreacha*le via router +. "imple split horion would do nothing to correct this misperception#

whereas a poisoned reverse update $rom router + would immediately stop the potential loop.'or this reason# most modern distance vector implementations use split horion with poisonedreverse.

Split horion #ill not prevent routing loops here.

1:.1.?.: 1:.1.9.:

/ink $ailure

1:.1.1.: 1:.1.C.:

1:.1.%.: I can reach1:.1.9.: via - one hop or via +

C hops away.


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&uestion

he graphic shows a network that is con$igured to use RIP routing protocol. Router% detectsthat the link to Router1 has gone down. It then advertises the network $or this link with a hopcount metric o$ 1F. hich routing loop prevention mechanism is in e$$ect>

!. split horion+. error condition8. holddown timer-. route poisoningE. count to in$inity

'ns#er D

route poisoning

*ounting to !ninity

"plit horion will *reak loops *etween neigh*ours# *ut it will not stop loops in a network suchas the one in a*ove. !gain# 1:.1.9.: has $ailed. Router - sends the appropriate updates to itsneigh*ours router ! (the dashed arrows) and router 8 (the solid arrows). Router 8 marks theroute via - as unreacha*le# *ut router + is advertising a next*est path to 1:.1.9.:# which is Chops away. 8 posts that route in its route ta*le.

8 now in$orms - that it has an alternative route to 1:.1.9.:. - posts this in$ormation andupdates !# saying that it has a ?hop route to the network. ! tells + that 1:.1.9.: is 9 hopsaway. + tells 8 that the network is now F hops away.

7!h#7 router 8 thinks# 7router +5s path to 1:.1.9.: has increased in length. 4onetheless# it5s theonly route I5ve got# so I5ll use it7

8 changes the hop count to H# updates -# and around it goes again. his situation is thecounting-to-infinitypro*lem *ecause the hop count to 1:.1.9.: will continue to increase toin$inity. !ll routers are implementing split horion# *ut it doesn5t help.

"etting a maximum hop count o$ 19 helps solve the countingtoin$inity pro*lem# *utconvergence will still *e very slow. Given an update period o$ C: seconds# a network couldtake up to H.9 minutes to reconverge and is suscepti*le to routing errors during this time. hetwo methods $or speeding up reconvergence are triggered updates and holddown timers.

+riggered pdates

Triggered updates# also known as flash updates# are very simple, I$ a metric changes $or*etter or $or worse# a router will immediately send out an update without waiting $or its updatetimer to expire. Reconvergence will occur $ar more 2uickly than i$ every router had to wait $orregularly scheduled updates# and the pro*lem o$ counting to in$inity is greatly reduced#although not completely eliminated. Regular updates may still occur along with triggeredupdates. hus a router might receive *ad in$ormation a*out a route $rom a notyet


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reconverged router a$ter having received correct in$ormation $rom a triggered update. "uch asituation shows that con$usion and routing errors may still occur while an internetwork isreconverging# *ut triggered updates will help to iron things out more 2uickly.

! $urther re$inement is to include in the update only the networks that actually triggered it#rather than the entire route ta*le. his techni2ue reduces the processing time and the impacton network *andwidth.

,olddo#n +imers

' holdo#n prevents regular update messages rom reinstating a route that is going upand do#n $lapping). ypically this happens on a serial link thats losing connectivity andthen coming *ack up. 3oldowns prevent routes $rom changing rapidly *y allowing time $oreither the downed route to come *ack up or the network to sta*lise *e$ore it tries toreconverge. Holddown timersintroduce a certain amount o$ skepticism to reduce theacceptance o$ *ad routing in$ormation.

I$ the distance to a destination increases ($or example# the hop count increases $rom % to ?)#the router sets a holddown timer $or that route.


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/inkstate routing uses,

Link-state advertisements $LS's% 0 ! linkstate advertisement (/"!) is a smallpacket o$ routing in$ormation that is sent *etween routers.

+opological database 0 ! topological data*ase is a collection o$ in$ormationgathered $rom /"!s.

SP1 algorithm 0 he shortest path $irst ("P') algorithm is a calculation per$ormed on

the data*ase resulting in the "P' tree. Routing tables 0! list o$ the known paths and inter$aces.

/ink state routing protocols only send out routing updates when a change in the internetwork$orces them to change their routing ta*le# these updates only contain the changes that haveoccurred in the internetwork they dont send out the whole routing ta*le. hese link statesupdates are called /ink "tate !dvertisements (/"!) they are much less *andwidth intensiveandfloodthroughout the network. !ll the routers note the change# and recompute their routesaccordingly.

hen the router receives an /"!# the data*ase is updated with the most recent in$ormationand computes a map o$ the internetwork using the accumulated data and calculates the


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shortest path to other networks using the "P' algorithm. Each time an /"! packet causes achange to the linkstate data*ase# "P' recalculates the *est paths and updates the routingta*le.

