cap 8 single.doc

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cap 8 Single-Area OSPF

Open Shortest Path First (OSPF) is a link-state routing protocol that was developed as a replacement forthe distance vector routing protocol, RIP RIP was an accepta!le routing protocol in the earl" da"s ofnetworking and the Internet #owever, RIP$s reliance on hop count as the onl" metric for determining !estroute %uickl" !ecame pro!lematic &sing hop count does not scale well in larger networks with multiple

paths of var"ing speeds OSPF has significant advantages over RIP in that it offers faster convergence andscales to much larger network implementations

OSPF is a classless routing protocol that uses the concept of areas for scalability. 'his chapter covers !asic, single-area OSPF implementations and configurations

Edsger Wybe Dijkstra was a famous computer programmer and theoretical physicist. One of his most famous quotes was: he question of whether computers can think is like thequestion of whether submarines can swim.! Dijkstra"s work has been applied# among otherthings# to routing protocols. $e created the %hortest &ath 'irst (%&') algorithm for networkrouting.

s shown in Figure , OSPF version * (OSPFv*) is availa!le for IPv+ while OSPF version (OSPFv ) isavaila!le for IPv

.lick the dates in Figure * to see historical events related to OSPF


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'he initial development of OSPF began in 198 !" the Internet /ngineering 'ask Force (I/'F) OSPF0orking 1roup t that time, the Internet was largel" an academic and research network funded !" the& S government

In 1989! the specification for OSPFv1 was pu!lished in RF. 'wo implementations were writtenOne implementation was developed to run on routers and the other to run on &2I3 workstations 'he latter implementation !ecame a widespread &2I3 process known as 1 '/4 OSPFv was an e5perimentalrouting protocol and was never deplo"ed

"n 1991! OSPFv# was introduced in RF. *+6 !" 7ohn 8o" OSPFv* offered significant technicalimprovements over OSPFv "t is classless by design$ therefore! it supports %&S' and (")*

t the same time the OSPF was introduced, ISO was working on a link-state routing protocol of their own,Intermediate S"stem-to-Intermediate S"stem (IS-IS) I/'F chose OSPF as their recommended Interior1atewa" Protocol (I1P)

In 1998! the OSPFv# specification was updated in RF. * *9, which remains the current RF. for OSPF

"n 1999! OSPFv+ for "Pv, was pu!lished in RF. *6+: OSPF for "Pv, , created !" 7ohn 8o", Ro!.oltun, and 4ennis Ferguson, is not onl" a new protocol implementation for IPv , !ut also a ma;or rewriteof the operation of the protocol

"n # 8! OSPFv+ was updated in RF. < +: as OSPF for IPv

ote = In this chapter, unless e5plicitl" identified as OSPFv* or OSPFv , the term OSPF is used to indicateconcepts that are shared !" !oth

OSPF features, as shown in Figure , include=

(lassless - It is classless !" design> therefore, it supports ?@S8 and .I4R

/fficient - Routing changes trigger routing updates (no periodic updates) It uses the SPF algorithm

to choose the !est path Fast convergence - It %uickl" propagates network changes

Scalable - It works well in small and large network siAes Routers can !e grouped into areas tosupport a hierarchical s"stem

Secure - It supports 8essage 4igest < (84


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ll routing protocols share similar components 'he" all use routing protocol messages to e5change routeinformation 'he messages help !uild data structures, which are then processed using a routing algorithm'he three main components of the OSPF routing protocol include=)ata StructuresOSPF creates and maintains three data!ases= (see Figure )

Ad0acency database - .reates the neigh!or ta!le &in -state database 2&S)34 - .reates the topolog" ta!le For5arding database - .reates the routing ta!le

'hese ta!les contain a list of neigh!oring routers to e5change routing information with and are kept andmaintained in R 8*outing Protocol 'essages OSPF e6changes messages to conve" routing information using five t"pes of packets 'hese packets, asshown in Figure *, are=

#ello packet 4ata!ase description packet @ink-state re%uest packet &in -state update pac et - &S7 @ink-state acknowledgment packet


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'hese packets are used to discover neigh!oring routers and also to e5change routing information tomaintain accurate information a!out the network

Algorithm

'he .P& processes the neigh!or and topolog" ta!les using )i0 stra s SPF algorithm 'he SPF algorithmis !ased on the cumulative cost to reach a destination

'he SPF algorithm creates an SPF tree !" placing each router at the root of the tree and calculating theshortest path to each node 'he SPF tree is then used to calculate the !est routes OSPF places the !estroutes into the forwarding data!ase, which is used to make the routing ta!le


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'o maintain routing information, OSPF routers complete the following generic link-state routing process toreach a state of convergence=

/stablish eighbor Ad0acencies (Figure ) - OSPF-ena!led routers must recogniAe each other on thenetwork !efore the" can share information n OSPF-ena!led router sends #ello packets out all OSPF-ena!led interfaces to determine if neigh!ors are present on those links If a neigh!or is present, the OSPF-ena!led router attempts to esta!lish a neigh!or ad;acenc" with that neigh!or

* /5change @ink-State dvertisements (Figure *) - fter ad;acencies are esta!lished, routers thene5change link-state advertisements ( &SAs) @S s contain the state and cost of each directl" connectedlink Routers flood their @S s to ad;acent neigh!ors d;acent neigh!ors receiving the @S immediatel"flood the @S to other directl" connected neigh!ors, until all routers in the area have all @S s

Build the 'opolog" 'a!le (Figure ) - fter @S s are received, OSPF-ena!led routers !uild the topolog"ta!le (@S4B) !ased on the received @S s 'his data!ase eventuall" holds all the information a!out thetopolog" of the network

+ /5ecute the SPF lgorithm (Figures + and


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'o make OSPF more efficient and scala!le, OSPF supports hierarchical routing using areas n OSPF areais a group of routers that share the same link-state information in their @S4Bs

OSPF can !e implemented in one of two wa"s=

Single-Area OSPF - In Figure , all routers are in one area called the !ack!one area (area :)

'ultiarea OSPF - In Figure *, OSPF is implemented using multiple areas, in a hierarchal fashionll areas must connect to the !ack!one area (area :) Routers interconnecting the areas are referred

to as Area 3order *outers 2A3*4.

