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Transcript of CCNAV3.3 302
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ObjectivesObjectives
Identify the key features of link-state routing Explain how link-state routing information ismaintained
Discuss the link-state routing algorithm
Examine the advantages and disadvantages of link-state routing
Compare and contrast link-state routing withdistance vector routing
Enable OSPF on a router Configure a loopback address to set router
priority
Change OSPF route preference by modifying thecost metric
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ObjectivesObjectives
Configure OSPF authentication Change OSPF timers Describe the steps to create and propagate a
default route Use show commands to verify OSPF operation Configure the OSPF routing process
Define key OSPF terms Describe the OSPF network types Describe the OSPF Hello protocol
Identify the basics steps in the operation of OSPF
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Table of ContentTable of Content
1 Link-state Routing Protocol2 OSPF Concepts3 OSPF Configuration
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LINK-STATE ROUTING PROTOCOLLINK-STATE ROUTING PROTOCOL
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Overview of link-state routingOverview of link-state routing
Protocol Description Characteristics
Distancevector
RIP v1 and RIPv2
InterriorGatewayRoutingProtocol (IGRP)
Copies routing table toneighbors
Updates frequently Slow converge RIP v1 and RIP v2 use hop
count as metric
View network from neighborsperspective Susceptible to routing loops Easy to configure and
administrate
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Overview of link-state routingOverview of link-state routing
Protocol Description Characteristics
Link-stateOpen ShortestPath First(OSPF)
Intermediate-system toIntermediate-system (IS-IS)
Use shortest path Updates are event triggered Fast to converge Send link-state packets to all
network routers Has common view of network Not as susceptible to routing loops Harder to configure Requires more memory and
processing power than distancevector
Consumes less bandwidth than
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Link-state routing protocol functionsLink-state routing protocol functions
Link-state routing protocols collect routinginformation from all other routers in the network.
Each router maintains its own complex database of topology information, the own view of the network.
Link-state routing protocols functions: Respond quickly to network changes Send triggered updates only when a network change has
occurred Send periodic updates known as link-state refreshes Use a hello mechanism to determine the reachability of
neighbors
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Link-state routing protocol featuresLink-state routing protocol features
Uses the hello information and Link-stateadvertisements (LSAs) it receives from otherrouters to build a database about thenetwork
A topological database
Uses the shortest path first (SPF) algorithm(Dijkstra algorithm) to calculate the shortestroute to each network
The resulting SPF tree
Stores this route information in its routing
table
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Hello mechanismHello mechanism
Perth1network
Perth2network
Perth3network
Perth4network
Hello Hello Hello Hello
HelloHello
Hello
P1 P2 P3
P4
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How routing information is maintainedHow routing information is maintained
When a failure occurs in the network, eachlink-state router take flowing action: Flood LSAs using a special multicast address
throughout an area. Copy of the LSA and updates its link-state, or
topological database. Then forward the LSA to all neighboring devices Recalculate their routing tables.
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Link-state routing algorithmsLink-state routing algorithms
They are known collectively as shortest pathfirst (SPF) protocols. They maintain a complex database of the
network topology. They are based on the Dijkstra algorithm.
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Link-state routing algorithmsLink-state routing algorithms
Cost=1
A B
DC
Net 20Net 10
Net 30 Net 40
Cost=1
Cost=1
Cost=1Cost=1
Router Destination Nexthop
Cost
A 20 B 1A 30 C 1
A 40 C 2A 40 B 2B 10 A 1B 30 C 1B 40 D 1B
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Advantages and disadvantages of link-state routingAdvantages and disadvantages of link-state routing
Advantages Disadvantages Fast convergence:
Changes are reportedimmediately by the source
affected Robustness against
routing loops Routers know the topology
Link-state packets aresequenced and aged
The link-state databasesizes can minimized withcareful network design
Significant demands onmemory and processingresources
Requires very strictnetwork design
Requires a knowledgeablenetwork administrator
Initial flooding can impedenetwork performance
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SINGLE AREA OSPF CONCEPTSSINGLE AREA OSPF CONCEPTS
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OSPF overviewOSPF overview
Open Shortest Path First (OSPF) is a link-staterouting protocol based on open standards.