/inkstate concerns

Processor overhead

;emory re2uirements

+andwidth 8onsumption

Routers running linkstate protocols re2uire more memory and per$orm more processing thandistance vector routing protocols. Routers must have su$$icient memory to *e a*le to hold allthe in$ormation $rom the various data*ases# the topology tree# and the routing ta*le. Initiallinkstate packet $looding consumes *andwidth. -uring the initial discovery process# allrouters using linkstate routing protocols send /"! packets to all other routers. his action$loods the internetwork and temporarily reduces *andwidth availa*le $or routed tra$$ic carryinguser data. !$ter this initial $looding# linkstate routing protocols generally re2uire only minimal*andwidth to send in$re2uent or event triggered /"! packets re$lecting topology changes.

/ink "tate protocols only send updates when a change occurs# which makes them moree$$icient $or larger networks. +andwidth and delay are the most widely used metrics whenusing /ink"tate protocols. Eg, &"P' 4/"P and I"I".

+ene$its o$ /ink "tate protocols,

1. !llows $or a larger scala*le network%. Reduces convergence timeC. !llows supernettingJ

&uestionhy is it di$$icult $or routing loops to occur in networks that use linkstate routing>

!. Each router *uilds a simple view o$ the network *ased on hop count.+. Routers $lood the network with /"!s to discover routing loops.

8. Each router *uilds a complete and synchronied view o$ the network.E. Routers use holddown timers to prevent routing loops.

'ns#er *

Each router *uilds a complete and synchronied view o$ the network.


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Exam Essentials

o con$igure RIP routing

enable

configt 1st

enter glo*al con$iguration mode.Router(config)#router rip type router rip

hen add all directly connected networks using a class$ul address ie i$ network 1:.1.1.: isdirectly connected the command would *enetwork 1:.:.:.:

Router(config-router)#network 10.0.0.0

Router(config-router)# Z

Router#

8on$iguring RIPv%

configt

Router(config)#router rip

Router(config-router)#network 10.0.0.0

Router(config-router)#version 2

Kust add version %

rou*leshooting Routing Protocols

Its important to veri$y your con$iguration o$ routing protocols the same commands used toveri$y are also used to trou*leshoot.

he show ip route coan!displays the current contents o$ the routing ta*le.

Router"#sh ip route

Codes: C connected etc , D EIGRP

Gateway of last resort is not set

D 192.16.!"."#2$ %9"#21&2$16' (ia 192.16.2".2, "":"$:!6, serial"#"

C 192.16.1"."#2$ is directly connected, )astEt*ernet"#"

D 192.16.$"."#2$ %9"#261+6' (ia 192.16.2".2, "":"$:!6, serial"#"

C 192.16.2"."#2$ is directly connected, serial"#"

D 192.16.+"."#2$ %9"#2&"&$+6' (ia 192.16.2".2, "":"$:!+, serial"#"

Roter-

&uestion!$ter typing the command show ip route# you see a list o$ IP addresses with L1%:MNO next tothem. hat does the L1%:MNO mean>

!. 8ostM;etric+. !dministrative -istanceM;etric8. ;etricM-istance-. 8ostM3op 8ount

'ns#er (

he routing ta*le in$ormation shown gives the administrative distance (!-) with the metric.! is incorrect# as the LMO shows the !- and metric. 8ost is a metric used *y &"P'.8 is incorrect# as the !- is shown $irst# then the metric# not the other way around.- is incorrect# as the L!-M;etricO is not shown as cost and hop count.


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+he sho# protocols command

he show protocols coan!displays all the routed protocols and the inter$ace upon

which the protocol is ena*led.

Router#show ip protocol

Glo/al (ales:

Internet Protocol rotin0 is ena/led)astEt*ernet" is , line rotocol is

Internet address is 192.16.!".1#2$

erial"#" is , line rotocol is

Internet address is 192.16.2".2#2$

erial"#1 is , line rotocol is

Internet address is 192.16.$".1#2$

Roter3

his output shows the IP address o$ the 'astEthernet :M:# "erial :M:# and "erial :M1 inter$aceso$ Router+.

he show ip protocols coan!.-isplays the routing protocols con$igured on your

router. he $ollowing output shows that *oth RIP and IGRP are running on the router. &nlythe IGRP shows on the routing ta*le *ecause it has a lower administrative distance !-.

Router#show ip protocols

Rotin0 Protocol is 4ri5

endin0 dates e(ery !" seconds net de in 6 seconds

In(alid after 1" seconds, *old down 1", fls*ed after 2$"

7t0oin0 date filter list for all interfaces is

Inco8in0 date filter list for all interfaces is

Redistri/tin0: ri

Defalt (ersion control: send (ersion1 recei(e any (ersionInterface end Rec( eyc*ain

)astEt*ernet 1 1 2

erial"#" 1 1 2

erial"#1 1 1 2

Rotin0 for networ;s

192.16.1"."

192.16.2"."

192.16.!"."

Rotin0 Infor8ation orces:

Gateway Distance defalt is 12"?

Rotin0 Protocol is 4i0r 1"5

endin0 dates e(ery 9" seconds net de in $2 seconds

In(alid after 2&" seconds, *old down 2", fls*ed after 6!"