0ith multiarea OSPF, OSPF can divide one large autonomous s"stem ( S) into smaller areas, to supporthierarchical routing 0ith hierarchical routing, routing still occurs !etween the areas (interarea routing),while man" of the processor intensive routing operations, such as recalculating the data!ase, are keptwithin an area

For instance, an" time a router receives new information a!out a topolog" change within the area(including the addition, deletion, or modification of a link) the router must rerun the SPF algorithm, createa new SPF tree, and update the routing ta!le 'he SPF algorithm is .P&-intensive and the time it takes forcalculation depends on the siAe of the area

ote = 'opolog" changes are distri!uted to routers in other areas in a distance vector format In other words,these routers onl" update their routing ta!les and do not need to rerun the SPF algorithm

'oo man" routers in one area would make the @S4Bs ver" large and increase the load on the .P&'herefore, arranging routers into areas effectivel" partitions a potentiall" large data!ase into smaller andmore managea!le data!ases


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'he hierarchical-topolog" possi!ilities of multiarea OSPF have these advantages= Smaller routing tables - Fewer routing ta!le entries !ecause network addresses can !e summariAed

!etween areas Route summariAation is not ena!led !" default *educed lin -state update overhead - 8inimiAes processing and memor" re%uirements *educed fre uency of SPF calculations - @ocaliAes the impact of a topolog" change within an

area For instance, it minimiAes routing update impact !ecause @S flooding stops at the area !oundar"

Figure illustrates these advantagesFor e5ample, R* is an BR for area < s an BR, it would summariAe the area < routes into area :0hen one of the summariAed links fails, @S s are e5changed within area < onl" Routers in area < mustrerun the SPF algorithm to identif" the !est routes #owever, the routers in area : and area do not receivean" updates> therefore, the" do not e5ecute the SPF algorithm'he focus of this chapter is on single- rea OSPF


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OSPF messages transmitted over an /thernet link contain the following information= )ata &in /thernet Frame :eader - Identifies the destination multicast 8 . addresses : -::-


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Characteristics of OSPF - OSPF Messages

OSPF uses link-state packets (@SPs) to esta!lish and maintain neigh!or ad;acencies and e5change routingupdates

'he figure shows the five different t"pes of @SPs used !" OSPF /ach packet serves a specific purpose inthe OSPF routing process=

;ype 1< :ello pac et - &sed to esta!lish and maintain ad;acenc" with other OSPF routers

;ype #< )atabase )escription 2)3)4 pac et - .ontains an a!!reviated list of the sending routerDs@S4B and is used !" receiving routers to check against the local @S4B 'he @S4B must !eidentical on all link-state routers within an area to construct an accurate SPF tree

;ype +< &in -State *e uest 2&S*4 pac et - Receiving routers can then re%uest more information

a!out an" entr" in the 4B4 !" sending an @SR

;ype =< &in -State 7pdate 2&S74 pac et - &sed to repl" to @SRs and to announce newinformation &S7s contain seven different types of &SAs

;ype >< &in -State Ac no5ledgment 2&SAc 4 pac et - 0hen an @S& is received, the routersends an @S ck to confirm receipt of the @S& 'he @S ck data field is empt"


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:ello Pac et 'he OSPF '"pe packet is the #ello packet #ello packets are used to=

4iscover OSPF neigh!ors and esta!lish neigh!or ad;acencies dvertise parameters on which two routers must agree to !ecome neigh!ors /lect the 4esignated Router (4R) and Backup 4esignated Router (B4R) on multiaccess networks

like /thernet and Frame Rela" Point-to-point links do not re%uire 4R or B4R'he figure displa"s the fields contained in the '"pe #ello packet Important fields shown in the figureinclude=

;ype - Identifies the t"pe of packet one ( ) indicates a #ello packet value * identifies a 4B4 packet, an @SR packet, + an @S& packet, and < an @S ck packet

*outer ") - *-!it value e5pressed in dotted decimal notation (an IPv+ address) used to uni%uel"identif"ing the originating router

Area ") - rea from which the packet originated et5or 'as - Su!net mask associated with the sending interface :ello "nterval - Specifies the fre%uenc", in seconds, at which a router sends #ello packets 'he

default #ello interval on multiaccess networks is : seconds 'his timer must !e the same onneigh!oring routers> otherwise, an ad;acenc" is not esta!lished

*outer Priority - &sed in a 4REB4R election 'he default priorit" for all OSPF routers is , !utcan !e manuall" altered from : to * otherwise, an ad;acenc" is notesta!lished

)esignated *outer 2)*4 - Router I4 of the 4R 3ac up )esignated *outer 23)*4 - Router I4 of the B4R

&ist of eighbors - @ist that identifies the router I4s of all ad;acent routers.lick each of the highlighted fields in the figure for more information


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s shown in the figure, OSPF #ello packets are transmitted to multicast address **+ : : < in IPv+ andFF:*==< in IPv (all OSPF routers) ever"=

: seconds (default on multiaccess and point-to-point networks)

: seconds (default on non!roadcast multiaccess 2B8 G networks> for e5ample, Frame Rela")

'he 4ead interval is the period that the router waits to receive a #ello packet !efore declaring the neigh!or down If the 4ead interval e5pires !efore the routers receive a #ello packet, OSPF removes that neigh!orfrom its @S4B 'he router floods the @S4B with information a!out the down neigh!or out all OSPF-ena!led interfaces

.isco uses a default of + times the #ello interval=

+: seconds (default on multiaccess and point-to-point networks)

*: seconds (default on 2B8 networks> for e5ample, Frame Rela")


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Routers initiall" e5change '"pe * 4B4 packets, which is an a!!reviated list of the sending routerDs @S4Band is used !" receiving routers to check against the local @S4B

'"pe @SR packet is used !" the receiving routers to re%uest more information a!out an entr" in the4B4'he '"pe + @S& packet is used to repl" to an @SR packet@S&s are also used to forward OSPF routing updates, such as link changes Specificall", an @S& packetcan contain different t"pes of OSPFv* @S s, as shown in the figure OSPFv renamed several of these@S s and also contains two additional @S s

ote = 'he difference !etween the @S& and @S terms can sometimes !e confusing !ecause these terms areoften used interchangea!l" #owever, an @S& contains one or more @S s