The most recent description is RFC 2328. TheOpen in OSPF means that it is open to thepublic and is non-proprietary.
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OSPF terminologyOSPF terminology
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OSPF terminology:OSPF terminology: LinksLinks
TokenRing
Links
An interface on Router
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OSPF terminology:OSPF terminology: Link stateLink state
The status of a link between two routers. Alsoa routers interface and its relationship to itsneighboring routers.
Neighbors
TokenRing
Links
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OSPF terminology:OSPF terminology: AreaArea
A collection of networks and routers that havethe same area identification. Each router within an area has the same link-
state information.
A router within an area is an internal router
TokenRing
Area 1Area 0
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OSPF terminology:OSPF terminology: Link CostLink Cost
The value assigned to a link. Rather than hops,link-state protocols assign a cost to a link that isbased on the speed of the media.
Interface Output Cost .
Neighbors
TokenRing
Interfaces
Cost = 10
Cost = 6Cost = 1785
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OSPF terminology:OSPF terminology: Adjacency databaseAdjacency database
A listing of all the neighbors to which a routerhas established bi-directional communication.Not every pair of neighboring routers becomeadjacent
Adjacencydatabase
Neighbors
TokenRing
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OSPF terminology:OSPF terminology: Link-state databaseLink-state database
Also known as a topological database A list of link-state entries of all other routers in
the internetwork
Token
Ring
TopologicalDatabase
Adjacencydatabase
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OSPF terminology:OSPF terminology: Routing tableRouting table
The routing table (also known as forwardingdatabase) generated when an algorithm is run onthe link-state database.
Each routers routing table is unique
AdjacencyDatabase
Lists neighbors
Token
Ring
TopologicalDatabase
Lists all routes
RoutingTable
Lists best routes
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OSPF terminology:OSPF terminology: DR and BDR routerDR and BDR router
Designated router (DR) and backup designated router (BDR):
A router that is elected by all other routers on the same LAN torepresent all the routers.
Each network has a DR and BDR
TokenRing
DRDR
BDR
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Comparing OSPF with distance vector routing protocolsComparing OSPF with distance vector routing protocols
Distance vector OSPF
View network topologyfrom neighborsperspective
Adds distance vectorsfrom router to router
Frequent, periodicupdate: Slow
convergence Passes copies routing
tables to neighbor routers Use flat topology
Gets common view of entire networktopology
Calculates the shortestpath to other routers
Event-triggeredupdate: Fast to
convergence Passes link-state
routing updates toother routers
Allow hierarchical
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Shortest path algorithmShortest path algorithm
1
4
1 4
2
22
ABC
D
E F G
The best path is the lowest cost path.
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Shortest Path First Tree for node BShortest Path First Tree for node B
1
(1,B)
1 4
2
22
A
B
C
D
E F G
(3,C)
(4,E)
2
(5,E)
(6,A)
(4,B)
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OSPF network typesOSPF network types
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OSPF network types: Fourth typeOSPF network types: Fourth type
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DR and BDR receive LSAsDR and BDR receive LSAs
Hellos elect DR and BDR to present segment
Each router then forms adjacency with DR
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OSPF Hello ProtocolOSPF Hello Protocol
The rules that govern the exchange of OSPF hellopackets are called the Hello protocol.
Hello packets use : 224.0.0.5 (all routers). Hello packets are sent at regular intervals (default):
Multi access and Point-to-point: 10s NBMA : 30s
On multi-access networks the Hello protocol elects
a designated router (DR) and a backup designatedrouter (BDR). The hello packet carries information that all
neighbors must agree upon before an adjacency isformed, and link-state information is exchanged.
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OSPF packet header
For the hello packet the type field is set to 1.