7t0oin0 date filter list for all interfaces is

Inco8in0 date filter list for all interfaces is

Defalt networ;s fla00ed in ot0oin0 dates

Defalt networ;s acceted fro8 inco8in0 dates

IGRP 8etric wei0*t 1@1, 2@", !@1, $@", +@"

IGRP 8ai88 *ocont 1""

IGRP 8ai88 8etric (ariance 1

Redistri/tin0: ei0r 1", i0r 1"

Rotin0 for networ;s192.16.1"."

192.16.2"."

192.16.!"."

Rotin0 Infor8ation orces:

Gateway Distance


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Distance: >defalt is 1""?

he show ip protocols command includes the autonomous system (!")# routing timers#

networks advertised# gateways and !- (1::)

hese commands can *e used on all IP routing protocols

Exam uestions Q !nswers

&uestionhe $ull command to see all de*ugs running on the router isRCsho# debug(IP packet de*ugging is on)

and the $ull command to turn o$$ all running de*ugs isRCundebug all(!ll possi*le de*ugging has *een turned o$$)

&uestionhe de$ault *ehaviour o$ a distance vector protocol that does not allow a router to send aroute advertisement out the same inter$ace upon it was originally learned a*out is>

'ns#er2 Split horion"plit horion prevents a router inter$ace $rom advertising a route out o$ the same inter$aceupon which it was learned in the $irst place.

&uestion=ou issue the command show ip routewhich o$ the $ollowing correctly descri*es the codesdisplayed in your route ta*le a$ter you issue this command> (8hoose two.)

!. IIndicates a route was learned through an internal protocol.+. "Indicates a route was learned through static command.

8. RIndicates a route was learned through RIP.-. "Indicates a route was learned through a serial port.E. RIndicates a route was learned through a relia*le port.

'ns#er (/ *

&uestion he network 1C1.1:H.?.:M%? was advertised *y a neigh*our router $rom RIP and IGRP. =ou


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also added a static route to 1C1.1:H.?.:M%? manually. hich route would *e used to $orwardtra$$ic>

!. he IGRP route.+. he static route.8. he RIP route.-. !ll three will load *alance.

'ns#er (

&uestion=our *oss asks you to explain the di$$erence *etween a routed protocol and a routingprotocol. hich o$ the $ollowing statements *est descri*e the di$$erence> (8hoose two.)

!. Routed protocols cannot cross routers.+. Routing protocols can identi$y data.8. Routed protocols help transport data *etween network segments.-. Routing protocols are used *y routers to communicate routing in$ormation.

'ns#ers * and D

Routed protocols such as IP or IP6 are used to help communicate data across networksegments *y using logical addressing# which identi$ies *oth the network and host that need tocommunicate.- is a correct answer# as routers use routing protocols such as RIP and &"P' tocommunicate changes in their routing ta*les.! is incorrect *ecause routed protocols can cross routers.+ is incorrect *ecause routing protocols populate only routing ta*lesD they cannot identi$y data

&uestion=our *oss is concerned a*out routing loops with the use o$ distance vector routing protocolssuch as RIP and IGRP in your network you would like to ensure him that there are

mechanisms used to prevent the possi*ility o$ a routing loop. hich o$ the $ollowing areexamples o$ this mechanism> (8hoose two.)

!. /inkstate advertisement (/"!)+. "panning ree Protocol.8. "hortest path $irst tree.-. "plit horion.E. 3olddown timers.

'ns#er D/ E

ExplanationRIP and IGRP are distance vector routing protocols. RIP and IGRP use holddown counters

and split horion to prevent route looping.

4ote, 3olddowns are used to prevent regular update messages $rom inappropriatelyreinstating a route that might have gone *ad. "plit horions derive $rom the premise that it isnever use$ul to send in$ormation a*out a route *ack in the direction $rom which it came.

Incorrect !nswers!# /inkstate routing protocols use /"!s. 3owever# /"!s are not used *y distance vectorrouting protocols like RIP or IGRP.+# 8, he spanning tree protocol and the shortest path $irst tree is used *y /ayer C routingprotocol such as Enhanced IGRP or &"P'# not *y RIP or IGRP.

&uestion

hat one o$ the $ollowing protocols is an example o$ a link state routing protocol that uses the8PMIP protocol stack>

!. IP+. I"I"8. 4/"P-. &"P'


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E. RIP ver %

'ns#er D

Explanation&"P' is a linkstate protocol that can *e used in routing that is part o$ the 8PMIP protocolstack.Incorrect !nswers

!. IP is a protocol# and a routed protocol# *ut 4& a routing protocol.+. I"I" is a routing algorithm used *y the I"& protocol stack.8. 4/"P is part o$ the 4ovell protocol stack.E. RIP (and RIP %) are distance vector protocols.

&uestionhere are three ma0or groups o$ routing protocols, distancevector protocols# linkstateprotocols# and hy*rid protocols. "elect two valid statements regarding routing protocols>(8hoose two)

!. -istance vector protocols send the entire routing ta*le to directly connected neigh*ors.+. /ink state protocols send the entire routing ta*le to all routers in the network.8. -istance vector protocols send updates a*out directory connected neigh*ors to allnetworks listed in the routing ta*le.-. /ink state protocols send updates containing the state o$ their own links to all other routerson the network.