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0hen an OSPF router is initiall" connected to a network, it attempts to= .reate ad;acencies with neigh!ors /5change routing information .alculate the !est routes Reach convergence

OSPF progresses through several states while attempting to reach convergence= 4own state Init state 'wo-0a" state /5Start state /5change state @oading state Full state

.lick the !lue !o5es in the figure for more information


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ospf operation

0hen OSPF is ena!led on an interface, the router must determine if there is another OSPF neigh!or on thelink 'o accomplish this, the router forwards a #ello packet that contains its router I4 out all OSPF-ena!ledinterfaces 'he OSPF router I4 is used !" the OSPF process to uni%uel" identif" each router in the OSPFarea router I4 is an IP address assigned to identif" a specific router among OSPF peers

0hen a neigh!oring OSPF-ena!led router receives a #ello packet with a router I4 that is not within itsneigh!or list, the receiving router attempts to esta!lish an ad;acenc" with the initiating router

Refer to R in Figure 0hen OSPF is ena!led, the ena!led 1iga!it /thernet :E: interface transitions fromthe 4own state to the Init state R starts sending #ello packets out all OSPF-ena!led interfaces to discover OSPF neigh!ors to develop ad;acencies with

In Figure *, R* receives the #ello packet from R and adds the R router I4 to its neigh!or list R* thensends a #ello packet to R 'he packet contains the R* Router I4 and the R Router I4 in its list ofneigh!ors on the same interface

In Figure , R receives the #ello and adds the R* Router I4 in its list of OSPF neigh!ors It also noticesits own Router I4 in the #ello packetDs list of neigh!ors 0hen a router receives a #ello packet with itsRouter I4 listed in the list of neigh!ors, the router transitions from the Init state to the 'wo-0a" state

'he action performed in 'wo-0a" state depends on the t"pe of inter-connection !etween the ad;acentrouters=

If the two ad;acent neigh!ors are interconnected over a point-to-point link, then the" immediatel"transition from the 'wo-0a" state to the data!ase s"nchroniAation phase

If the routers are interconnected over a common /thernet network, then a designated router 4R anda B4R must !e elected


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Because R and R* are interconnected over an /thernet network, a 4R and B4R election takes place sshown in Figure +, R* !ecomes the 4R and R is the B4R 'his process onl" occurs on multi-accessnetworks such as /thernet @ 2s#ello packets are continuall" e5changed to maintain router information

0h" is a 4R and B4R election necessar"H


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8ultiaccess networks can create two challenges for OSPF regarding the flooding of @S s= (reation of multiple ad0acencies - /thernet networks could potentiall" interconnect man" OSPF

routers over a common link .reating ad;acencies with ever" router is unnecessar" and undesira!leIt would lead to an e5cessive num!er of @S s e5changed !etween routers on the same network

/6tensive flooding of &SAs - @ink-state routers flood their @S s an" time OSPF is initialiAed, orwhen there is a change in the topolog" 'his flooding can !ecome e5cessive

'o understand the pro!lem with multiple ad;acencies, we must stud" a formula=For an" num!er of routers (designated as n) on a multiaccess network, there are n (n ) E * ad;acenciesFigure shows a simple topolog" of five routers, all of which are attached to the same multiaccess/thernet network 0ithout some t"pe of mechanism to reduce the num!er of ad;acencies, collectivel" theserouters would form : ad;acencies=< (< ) E * J :'his ma" not seem like much, !ut as routers are added to the network, the num!er of ad;acencies increasesdramaticall", as shown in Figure *'o understand the pro!lem of e5tensive flooding of @S s, pla" the animation in Figure In the animation,R* sends out an @S 'his event triggers ever" other router to also send out an @S 2ot shown in theanimation are the re%uired acknowledgments sent for ever" @S received If ever" router in a multiaccessnetwork had to flood and acknowledge all received @S s to all other routers on that same multiaccess

network, the network traffic would !ecome %uite chaotic'he solution to managing the num!er of ad;acencies and the flooding of @S s on a multiaccess network isthe 4R On multiaccess networks, OSPF elects a 4R to !e the collection and distri!ution point for @S ssent and received B4R is also elected in case the 4R fails ll other routers !ecome 4RO'#/Rs 4RO'#/R is a router that is neither the 4R nor the B4RPla" the animation in Figure + to see the role of 4R


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fter the 'wo-0a" state, routers transition to data!ase s"nchroniAation states 0hile the #ello packet wasused to esta!lish neigh!or ad;acencies, the other four t"pes of OSPF packets are used during the process ofe5changing and s"nchroniAing @S4Bs

In the /5Start state, a master and slave relationship is created !etween each router and its ad;acent 4R andB4R 'he router with the higher router I4 acts as the master for the /5change state In Figure , R*

!ecomes the master

In the /5change state, the master and slave routers e5change one or more 4B4 packets 4B4 packetincludes information a!out the @S entr" header that appears in the routerDs @S4B 'he entries can !ea!out a link or a!out a network /ach @S entr" header includes information a!out the link-state t"pe, theaddress of the advertising router, the linkDs cost, and the se%uence num!er 'he router uses the se%uencenum!er to determine the newness of the received link-state information

In Figure *, R* sends a 4B4 packet to R 0hen R receives the 4B4, it performs the following actions=

It acknowledges the receipt of the 4B4 using the @S ck packet

* R then sends 4B4 packets to R*

R* acknowledges R

R compares the information received with the information it has in its own @S4B If the 4B4 packet hasa more current link-state entr", the router transitions to the @oading state

For e5ample, in Figure , R sends an @SR regarding network 6* : to R* R* responds with the

complete information a!out 6* : in an @S& packet gain, when R receives an @S&, it sends an@S ck R then adds the new link-state entries into its @S4B

fter all @SRs have !een satisfied for a given router, the ad;acent routers are considered s"nchroniAed andin a full state

s long as the neigh!oring routers continue receiving #ello packets, the network in the transmitted @S sremain in the topolog" data!ase fter the topological data!ases are s"nchroniAed, updates (@S&s) are sentonl" to neigh!ors when=

change is perceived (incremental updates)