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OSPF Hello Protocol -OSPF Hello Protocol - Hello headerHello header
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Steps in the operation of OSPF
5 steps of operation:1. Establish router adjacencies.2. Elect a DR and BDR (if necessary).3. Discover routes.4. Select the appropriate routes to
use.5. Maintain routing information.
d l
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3 2000, Cisco Systems, Inc. www.cisco.com 4-39
OSPF Operation in a Broadcast MultiaccessOSPF Operation in a Broadcast MultiaccessTopologyTopology
BroadcastMultiaccess
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Step 1: Establish router adjacenciesStep 1: Establish router adjacencies
First step in OSPF operation is to establishrouter adjacencies
RTB sends hello packets, advertising its ownrouter ID highest IP address:10.6.0.1(noloopback)
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Step 1: Establish router adjacencies (cont.)Step 1: Establish router adjacencies (cont.)
Router IDHello/dead intervalsNeighborsArea-IDRouter priorityDR IP address
BDR IP addressAuthentication passwordStub area flag
* *
* *
* Entry must match on neighboring routers
Hello
afadjfjorqpoeru39547439070713
Hello
AA
DD EE
CCBB
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Step 2: Electing the DR and BDR (if necessary).
P=1 P=0P=1
P=3 P=2
DRDR BDRBDR
Hello
The router with the highest priority value isthe DR. The router with the second highest priority
value is the BDR.
The default for the interface OSPF priority is
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Step 3: Discover routes
On difference network have differ discover process.
On multi-access network, the exchange of routing
information occurs between the DR or BDR and everyother router on the network.
Link partners on a point-to-point or point-to-multipoint network also engage in the exchangeprocess.
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Exchange ProcessExchange Process
Router BNeighbors List
172.16.5.1/24, int E1
Router BNeighbors List
172.16.5.1/24, int E1
172.16.5.1/24E0
I am router ID 172.16.5.2, and I see 172.16.5.1.
Router ANeighbors List
172.16.5.2/24, int E0
172.16.5.2/24E1
I am router ID 172.16.5.1 and I see no one.Down State
Init State
A B
Two-way State
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Step 3: Discover routes (cont.)Step 3: Discover routes (cont.)
DBD
afadjfjorqpoeru39547439070713
Here is a summary of my link-state database.
Here is a summary of my link-state database.DBD
afadjfjorqpoeru39547439070713
E0172.16.5.1
DRE0172.16.5.3
No, I will start exchange because I have ahigher router ID.
I will start exchange because I have router ID 172.16.5.1.Hello
afadjfjorqpoeru39547439070713
Hello
afadjfjorqpoeru39547439070713
Exstart StateExstart State
Exchange StateExchange State
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Step 3: Discover routes (cont.)Step 3: Discover routes (cont.)
Full StateFull State
I need the complete entry for network 172.16.6.0/24.
Here is the entry for network 172.16.6.0/24.
Thanks for the information!
LSR
afadjfjorqpoeru39547439070713
LSAck
afadjfjorqpoeru39547439070713
LSU
afadjfjorqpoeru39547439070713
Loading StateLoading State
E0172.16.5.1
E0172.16.5.3
LSAck
afadjfjorqpoeru39547439070713
Thanks for the information! LSAck
afadjfjorqpoeru39547439070713
DR
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Step 5: Maintaining Routing Information
Router A tells all OSPF DRs on 224.0.0.6
xxLSU1
Link-State ChangeDRDR
AA BB
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Step 5: Maintaining Routing Information
Router A tells all OSPF DRs on 224.0.0.6 DR tells all others on 224.0.0.5
LSU
2
xx
Link-State Change
LSU1
DRDR
AA BB
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Step 5: Maintaining Routing InformationStep 5: Maintaining Routing Information
I need to updatemy routing table.
4
LSU
3
LSU
2
xx
Link-State Change
LSU1
DRDR
AA BB
Router A tells all OSPF DRs on 224.0.0.6 DR tells all others on 224.0.0.5
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Step 5: Maintaining Routing Information (cont.)
No
Send LSUwith newerinformation to
source
Is seq. #higher?
No
Yes
Is seq. #the same?
Yes
Ignore LSAIs entry inlink-state
database?LSA
LSU
No
Run SPF to calculatenew routing table
Add to database
Flood LSA
Send LSAckto DR
End End
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Step 5: Maintaining Routing Information (cont.)
Yes
GotoA
A
No
Send LSUwith newerinformation to
source
Is seq. #higher?
No
Yes
Is seq. #the same?