'ns#er '/ D

Explanation-istance vector protocols send their entire routing ta*le to ad0acent routers. /inkstateprotocols only send linkstate updates to all routers on their network (or autonomous system).Incorrect !nswers+. /ink state protocols do not send their entire routing ta*le rather only updates on their ownlinks.8. -istance vector protocols in $act send their entire routing ta*les.

&uestionhen /ayer % devices were $irst created there was no way o$ detecting *ridging loops. !protocol was created *y -E8 to stop these loops. hat protocol did -E8 create>

!. irtual /!4s.+. 'rame $iltering.8. 8ut through switching.-. "panning tree protocol.

'ns#er D

Explanation"panning ree protocol *uilds &4E path through all the nodes# and eliminates any loops.!nything sent along the tree will not encounter any loops *ecause the protocol will eliminateany loops.Incorrect !nswers!. /!4s is not a loop resolution technology.+. 'rame 'iltering is not a loop resolution technology.8. 8ut through switching is not a loop resolution technology. Rather it is a $orm o$ $orwarding.

&uestionhich o$ the $ollowing statements regarding routed and routing protocols are true>(8hoose two)

!. ! routed protocol is assigned to an inter$ace and determines the method o$ packet delivery.+. ! routing protocol determines the path o$ a packet through a network.8. ! routed protocol determines the path o$ a packet through a network.-. ! routing protocol operates at the transport layer o$ the &"I model.E. ! routed protocol updates the routing ta*le o$ a router.

'ns#er '/ (


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Explanation!. Routed Protocol delivers data to an inter$ace or host.+. Routing Protocol Routes data.Incorrect answers,8# E. Routed protocol only delivers data# Routed Protocol will not update any routing ta*les.-. ! Routing Protocol operates on /ayer C o$ &"I ;odel. ie 4etwork layer.

&uestion=ou are con$iguring a network at a main site in oronto. =ou use a distance vector routingprotocol.hat could you use to prevent routing loops in the network> (8hoose two)

!. /inkstate advertisements (/"!)+. "panning ree Protocol8. "hortest path $irst tree-. "plit horionE. 3olddown timers

'ns#er D/ E

Explanation"plit horion S the routing protocol advertises routes out an inter$ace only i$ they were notlearned $rom updates entering that inter$ace.3olddown timer S !$ter $inding out that a router to a su*net has $ailed# a router waits a certainperiod o$ time *e$ore *elieving any other routing in$ormation a*out that su*net.

&uestionhich statement descri*es the rule o$ split horion>

!. &nly routers can split *oundaries (horions) *etween concentric networks.+. !ll distance vector protocols re2uire $all *ack routers that may cause momentary loops asthe topology changes.8. 4etworks can only remain $ully converged i$ all in$ormation a*out routers is sent out allactive inter$aces.-. In$ormation a*out a route should not *e sent *ack in the direction $rom which the originalupdate come.E. Each !" must keep routing ta*les converged to prevent dead routes $rom *eing advertisedacross the !" *oundary.

'ns#er D

Explanation"plit horion includes two related concepts that a$$ect what routes are included in a routingupdate!n update does not include the su*net o$ the inter$ace out which the update is sent

!ll routes with outgoing inter$ace o$ inter$ace x are not included in updates sent out that sameinter$ace x.

&uestionhich o$ the $ollowing technologies can *e used in distance vector routing protocols toprevent routing loops> ("elect all valid answer choices)

!. "panning ree Protocol+. "hortest path $irst tree8. /inkstate advertisements (/"!)-. 3olddown timersE. "plit horion'. RRP

'ns#er D/ E

Explanation -istance vector routing protocols use the rule o$ split horions and hold downtimers to prevent routing loops a$ter a topology change. T "plithorion the routing protocoladvertises routes out an inter$ace only i$ they were not learned $rom updates entering thatinter$ace. T 3olddown timer !$ter $inding out that a router to a su*net has $ailed# a routerwaits a certain period o$ time *e$ore *elieving any other routing in$ormation a*out that su*net.


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Incorrect !nswers!. "P is used in *ridged /!4s to prevent *ridging loops. It is a means $or preventing loopsat layer two# not layer C.+# 8. hese are two o$ the mechanisms o$ /ink "tate Protocols# not distance vector protocols.'. RRP is the irtual Router Redundancy Protocol# which is a standards *ased methodsimilar to 8isco5s proprietary 3"RP. 4either o$ these two methods deal with distance vectorrouting protocols.

&uestionhat is the reason $or con$iguring a passive inter$ace on a router> ("elect only one answer)

!. !llows inter$aces to share common IP addresses.+. !llows an inter$ace to remain up without the aid o$ keepalives.8. !llows a router to send routing and not receive updates via that inter$ace.-. !llows a routing protocol to $orward updates that is missing its IP address.E. !llows a router to receive routing updates on an inter$ace *ut not send updates via thatinter$ace.