/ver" : minutes


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Con guring Single-Area OSPFv2 - OSPF Router ID

Introduced in CC , OSPFv* is a link-state routing protocol for IPv+ OSPF was designed as an alternative

to another IPv+ routing protocol, RIP

'he figure shows the topolog" used for configuring OSPFv* in this section 'he t"pes of serial interfacesand their associated !andwidths ma" not necessaril" reflect the more common t"pes of connections foundin networks toda" 'he !andwidths of the serial links used in this topolog" were chosen to help e5plain thecalculation of the routing protocol metrics and the process of !est path selection

'he routers in the topolog" have a starting configuration, including interface addresses 'here is currentl"no static routing or d"namic routing configured on an" of the routers ll interfaces on routers R , R*, andR (e5cept the loop!ack on R*) are within the OSPF !ack!one area 'he ISP router is used as the routingdomainDs gatewa" to the Internet

ote = In this topolog" the loop!ack interface is used to simulate the 0 2 link to the Internet


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Figure is the reference topolog" for this topic OSPFv* is ena!led using the router ospf process-id glo!alconfiguration mode command 'he process-id value represents a num!er !etween and


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/ver" router re%uires a router I4 to participate in an OSPF domain 'he router I4 can !e defined !" anadministrator or automaticall" assigned !" the router 'he router I4 is used !" the OSPF-ena!led router to=

7ni uely identify the router - 'he router I4 is used !" other routers to uni%uel" identif" eachrouter within the OSPF domain and all packets that originate from them

Participate in the election of the )* - In a multiaccess @ 2 environment, the election of the 4Roccurs during initial esta!lishment of the OSPF network 0hen OSPF links !ecome active, therouting device configured with the highest priorit" is elected the 4R ssuming there is no priorit"configured, or there is a tie, then the router with the highest router I4 is elected the 4R 'he routingdevice with the second highest router I4 is elected the B4R

But how does the router determine the router I4H s illustrated in the figure, .isco routers derive the router I4 !ased on one of three criteria, in the following preferential order=

'he router I4 is e5plicitl" configured using the OSPF router-id rid router configuration modecommand 'he rid value is an" *-!it value e5pressed as an IPv+ address 'his is the recommendedmethod to assign a router I4

If the router I4 is not e5plicitl" configured, the router chooses the highest IPv+ address of an" ofconfigured loop!ack interfaces 'his is the ne5t !est alternative to assigning a router I4

If no loop!ack interfaces are configured, then the router chooses the highest active IPv+ address ofan" of its ph"sical interfaces 'his is the least recommended method !ecause it makes it moredifficult for administrators to distinguish !etween specific routers

If the router uses the highest IPv+ address for the router I4, the interface does not need to !e OSPF-ena!led 'his means that the interface address does not need to !e included in one of the OSPF net5or commands for the router to use that IP address as the router I4 'he onl" re%uirement is that the interface is

active and in the up state

ote = 'he router I4 looks like an IP address, !ut it is not routa!le and, therefore, is not included in therouting ta!le, unless the OSPF routing process chooses an interface (ph"sical or loop!ack) that isappropriatel" defined !" a net5or command


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&se the router-id rid router configuration mode command to manuall" assign a *-!it value e5pressed asan IPv+ address to a router n OSPF router identifies itself to other routers using this router I4

s shown in Figure , R is configured with a router I4 of , R* with * * * *, and R with

In Figure *, the router I4 is assigned to R &se the sho5 ip protocols command to verif" therouter I4

ote = R had never !een configured with an OSPF router I4 If it had, then the router I4 would have to !emodified

If the router I4 is the same on two neigh!oring routers, the router displa"s an error message similar to theone !elow=

KOSPF-+-4&PLR'RI4 = 4etected router with duplicate router I4

'o correct this pro!lem, configure all routers so that the" have uni%ue OSPF router I4s&se the S"nta5 .hecker in Figure to assign the router I4 to R* and R


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Sometimes a router I4 needs to !e changed, for e5ample, when a network administrator esta!lishes a new

router I4 scheme for the network #owever, after a router selects a router I4, an active OSPF router doesnot allow the router I4 to !e changed until the router is reloaded or the OSPF process cleared


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router I4 can also !e assigned using a loop!ack interface'he IPv+ address of the loop!ack interface should !e configured using a *-!it su!net mask(*


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'he net5or command determines which interfaces participate in the routing process for an OSPF arean" interfaces on a router that match the network address in the net5or command are ena!led to send

and receive OSPF packets s a result, the network (or su!net) address for the interface is included inOSPF routing updates

'he !asic command s"nta5 is net5or network-address wildcard-mask area area-id

'he area area-id s"nta5 refers to the OSPF area 0hen configuring single-area OSPF, the net5orcommand must !e configured with the same area-id value on all routers lthough an" area I4 can !e used,it is good practice to use an area I4 of : with single-area OSPF 'his convention makes it easier if thenetwork is later altered to support multiarea OSPF

'he figure displa"s the reference topolog"

OSPFv* uses the argument com!ination of network-address wildcard-mask to ena!le OSPF on interfacesOSPF is classless !" design> therefore, the wildcard mask is alwa"s re%uired 0hen identif"ing interfacesthat are participating in a routing process, the wildcard mask is t"picall" the inverse of the su!net maskconfigured on that interface

wildcard mask is a string of * !inar" digits used !" the router to determine which !its of the address toe5amine for a match In a su!net mask, !inar" is e%ual to a match and !inar" : is not a match In awildcard mask, the reverse is true=

?ildcard mas bit - 8atches the corresponding !it value in the address

?ildcard mas bit 1 - Ignores the corresponding !it value in the address


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'he easiest method for calculating a wildcard mask is to su!tract the network su!net mask from*


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Some IOS versions allow the su!net mask to !e entered instead of the wildcard mask 'he IOS thenconverts the su!net mask to the wildcard mask format