Yes
Ignore LSAIs entry inlink-state
database?LSA
LSU
No
Run SPF to calculatenew routing table
Add to database
Flood LSA
Send LSAckto DR
End End
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OSPF Operation in a Point-to-PointOSPF Operation in a Point-to-Point
Point-to-Point Neighborship Router dynamically detects its neighboring
router using the Hello protocol
No election: Adjacency is automatic assoon as the two routers can communicate
OSPF packets are always sent as multicast224.0.0.5
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OSPF Operation in an NBMAOSPF Operation in an NBMA
NBMA Topology
Single interface interconnects multiple sites NBMA topologies support multiple routers
but without broadcasting capabilities
X.25Frame Relay
ATM
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SINGLE AREA OSPF ConfigurationSINGLE AREA OSPF Configuration
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interface Ethernet0ip address 10.64.0.2 255.255.255.0
!interface Serial0ip address 10.2.1.2 255.255.255.0
interface Ethernet0ip address 10.64.0.2 255.255.255.0
!interface Serial0ip address 10.2.1.2 255.255.255.0
interface Ethernet0ip address 10.64.0.1 255.255.255.0
!
interface Ethernet0ip address 10.64.0.1 255.255.255.0
!
router ospf 1
Basic OSPF ConfigurationBasic OSPF ConfigurationBasic OSPF ConfigurationBasic OSPF Configuration
Can Assign Network or
Interface Address.
Broadcast Network Point-to-Point Network E010.64.0.1
10.64.0.2E0
S010.2.1.2 10. 2.1.1
S1AA BB CC
network 10.0.0.0 0.255.255.255 area 0 router ospf 50network 10.2.1.2 0.0.0.0 area 0
network 10.64.0.2 0.0.0.0 area 0
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Configuring OSPF loopback addressConfiguring OSPF loopback address
Router ID: Number by which the router is known to OSPF
Default: The highest IP address on an active interface at themoment of OSPF process startup
Can be overridden by a loopback interface: Highest IP addressof any active loopback interface
! Create the loopback 0 interface
Router(configf)#Interface loopback 0Router(configf-if)#ip address 192.168.31.33 255.255.255.255
! C reate the loopback 0 interface
Router(configf)#Interface loopback 0Router(configf-if)#ip address 192.168.31.33 255.255.255.255
! Remove loopback 0 interface
Router(configf)#no Interface loopback 0
! Remove loopback 0 interface
Router(configf)#no Interface loopback 0
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Configuring OSPF router priorityConfiguring OSPF router priority
The router with the highest priority value is the DR. The default for the interface OSPF priority is 1. In case
of a tie, the routers router ID is used.
! Setting OSPF Priority
Router(configf)#Interface Fastethernet 0/0Router(configf-if)#ip ospf priority 50
! Setting OSPF Priority
Router(configf)#Interface Fastethernet 0/0Router(configf-if)#ip ospf priority 50
The priorities can be set to any value from 0 to 255 The command show ip ospf interface will display the interface
priority value as well as other key information.
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Modifying OSPF cost metricModifying OSPF cost metric
Cost is calculated using the formula10 8/bandwidth, where bandwidth is
expressed in bps. Bandwidth dividend is user configurable:
Interface subcommand: bandwidth 64
Interface subcommand: ip ospf cost
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Configuring OSPF authenticationConfiguring OSPF authentication
! Create a key that is used to generate the authentication data! in the OSPF packet header.
Router(config-if)#ip ospf authentication-key password
! C reate a key that is used to generate the authentication data! in the OSPF packet header.
Router(config-if)#ip ospf authentication-key password
After the password is configured, authentication must be enabled:
outer(config-router)#area area-number authentication
After the password is configured, authentication must be enabled:
outer(config-router)#area area-number authentication
The authentication key, known as a password, is a
shared secret between the routers. The password can be up to eight characters. The password is sent as plain text.