'ns#er E

Explanation, he passiveinter$ace command is used to control the advertisement o$ routingin$ormation. he command ena*les the suppression o$ routing updates over some inter$aceswhile allowing updates to *e exchanged normally over other inter$aces. 'or any inter$acespeci$ied as passive# no routing in$ormation will *e sent. Routing in$ormation received on thatinter$ace will *e accepted and processed *y the router. his is o$ten use$ul $or --R links suchas I"-4.

&uestionhich one o$ the $ollowing statements *est explains the split horion rule>

!. &nly routers can split *oundaries (horions) *etween networks in separate !" num*ers.+. Each !" must keep routing ta*les converged to prevent dead routes $rom *eing advertisedacross *oundaries.8. &nce a route is received on an inter$ace# advertise that route as unreacha*le *ack out thesame inter$ace.-. In$ormation a*out a route should never *e sent *ack in the direction $rom which the originalupdate came.

'ns#er D

Explanation he split horion rule states, T 4ever advertise a route out o$ the inter$acethrough which you learned it. 'or instance# in 'igure ?a *elow# i$ Router &ne is connected toRouters wo and hree through a single multipoint inter$ace (such as 'rame Relay)# and

Router &ne learned a*out 4etwork ! $rom Router wo# it will not advertise the route to4etwork ! *ack out the same inter$ace to Router hree. Router one assumes that Routerhree would learn a*out 4etwork ! directly $rom Router wo.

Incorrect !nswers!. here is no such re2uirement +. -istance vector protocols updates routing ta*le at regularintervals instead o$ opology changes 8. his is the de$inition o$ the poison reverse rule# notthe split horion rule.

&uestionhe statements *elow compare and contrast link state and distance vector routing protocols.hich o$ these are true> (8hoose two)

!. -istance vector protocols send the entire routing ta*le to directly connected neigh*ors.+. -istance vector protocols are responsi*le $or sending updates to all networks listed in therouting ta*le.8. /ink state protocols are responsi*le $or sending the entire routing ta*le to the wholenetwork.-. /ink state protocols send updates regarding their own links status to all other routers onthe network.


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'ns#er '/ D

Explanation-istance ector Protocols advertise routing in$ormation *y sending messages# called routingupdates# out the inter$aces on a router. hese updates contain a series o$ entries# with eachentry representing a su*net and a metric. /ink"tate Protocols, "end partial updates when

link status changes and $loods $ull routing ta*le updates every C: minutes. he $looding#however# does not happen all at once# so the overhead is minimal.

Incorrect !nswers+. -istance ector protocols only send in$ormation to ad0acent neigh*ors. 8. &nly partialrouting updates and sent to neigh*ors on a regular *asis. he entire ta*le is not sent to allneigh*ors. his would o*viously create $ar too much overhead tra$$ic.

&uestionhat are the characteristic o$ link state routing protocols> (8hoose all that apply.)

!. he exchange o$ advertisement is triggered *y a change in the network.+. !ll routers exchange routing ta*les with each other in a multipoint network.8. Packets are routed *ased upon the shortest path to the destination.-. Paths are chosen depending on the cost e$$iciency $actor.E. Every router in an &"P' area is capa*le o$ representing the entire network topology.'. &nly the designated router in an &"P' area can represent the entire network topology.

'ns#er '/ */ E

Explanation, &pen "hortest Path 'irst

T Each router discovers its neigh*ors on each inter$ace. he list o$ neigh*ors is kept in aneigh*or ta*le. T Each router uses a relia*le protocol to exchange topology in$ormation withits neigh*ors. T Each router places the learned topology in$ormation into its topologydata*ase. T Each router runs the "P' algorithm against its own topology data*ase. T Eachrouter runs the "P' algorithm against its own topology data*ase to calculate the *est routesto each su*net in the data*ase. T Each router places the *est rouU to each su*net into the IProuting ta*le. he $ollowing list points out some o$ the key $eatures o$ &"P',T 8onverges very 2uickly $rom the point o$ recogniing a $ailure# it o$ten can converge in lessthan 1: seconds.T "upports /";.T


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8 is incorrect *ecause distancevector routing protocols are notoriously slow in updating# as aresult o$ their periodic nature. his can cause convergence issues.

&uestionhen a router sets the metric $or a network that has gone down to the maximum value# whatis it doing>!. !pplying splithorion+. Putting the route in holddown8. Poisoning the route-. "ending a triggered update

'ns#er *

he distancevector mechanism to set the metric to the maximum value is called routepoisoning.! is incorrect# even though route poisoning is part o$ the splithorion $amily. "plithorion# *yde$inition# does not allow updates to travel *ack out the inter$ace on which they arrived.+ is incorrect# as holddown timers are not activated *y route poisoning.- is incorrect# as triggered updates occur when a change occurs on a network and re2uire anupdate that is earlier than the de$ault timer.

&uestionhich o$ these statements is true regarding distancevector routing protocols>

!. hey send the entire routing ta*le to directly connected neigh*ors.+. hey send the entire routing ta*le to every router in the network.8. hey send the changes to the routing ta*le to directly connected neigh*ors.-. hey send the changes to the routing ta*le to every router in the network.