&se the S"nta5 .hecker in Figure to advertise the networks connected to R*

ote = 0hile completing the s"nta5 checker, o!serve the informational messages descri!ing the ad;acenc" !etween R ( ) and R* (* * * *) 'he IPv+ addressing scheme used for the router I4 makes it eas" toidentif" the neigh!or


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B" default, OSPF messages are forwarded out all OSPF-ena!led interfaces #owever, these messagesreall" onl" need to !e sent out interfaces connecting to other OSPF-ena!led routers

Refer to the topolog" in the figure OSPF messages are forwarded out of all three routers 1:E: interfaceeven though no OSPF neigh!or e5ists on that @ 2 Sending out unneeded messages on a @ 2 affects thenetwork in three wa"s=

"nefficient 7se of 3and5idth - vaila!le !andwidth is consumed transporting unnecessar"

messages 8essages are multicasted> therefore, switches are also forwarding the messages out all ports

"nefficient 7se of *esources - ll devices on the @ 2 must process the message and eventuall"discard the message

"ncreased Security *is - dvertising updates on a !roadcast network is a securit" risk OSPFmessages can !e intercepted with packet sniffing software Routing updates can !e modified andsent !ack to the router, corrupting the routing ta!le with false metrics that misdirect traffic


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&se the passive-interface router configuration mode command to prevent the transmission of routingmessages through a router interface, !ut still allow that network to !e advertised to other routers, as shownin Figure Specificall", the command stops routing messages from !eing sent out the specified interface#owever, the network that the specified interface !elongs to is still advertised in routing messages that aresent out other interfaces

For instance, there is no need for R , R*, and R to forward OSPF messages out of their @ 2 interfaces'he configuration identifies the R 1:E: interface as passive

It is important to know that a neigh!or ad;acenc" cannot !e formed over a passive interface 'his is !ecause link-state packets cannot !e sent or acknowledged

'he sho5 ip protocols command is then used to verif" that the 1iga!it /thernet interface was passive, asshown in Figure * 2otice that the 1:E: interface is now listed under the Passive Interface(s) section 'henetwork 6* : is still listed under Routing for 2etworks, which means that this network is stillincluded as a route entr" in OSPF updates that are sent to R* and R

ote = OSPFv* and OSPFv !oth support the passive-interface command

&se the S"nta5 .hecker in Figure to configure the @ 2 interface as a passive interface on R*

s an alternative, all interfaces can !e made passive using the passive-interface default commandInterfaces that should not !e passive can !e re-ena!led using the no passive-interface command

.ontinue using the S"nta5 .hecker in Figure and configure the @ 2 interface as a passive interface onR

ote = 0hile completing the s"nta5 checker, notice the OSPF informational state messages as the interfacesare all rendered passive and then the two serial interfaces are made non-passive


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Recall that a routing protocol uses a metric to determine the !est path of a packet across a network metric gives indication of the overhead that is re%uired to send packets across a certain interface OSPFuses cost as a metric lower cost indicates a !etter path than a higher cost

'he cost of an interface is inversel" proportional to the !andwidth of the interface 'herefore, a higher !andwidth indicates a lower cost 8ore overhead and time dela"s e%ual a higher cost 'herefore, a :-8!Es/thernet line has a higher cost than a ::-8!Es /thernet line

'he formula used to calculate the OSPF cost is=

(ost J reference bandwidth Einterface bandwidth

'he default reference !andwidth is :M9 ( ::,:::,:::)> therefore, the formula is=

(ost J ::,:::,::: !ps E interface bandwidth in bps

Refer to the ta!le in the figure for a !reakdown of the cost calculation 2otice that Fast/thernet, 1iga!it/thernet, and : 1ig/ interfaces share the same cost, !ecause the OSPF cost value must !e an integer.onse%uentl", !ecause the default reference !andwidth is set to :: 8!Es, all links that are faster than Fast/thernet also have a cost of


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'he cost of an OSPF route is the accumulated value from one router to the destination network

For e5ample, in Figure , the cost to reach the R* @ 2 6* * :E*+ from R should !e as follows=

Serial link from R to R* cost J +

1iga!it /thernet link on R* cost J

'otal cost to reach 6* * :E*+ J ,>

'he routing ta!le of R in Figure * confirms that the metric to reach the R* @ 2 is a cost of


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OSPF uses a reference !andwidth of :: 8!Es for an" links that are e%ual to or faster than a fast /thernetconnection 'herefore, the cost assigned to a fast /thernet interface with an interface !andwidth of ::8!Es would e%ual

(ost @ 100,000,000 bps E100,000,000 = 1

0hile this calculation works for fast /thernet interfaces, it is pro!lematic for links that are faster than ::8!Es> !ecause the OSPF metric onl" uses integers as its final cost of a link If something less than aninteger is calculated, OSPF rounds up to the nearest integer For this reason, from the OSPF perspective, aninterface with an interface !andwidth of :: 8!Es (a cost of ) has the same cost as an interface with a

!andwidth of :: 1!Es (a cost of )


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'o assist OSPF in making the correct path determination, the reference !andwidth must !e changed to ahigher value to accommodate networks with links faster than :: 8!Es

Ad0usting the *eference 3and5idth

.hanging the reference !andwidth does not actuall" affect the !andwidth capacit" on the link> rather, itsimpl" affects the calculation used to determine the metric 'o ad;ust the reference !andwidth, use the auto-cost reference-band5idth Mb/s router configuration command 'his command must !e configuredon ever" router in the OSPF domain 2otice that the value is e5pressed in 8!Es> therefore, to ad;ust thecosts for=

igabit /thernet - auto-cost reference-band5idth 1

1 igabit /thernet - auto-cost reference-band5idth 1

'o return to the default reference !andwidth, use the auto-cost reference-band5idth 1 command

'he ta!le in Figure displa"s the OSPF cost if the reference !andwidth is set to 1iga!it /thernet lthough

the metric values increase, OSPF makes !etter choices !ecause it can now distinguish !etweenFast/thernet and 1iga!it /thernet links

Figure * displa"s the OSPF cost if the reference !andwidth is ad;usted to accommodate : 1iga!it /thernetlinks 'he reference !andwidth should !e ad;usted an"time there are links faster than Fast/thernet ( ::8!Es)

ote = 'he costs represent whole num!ers that have !een rounded down

In Figure , all routers have !een configured to accommodate the 1iga!it /thernet link with the auto-costreference-band5idth 1 router configuration command 'he new accumulated cost to reach the R*