'ns#er '

-istancevector routing protocols send the entire routing ta*le to directly connectedneigh*ors.+ is incorrect# as the routers send the ta*les only to their directly connected neigh*ors. I$ arouter learns o$ another route $rom its neigh*or# it then passes that in$ormation on to anotherneigh*or.

8 is incorrect# as distancevector routing protocols send the entire ta*le# not 0ust the changes.- is incorrect# as the routers send the entire ta*le# and they send it only to directly connectedneigh*ors.

&uestiono prevent routing loops# distancevector routing protocols use a maximum metric. hat isthe maximum reacha*le metric $or RIP>

!. 1:+. 198. 1F-. F:

E. 1::'. %99

'ns#er (

he maximum hop count that RIP allows is 19.


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8 is incorrect *ecause a hop count o$ 1F is the point that is considered unreacha*le *y theRIP protocol. !ll other answers are incorrect *ecause they either overshoot or underestimatethe maximum hop count.

&uestionhat routing loop solution prevents a router $rom sending in$ormation *ack to the neigh*orthat originally sent the in$ormation>

!. "plithorion+. 3olddown timer8. ;aximum hop count-. Route poisoningE. 8ounting to in$inity

'ns#er '

"plithorion is a mechanism in distancevector routing protocols that prevents routers $romsending updates *ack the way they came.+ is incorrect# as holddown timers are used to prevent routing ta*les $rom responding too2uickly to sudden changes.8 is incorrect# as the ;aximum 3op count re$ers to the maximum metric that RIP can use.- is incorrect# as route poisoning is the process o$ taking a route and making it the maximumdistance# thus 7poisoning7 the route.E is incorrect# as 7counting to in$inity7 is a pro*lem i$ a routing update loop occurs.

&uestion=ou type show ip route on Router ! to $ind out what entries are in your RIP routing ta*le.hich o$ the $ollowing routes would not *e $ound on a separate router receiving an RIPupdate $rom Router !>

!. R 1H%.1F.:.:M1F L1%:M?O+. R 1A%.1FN.A.:M%? L1%:M%O8. 8 1A%.1FN.?.:M%?-. R 1A%.1FN.H.:M%? L1%:M19OE. R 1A%.1FN.N.:M%? L1%:MNO

'ns#er D

he metric shown on this route is 19. I$ it is passed to a neigh*oring router it increments *yone e2ualing 1F# which is unreacha*le.! is incorrect# as this route could *e passed to neigh*ors.+ is incorrect# as this route could *e passed to a neigh*oring router.8 is incorrect# as a directly connected network could *e advertised to a neigh*oring router.E is incorrect# as this route is also within the 19hop limit.

&uestion=ou need to know what IP routing protocol is in use on your router. hich command shouldyou use>

!. show protocol

+. show routing protocol8. show runningcon$ig-. show ip protocol

'ns#er D

he show ip protocol command lists all routing protocols that are running on your router.


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! is incorrect *ecause it shows you only the routed protocols# not the routing protocols# onyour router.+ is incorrect# *ecause this is not a valid command.8 is incorrect# as the show runningcon$ig command shows you what routing protocols arerunning# *ut it also gives you all the other con$iguration in$ormation currently operating inR!;. his is too much in$ormation# when all you wanted was to $ind out which routingprotocol was running on the machine.

&uestion=ou are con$iguring a network at your head2uarters in Phoenix# !riona. =ou decide to use adistancevector routing protocol. hat command do you need to type to activate themechanisms that stop routing loops> (8hoose two.)

-. P*oeniRAR1>confi0roter?slit*oriBon ri

3. P*oeniRAR1>confi0roter?slit*oriBon distance (ector

C. P*oeniRAR1>confi0roter?distance (ector

D. P*oeniRAR1>confi0roter?roter i0r 1""

E. P*oeniRAR1>confi0roter?no rotin0loos

). P*oeniRAR1>confi0roter?roter ri

'ns#ers D and 1

3ou merely need to activate the routing protocols to turn on the routing loop-

prevention mechanisms4 they are activated by deault.! and + are incorrect# as you actually use the no splithorion command to turn it o$$ and 0ustthe splithorion command to turn it *ack on. Regardless# it is on *y de$ault.8 and E are incorrect# as there are no such commands.

&uestion=ou are training some new network engineers when one o$ them asks you the di$$erence*etween class$ul and classless routing protocols. hat do you tell him>

!. hey are pretty much the same# except class$ul protocols have more options.+. 8lassless supports /";# whereas class$ul supports '/";.8. 8lass$ul can *e used only *etween autonomous systems.-. RIPv1 and IGRP are the standard classless protocols# as they were some o$ the $irstprotocols in existence.

'ns#er (&ne o$ the main $eatures o$ classless protocols is to include the su*net mask in$ormation intheir updates# whereas class$ul does not. his means that you can use only $ixedlengthsu*net masks ('/";s)# not varia*lelength su*net masks (/";s) in a class$ul routingenvironment. ! is incorrect# as classless protocols actually have more options.8 is incorrect# as *oth class$ul and classless protocols can cross autonomous systems.- is incorrect# as RIPv1 and IGRP are actually class$ul protocols.

&uestionhich o$ the $ollowing are valid routing protocol trou*leshooting commands> (8hoose two.)

!. R1show ip route+. R1(con$igi$)show ip protocols


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8. R1Vde*ug ip rip-. R1show ip protocols

'ns#ers ' and D

he show ip route and show ip protocols commands give you in$ormation on the routingprotocols and what routes they have placed into the routing ta*le.

+ is incorrect# as you cannot issue the show ip protocols command $rom Inter$ace8on$iguration mode.8 is incorrect# as you cannot use de*ug commands $rom the (8hoose three.)!. Provides common view o$ entire topology+. Exchanges routing ta*les periodically with neigh*ors8. 8alculates shortest path-.


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'ns#ers '/ (/ */ and E

! is correct# as RIPv1 is an Interior Gateway Protocol (IGP)# meaning it exchanges routingin$ormation with routers in the same autonomous system (!"). + is correct# as RIPv% is alsoan IGP. 8 is correct# as IGRP actually gets its name $rom *eing an IGP.E is correct# as &"P' is an industry standard IGP.- is incorrect# as +order Gateway Protocol is an External Gateway Protocol (EGP)# whichmeans it exchanges routing in$ormation *etween autonomous systems.

&uestion"plithorion states that no advertisements will *e sent *ack through the inter$ace on whichthey were received. hat mechanism overrides that *ehaviour>

!. riggered updates+. 3olddown timers8. Poison reverse-. 4othing overrides splithorion.

'ns#er *

Poison reverse# or 7reverse poisoning#7 overrides splithorion and sends a route poison*roadcast *ack out the inter$ace on which the poisoned route came in. ! is incorrect# astriggered updates override the periodic update *ehavior o$ distancevector protocols. + isincorrect# as holddown timers maintain sta*ility o$ the routing ta*le *y waiting *e$ore makingany changes. - is incorrect# as poison reverse overrides splithorion.

&uestion=our *oss asks you what 7count to in$inity7 means on a router. hich o$ the $ollowingexplanations is the most accurate>

!. It is when a router continues to send out packets to an unknown destination.+. It is when a routing ta*le entry continues to increment its metric without stopping.8. It is when a router calculates an unknown destination.-. It is when a router receives a poisoned route.

'ns#er (

! router without active de$ault mechanisms could theoretically increment its metric $orever i$ arouting update loop occurs# thus the phrase 7counting to in$inity.7 !ll distancevector routingprotocols now have *uiltin mechanisms to stop this *ehavior.! is incorrect# as a router# i$ it does not know where to send a packet# either discards thepacket or sends it to a 7de$ault route.78 is incorrect# as a router does not run a calculation on a route it does not know.- is incorrect# as a router marks a route as 7possi*ly down7 when it receives a poisoned routeupdate. It then activates a holddown timer until it either receives an update that the route is*ack up# or it hears nothing and $lushes the route $rom its ta*le.

&uestionhere are some di$$erences *etween routed and routing protocols. hich o$ the $ollowing areexamples o$ those di$$erences> (8hoose two.)

!. ! routed protocol is assigned to an inter$ace and determines the method o$ packet delivery.+. ! routing protocol determines the path o$ a packet through a network.8. ! routed protocol determines the path o$ a packet through a network.


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-. ! routing protocol operates at the transport layer o$ the &"I model.E. ! routed protocol updates the routing ta*le o$ a router.

'ns#er2 '/ (

Explanation,!, ! routed protocol delivers data.+, ! routing protocol routes data.

Incorrect !nswers8# E, ! routed protocol only delivers data# it does not route data or update any routing ta*les.-, ! routing protocol does not work at transport layer# layer ?. ;any routing protocols work atlayer C# the network layer.

&uestionhat is the total *andwidth o$ an I"-4 +RI circuit>

!. 9? kps+. F? kps8. 11% kps-. 1%N kpsE. 1?? kps

'ns#er E

Explanation,! +RI 8ircuit is % x F?W data channels plus one 1FW control channel @ 1??W! PRI 8ircuit is %C F?W data channels plus one F?W control channel @ 1.9??; @ 1 ($or

!. he IP host ta*le.


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+. he inter$aces output.8. Each section o$ the output.-. Each section o$ the output.E. he glo*al con$iguration statements.'. he section under the autonomous system num*er.

'ns#er (

Explanation,Each inter$ace that has the IP protocol# will show all IP addresses that were con$igured onthat inter$ace# with su*net masks.Incorrect !nswers!. his only shows the IP to host mappings# like the mapping o$ a 3&"" ta*le.8# -# E# and ', hese options do not provide this in$ormation.

&uestionhich characteristics are representative o$ a linkstate routing protocol> (8hoose three)!. Provide common view o$ entire topology+. Exchange routing ta*les with neigh*ors.8. 8alculates shortest path.-.


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(8hoose two)

!. -istance vector protocols send the entire routing ta*le to directly connected neigh*ors.