@ 2 6* * :E*+ from R =

Serial link from R to R* cost J +6

1iga!it /thernet link on R* cost J

'otal cost to reach 6* * :E*+ J ,=8

&se the sho5 ip ospf interface s B B command to verif" the current OSPF cost assigned to the R serial:E:E: interface, as shown in Figure + 2otice how it displa"s a cost of +6

'he routing ta!le of R in Figure < confirms that the metric to reach the R* @ 2 is a cost of +9


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OSPF (ost

ll interfaces have default !andwidth values assigned to them s with reference !andwidth, interface !andwidth values do not actuall" affect the speed or capacit" of the link Instead, the" are used !" OSPF tocompute the routing metric 'herefore, it is important that the !andwidth value reflect the actual speed ofthe link so that the routing ta!le has accurate !est path information

lthough the !andwidth values of /thernet interfaces usuall" match the link speed, some other interfacesma" not For instance, the actual speed of serial interfaces is often different than the default !andwidth On.isco routers, the default !andwidth on most serial interfaces is set to


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Ad0usting the "nterface 3and5idth

'o ad;ust the interface !andwidth use the band5idth kilobits interface configuration command &se the noband5idth command to restore the default value

'he e5ample in Figure ad;usts the R Serial :E:E interface !andwidth to + k!Es %uick verificationconfirms that the interface !andwidth setting is now + k!Es

'he !andwidth must !e ad;usted at each end of the serial links, therefore=

R* re%uires its S:E:E interface to !e ad;usted to ,:*+ k!Es

R re%uires its serial :E:E: to !e ad;usted to + k!Es and its serial :E:E to !e ad;usted to ,:*+ k!Es


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&se the S"nta5 .hecker in Figure * to ad;ust the serial interface of R* and R

ote = common misconception for students who are new to networking and the .isco IOS is to assumethat the band5idth command changes the ph"sical !andwidth of the link 'he command onl" modifies the

!andwidth metric used !" /I1RP and OSPF 'he command does not modif" the actual !andwidth on thelink


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s an alternative to setting the default interface !andwidth, the cost can !e manuall" configured on aninterface using the ip ospf cost value interface configuration command


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%erify OSPF

Figure shows the reference topolog"

&se the sho5 ip ospf neighbor command to verif" that the router has formed an ad;acenc" with itsneigh!oring routers If the router I4 of the neigh!oring router is not displa"ed, or if it does not show as

!eing in a state of F&@@, the two routers have not formed an OSPF ad;acenc"

If two routers do not esta!lish ad;acenc", link-state information is not e5changed Incomplete @S4Bs cancause inaccurate SPF trees and routing ta!les Routes to destination networks ma" not e5ist, or ma" not !ethe most optimum path

Figure * displa"s the neigh!or ad;acenc" of R For each neigh!or, this command displa"s the followingoutput=

eighbor ") - 'he router I4 of the neigh!oring router

Pri - 'he OSPF priorit" of the interface 'his value is used in the 4R and B4R election

State - 'he OSPF state of the interface F&@@ state means that the router and its neigh!or haveidentical OSPF @S4Bs On multiaccess networks, such as /thernet, two routers that are ad;acentma" have their states displa"ed as *0 N 'he dash indicates that no 4R or B4R is re%uired

!ecause of the network t"pe

)ead ;ime - 'he amount of time remaining that the router waits to receive an OSPF #ello packetfrom the neigh!or !efore declaring the neigh!or down 'his value is reset when the interfacereceives a #ello packet

Address - 'he IPv+ address of the neigh!orDs interface to which this router is directl" connected "nterface - 'he interface on which this router has formed ad;acenc" with the neigh!or

&se the S"nta5 .hecker in Figure to verif" the R* and R neigh!ors using the sho5 ip ospf neighborcommand

'wo routers ma" not form an OSPF ad;acenc" if=

'he su!net masks do not match, causing the routers to !e on separate networks

OSPF #ello or 4ead 'imers do not match

OSPF 2etwork '"pes do not match

'here is a missing or incorrect OSPF net5or command


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s shown in Figure , the sho5 ip protocols command is a %uick wa" to verif" vital OSPF configurationinformation 'his includes the OSPF process I4, the router I4, networks the router is advertising, theneigh!ors the router is receiving updates from, and the default administrative distance, which is : forOSPF&se the S"nta5 .hecker in Figure * to verif" the OSPF protocol settings of R* and R using the sho5 ipprotocols command


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'he sho5 ip ospf command can also !e used to e5amine the OSPF process I4 and router I4, as shown inFigure 'his command displa"s the OSPF area information and the last time the SPF algorithm wascalculated

&se the S"nta5 .hecker in Figure * to verif" the OSPF process of R* and R using the sho5 ip ospfcommand

'he %uickest wa" to verif" OSPF interface settings is to use the sho5 ip ospf interface command 'his

command provides a detailed list for ever" OSPF-ena!led interface 'he command is useful to determinewhether the net5or statements were correctl" composed


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'o get a summar" of OSPF-ena!led interfaces, use the sho5 ip ospf interface brief command, as shown inFigure

&se the S"nta5 .hecker in Figure * to retrieve and view a summar" of OSPF-ena!led interfaces on R*using the sho5 ip ospf interface brief command 2otice that specif"ing the interface name as done in the sho5 ip ospf interface serial B B1 command provides detailed OSPF information

.ontinue using the S"nta5 .hecker in Figure * to get a summar" of OSPF-ena!led interfaces on R usingthe sho5 ip ospf interface brief command Retrieve and view additional information for the Serial :E:E:interface using the sho5 ip ospf interface serial B B command


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OSPFv* and OSPFv each have separate ad;acenc" ta!les, OSPF topolog" ta!les, and IP routing ta!les, asshown in the figure

'he OSPFv configuration and verification commands are similar those used in

s shown in the figure, the following are similarities !etween OSPFv* and OSPFv = &in -state - OSPFv* and OSPFv are !oth classless link-state routing protocols