+. /ink state protocols send the entire routing ta*le to all routers in the network.8. -istance vector protocols send updates a*out directory connected neigh*ors to allnetworks listed in the routing ta*le.-. /ink state protocols send updates containing the state o$ their own links to all other routerson the network.

'ns#er '/ D

Explanation

-istance ector Protocols advertise routing in$ormation *y sending messages# called routingupdates# out the inter$aces on a router. hese updates contain a series o$ entries# with eachentry representing a su*net and a metric.

/ink"tate Protocols"ends partial updates when link status changes and $loods $ull updates every C: minutes.he $looding# however# does not happen all at once# so the overhead is minimal.

&uestionhich o$ the $ollowing routing protocols are less likely prone routing loops and networkreacha*ility pro*lems when used in discontiguous networks> ("elect all valid responses)

!. IGRP+. 8-P8. &"P'-. RIP v1E. RIP v%'. EIGRP

'ns#er */ E/ 1

Explanation, &nly &"P'# RIP version %# and EIGRP carry /"; in$ormation. In adiscontiguous network# su*net masks o$ di$$erent lengths can *e used# *ut this in$ormation willneed to *e propagated via the routing protocol i$ all networks are to *e reached.

Incorrect !nswers, !# -. ith RIP version one and IGRP# discontiguous networks can *epro*lematic# as /"; is not supported.+. 8-P is the 8isco -iscovery Protocol# which is used to exchange in$ormation *etween8isco devices. It can only *e used *etween 8isco routers and switches# and it is not a routingprotocol.

&uestion! router learns a*out a remote network $rom EIGRP# &"P'# and a static route. !ssuming allrouting protocols are using their de$ault administrative distance# which route will the router useto $orward data to the remote network>

!. he router will use the static route.+. he router will use the &"P' route.8. he route will use the EIGRP route.-. he router will load *alance and use all three routes.


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'ns#er '

Explanation, hen a router learns a*out the same network via multiple sources# the routerwill choose the source with the lowest administrative distance (!-). +y de$ault# the !- $orthese routing protocols are, 8onnected Inter$ace has : !- "tatic Route, 1 EIGRP, A:&"P', 11: "o# the static route will *e chosen since it has the lowest !-.

&uestion

hich o$ the $ollowing dynamic routing protocols are considered classless>(8hoose all that apply.)

!. IGRP

+. EIGRP

8. RIP v1

-. &"P'

E. I"I"

'. +GP

G. RIP v%

'ns#er (/ D/ E/ 1/ "

RIPv1 and IGRP are class$ul!ll the rest are 8lassless

Exam 8ram

Routed and Routing protocols,


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Routing protocols0o* is to maintain routing ta*les and route packets appropriately.Examples o$ routing protocols are RIP# IGRP# EIGRP# &"P'. Routers can support multipleindependent routing protocols and can update and maintain routing ta*les $or each protocolindependently.

Routed protocolsare used to transport user tra$$ic $rom source node to destination node.Examples o$ routed protocols are IP# IP6# !pplealk.

here are three ways a router learns how to $orward a packet,

Static Routes 8on$igured *y the administrator manually. he administrator must alsoupdate the ta*le manually every time a change to the network takes place. "tatic routes arecommonly used when routing $rom a network to a stu* (a network with a single route)network.

he command is

ip route network mask addressMinter$ace LdistanceO

ip route 1F9.??.C?.: %99.%99.%99.: 1F9.??.9F.9

3ere# 1F9.??.C?.: is the destination network or su*net %99.%99.%99.: is the su*net mask

1F9.??.9F.9 is the de$ault gateway.

Deault Routes he de$ault route (gateway o$ last resort) is used when a route is not knownor is in$easi*le. he command is

ip route :.:.:.: :.:.:.: 1F9.??.9F.

he de$ault gateway is set to 1F9.??.9F.9

-ynamic Routes In dynamic routing# the routing ta*les are automatically updated.-ynamic routing uses *roadcasts and multicasts to communicate with other routers.

he commands to ena*le rip are,

router rip

network Xma0or network num*erV.

Sho# !P protocol2his command will show in$ormation on RIP timers including routingupdate timer (C:sec de$ault)# holddown timer (de$ault 1N:sec). It also displays the num*er o$

seconds due $or next update (this is $raction o$ update timer). his command also gives thenetwork num*er $or which IP RIP is ena*led# Gateway# and the de$ault metric.

Sho# !P route, his command will display the IP routing ta*le entries. In addition# it displaysthe Gateway o$ last resort (i$ one is assigned). It also displays the codes used $or varioustypes o$ routes. "ome o$ the important codes are,

stands for static.

R stands for RIP.C directly connected

static

I IGRP

R RIP

8o/ile

3 3GP

D EIGRP


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E EIGRP eternal

7 7P)

I- 7P) inter area

1 7P) - eternal tye 1

2 7P) - eternal tye 2

E1 7P) eternal tye 1

E2 7P) eternal tye 2

E EGP

i II

33596497 Routing Protocols

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Transcript of 33596497 Routing Protocols