*outing algorithm - OSPFv* and OSPFv use the SPF algorithm to make routing decisions

'etric - 'he RF.s for !oth OSPFv* and OSPFv define the metric as the cost of sending packetsout the interface OSPFv* and OSPFv can !e modified using the auto-cost reference-band5idthref-bw router configuration mode command 'he command onl" influences the OSPF metric whereit was configured For e5ample, if this command was entered for OSPFv , it does not affect theOSPFv* routing metrics

Areas - 'he concept of multiple areas in OSPFv is the same as in OSPFv* 8ultiareas thatminimiAe link-state flooding and provide !etter sta!ilit" with the OSPF domain

OSPF pac et types - OSPFv uses the same five !asic packet t"pes as OSPFv* (#ello, 4B4, @SR,@S&, and @S ck)

eighbor discovery mechanism - 'he neigh!or state machine, including the list of OSPF neigh!or states and events, remains unchanged OSPFv* and OSPFv use the #ello mechanism to learna!out neigh!oring routers and form ad;acencies #owever, in OSPFv , there is no re%uirement formatching su!nets to form neigh!or ad;acencies 'his is !ecause neigh!or ad;acencies are formedusing link-local addresses, not glo!al unicast addresses


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)*B3)* election process - 'he 4REB4R election process remains unchanged in OSPFv

*outer ") - Both OSPFv* and OSPFv use a *-!it num!er for the router I4 represented in dotted-decimal notation '"picall" this is an IPv+ address 'he OSPF router-id command must !e used toconfigure the router I4 'he process in determining the *-!it Router I4 is the same in !oth

protocols &se an e5plicitl"-configured router I4> otherwise, the highest loop!ack IPv+ address !ecomes the router I4

'he figure shows the differences !etween OSPFv* and OSPFv =

Advertises - OSPFv* advertises IPv+ routes, whereas OSPFv advertises routes for IPv

Source address - OSPFv* messages are sourced from the IPv+ address of the e5it interface InOSPFv , OSPF messages are sourced using the link-local address of the e5it interface

All OSPF router multicast addresses - OSPFv* uses **+ : : whereas, OSPFv uses FF:*== whereas, OSPFv uses FF:*==

Advertise net5or s - OSPFv* advertises networks using the net5or router configurationcommand> whereas, OSPFv uses the ipv, ospf process-id area area-id interface configurationcommand

"P unicast routing - /na!led, !" default, in IPv+> whereas, the ipv, unicast-routing glo!alconfiguration command must !e configured

Authentication - OSPFv* uses either plainte5t authentication or 84< authentication OSPFv usesIPv authentication


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Routers running a d"namic routing protocol, such as OSPF, e5change messages !etween neigh!ors on thesame su!net or link Routers onl" need to send and receive routing protocol messages with their directl"connected neigh!ors 'hese messages are alwa"s sent from the source IPv+ address of the router doing theforwarding

IPv link-local addresses are ideal for this purpose n IPv link-local address ena!les a device tocommunicate with other IPv -ena!led devices on the same link and onl" on that link (su!net) Packets witha source or destination link-local address cannot !e routed !e"ond the link from where the packetoriginated

s shown in the figure, OSPFv messages are sent using=

Source "Pv, address - 'his is the IPv link-local address of the e5it interface

)estination "Pv, address - OSPFv packets can !e sent to a unicast address using the neigh!orIPv link-local address 'he" can also !e sent using a multicast address 'he FF:*==< address is theall OSPF router address, while the FF:*== is the 4REB4R multicast address


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(onfiguring OSPFv+

Figure displa"s the network topolog" that is used to configure OSPFv

Figure * shows IPv unicast routing and the configuration of the glo!al unicast addresses of R , asidentified in the reference topolog" ssume that the interfaces of R* and R have also !een configuredwith their glo!al unicast addresses, as identified in the referenced topolog"

In this topolog", none of the routers have IPv+ addresses configured network with router interfacesconfigured with IPv+ and IPv addresses is referred to as dual-stacked dual-stacked network can haveOSPFv* and OSPFv simultaneousl"-ena!led

Figure displa"s the steps to configure !asic OSPFv in a single area


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In the figure, the output of the sho5 ipv, interface brief command confirms that the correct glo!al IPvaddresses have !een successfull" configured and that the interfaces are ena!led lso, notice that eachinterface automaticall" generated a link-local address, as highlighted in the figure

@ink-local addresses are automaticall" created when an IPv glo!al unicast address is assigned to theinterface 1lo!al unicast addresses are not re%uired on an interface> however, IPv link-local addresses are

&nless configured manuall", .isco routers create the link-local address using F/9:==E : prefi5 and the/&I- + process /&I- + involves using the +9-!it /thernet 8 . address, inserting FFF/ in the middle andflipping the seventh !it For serial interfaces, .isco uses the 8 . address of an /thernet interface 2oticein the figure that all three interfaces are using the same link-local address

@ink-local addresses created using the /&I- + format or in some cases, random interface I4s, make itdifficult to recogniAe and remem!er those addresses Because IPv routing protocols use IPv link-localaddresses for unicast addressing and ne5t-hop address information in the routing ta!le, it is common

practice to make it an easil" recogniAa!le address

.onfiguring the link-local address manuall" provides the a!ilit" to create an address that is recogniAa!leand easier to remem!er s well, a router with several interfaces can assign the same link-local address toeach IPv interface 'his is !ecause the link-local address is onl" re%uired for local communications

@ink-local addresses can !e configured manuall" using the same interface command used to create IPvglo!al unicast addresses, !ut appending the lin -local ke"word to the ipv, address command

link-local address has a prefi5 within the range F/9: to F/BF 0hen an address !egins with this he5tet( -!it segment) the lin -local ke"word must follow the address

'he e5ample in Figure configures the same link-local address F/9:== on the three R interfacesF/9:== was chosen to make it eas" to remem!er the link-local addresses of R

%uick look at the interfaces as shown in Figure * confirms that the R interface link-local addresses have !een changed to F/9:==

&se the S"nta5 .hecker in Figure to configure and verif" link-local address F/9:==* on R*, and the link-local address F/9:== on R


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&se the ipv, router ospf process-id glo!al configuration mode command to enter router configurationmode 'he IPv router configuration mode prompt is different than the IPv+ router configuration mode

prompt &se the IPv router confirmation mode to configure glo!al OSPFv parameters, such as aassigning a *-!it OSPF router I4 and reference !andwidth

IPv routing protocols are ena!led on an interface, and not from router configuration mode, like their IPv+counterparts 'he net5or IPv+ router configuration mode command does not e5ist in IPv

@ike OSPFv*, the process-id value is a num!er !etween and


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If none are configured, then the router uses the highest configured IPv+ address of a loop!ackinterface

If none are configured, then the router uses the highest configured IPv+ address of an activeinterface

If there are no sources of IPv+ addresses on a router, then the router displa"s a console message toconfigure the router I4 manuall"

ote = For consistenc", all three routers use the process I4 of :

s shown in the topolog" in Figure *, routers R , R*, and R are to !e assigned the router I4s indicated'he router-id rid command used to assign a router I4 in OSPFv* is the same command used in OSPFv

'he e5ample in Figure =

/nters the router OSPFv configuration mode 2otice how the router prompt is different than thedefault IPv+ routing protocol mode router prompt lso notice how an informational console

message appeared when the OSPFv router configuration mode was accessed ssigns the router I4

d;usts the reference !andwidth to ,:::,:::,::: !ps ( 1!Es), !ecause there are 1iga!it /thernetlinks in the network 2otice the information console message that this command must !econfigured on all routers in the routing domain

'he sho5 ipv, protocols command is used to verif" that the OSPFv process I4 : is using therouter I4

&se the S"nta5 .hecker in Figure + to configure glo!al OSPFv settings on R* and R


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Router I4s sometimes must !e changed, for e5ample, if the network administrator has esta!lished a newrouter I4 identification scheme #owever, after an OSPFv router esta!lishes a router I4, that router I4cannot !e changed until the router is reloaded or the OSPF process is clearedIn Figure , notice that the current router I4 is : 'he OSPFv router I4 should !e In Figure *, the router I4 is !eing assigned to R

ote = .learing the OSPF process is the preferred method to reset the router I4In Figure , the OSPF routing process is cleared using the clear ipv, ospf process privileged /3/. modecommand 4oing this forces OSPF on R to renegotiate neigh!or ad;acencies using the new router I4'he sho5 ipv, protocols command verifies that the router I4 has changed&se the S"nta5 .hecker in Figure + to modif" the router I4 for R


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OSPFv uses a different method to ena!le an interface for OSPF Instead of using the net5or routerconfiguration mode command to specif" matching interface addresses, OSPFv is configured directl" onthe interface

'o ena!le OSPFv on an interface, use the ipv, ospf process-id area area-id interface configuration modecommand

'he process-id value identifies the specific routing process and must !e the same as the process I4 used tocreate the routing process in the ipv, router ospf process-id command

'he area-id value is the area to !e associated with the OSPFv interface lthough an" value could have !een configured for the area, : was selected, !ecause area : is the !ack!one area to which all other areasmust attach, as shown in Figure 'his helps in the migration to multiarea OSPF, if the need arises

In Figure *, OSPFv is ena!led on the R interfaces using the ipv, ospf 1 area command 'he sho5ipv, ospf interface brief command displa"s the active OSPFv interfaces

&se the S"nta5 .hecker in Figure to ena!le OSPFv on the R* and R interfaces


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"nterface ") - 'he interface I4 or link I4

"nterface - 'he interface on which this router has formed ad;acenc" with the neigh!or

&se the S"nta5 .hecker in Figure * to verif" the R* and R neigh!ors using the sho5 ipv, ospf neighborcommand

s shown in Figure , the sho5 ipv, protocols command is a %uick wa" to verif" vital OSPFvconfiguration information, including the OSPF process I4, the router I4, and the interfaces ena!led forOSPFv

&se the S"nta5 .hecker in Figure * to verif" the OSPF protocol settings of R* and R using the sho5 ipv,protocols command

&se the sho5 ipv, ospf command to also e5amine the OSPFv process I4 and router I4 'his command

displa"s the OSPF area information and the last time the SPF algorithm was calculated


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'he %uickest wa" to verif" OSPF interface settings is to use the sho5 ipv, ospf interface command 'hiscommand provides a detailed list for ever" OSPF-ena!led interface


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summary'he current version of OSPF for IPv+ is OSPFv* introduced in RF. *+6 and updated in RF. * *9 !"7ohn 8o" In CCC, OSPFv for IPv was pu!lished in RF. *6+:

OSPF is a classless, link-state routing protocol with a default administrative distance of :, and is denotedin the routing ta!le with a route source code of O

OSPF is ena!led with the router ospf process-id glo!al configuration mode command 'he process-idvalue is locall" significant, which means that it does not need to match other OSPF routers to esta!lishad;acencies with those neigh!ors

'he net5or command used with OSPF has the same function as when used with other I1P routing protocols, !ut with slightl" different s"nta5 'he wildcard-mask value is the inverse of the su!net mask,and the area-id value should !e set to

B" default, OSPF #ello packets are sent ever" : seconds on multiaccess and point-to-point segments andever" : seconds on 2B8 segments (Frame Rela", 3 *


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For routers to !ecome ad;acent, their #ello interval, 4ead interval, network t"pes, and su!net masks mustmatch &se the sho5 ip ospf neighbors command to verif" OSPF ad;acencies

OSPF elects a 4R to act as collection and distri!ution point for @S s sent and received in the multiaccessnetwork B4R is elected to assume the role of the 4R should the 4R fail ll other routers are known as4RO'#/Rs ll routers send their @S s to the 4R, which then floods the @S to all other routers in themultiaccess network

'he sho5 ip protocols command is used to verif" important OSPF configuration information, includingthe OSPF process I4, the router I4, and the networks the router is advertising

OSPFv is ena!led on an interface and not under router configuration mode OSPFv needs link-localaddresses to !e configured IPv &nicast routing must !e ena!led for OSPFv *-!it router-I4 isre%uired !efore an interface can !e ena!led for OSPFv

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