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Configuring OSPF authentication:Configuring OSPF authentication: with MD5with MD5
pecifies the type of message-digest hashing algorithm to usend key value
uter(config-if)#ip ospf message-digest-key key-id md5 encryption-typ
pe cifies the type of message-digest hashing algorithm to usend key value
uter(config-if)#ip ospf message-digest-key key-id md5 encryption-typ
! After that, authentication must be enabled:
Router(config-router)#area area-id authentication message-digest
! Af ter that, authentication must be enabled:
Router(config-router)#area area-id authentication message-digest
The value of encryption-type field is 0 means none
and 7 means proprietary. The key-id is an identifier (1 to 255) The key is an alphanumeric password up to sixteen
characters. Neighbor routers must use the same key identifier
with the same key value.
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Configuring OSPF timersConfiguring OSPF timers
! To configure the hello and dead intervals on an interface
Router(config-if)#ip ospf hello-interval secondsRouter(config-if)#ip ospf dead-interval seconds
! To configure the hello and dead intervals on an interface
Router(config-if)#ip ospf hello-interval secondsRouter(config-if)#ip ospf dead-interval seconds
OSPF networks, the default Hello interval is 10 seconds Dead interval is 40 seconds.
On nonbroadcast networks, the default Hello interval is 30 seconds Dead interval is 120 seconds.
These timers must be configured to matchthose of any neighboring router.
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OSPF, propagating a default routeOSPF, propagating a default route
!Configure a gateway of last resort
Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next-hop address
!Configure a gateway of last resort
Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next-hop address
! To propagate this route to all the routers in a normal OSPF area
Router(config-router)#default-information originate
! To propagate this route to all the routers in a normal OSPF area
Router(config-router)#default-information originate
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Common OSPF configuration issuesCommon OSPF configuration issues
Failure to establish a neighbor relationship is caused byany of the following reasons: Hellos are not sent from both neighbors. Hello and dead interval timers are not the same.
Interfaces are on different network types. Authentication passwords or keys are different.
In OSPF routing it is also important to ensure the following: All interfaces have the correct addresses and subnet
mask. network area statements have the correct wildcard
masks. network area statements put interfaces into the
correct area.
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Router#show ip ospf interface
Verifying OSPF OperationVerifying OSPF Operation
Displays area ID and adjacency information
Router#
show ip protocols
Verifies that OSPF is configuredRouter#
show ip route
Displays all the routes learned by the router
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Displays OSPF timers and statistics
Displays information about DR, BDR andneighbors
Displays the link-state database
Verifying OSPF Operation (cont.)Verifying OSPF Operation (cont.)
Router#
show ip ospf neighbor detail
Router#
show ip ospf database
Router#show ip ospf
f
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Allows you to clear the IP routing table
Router#
clear ip route *
Router#
debug ip ospf option
Displays router interaction during thehello, exchange, and floodingprocesses
Verifying OSPF Operation (cont.)Verifying OSPF Operation (cont.)Verifying OSPF Operation (cont.)Verifying OSPF Operation (cont.)
h f f
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show ip ospf interface
R2#sh ip ospf int e0Ethernet0 is up, line protocol is up
Internet Address 192.168.0.12/24, Area 0
Process ID 1, Router ID 192.168.0.12, Network Type BROADCAST, Cost: 10Transmit Delay is 1 sec, State DROTHER, Priority 1Designated Router (ID) 192.168.0.11, Interface address 192.168.0.11Backup Designated router (ID) 192.168.0.13, Interface address
192.168.0.13Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Neighbor Count is 3, Adjacent neighbor count is 2Adjacent with neighbor 192.168.0.13 (Backup Designated Router)Adjacent with neighbor 192.168.0.11 (Designated Router)
Suppress hello for 0 neighbor(s)
h i f i hb
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show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface192.168.0.13 1 2WAY/DROTHER 00:00:31 192.168.0.13 Ethernet0192.168.0.14 1 FULL/BDR 00:00:38 192.168.0.14 Ethernet0192.168.0.11 1 2WAY/DROTHER 00:00:36 192.168.0.11 Ethernet0
192.168.0.12 1 FULL/DR 00:00:38 192.168.0.12 Ethernet0
OSPF over Ethernet - Multiaccess Network
Neighbor ID Pri State Dead Time Address Interface192.168.0.11 1 FULL/ - 00:00:39 10.1.1.2 Serial1
OSPF over HDLC - Point-to-Point Network
h i f d b
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R2# show ip ospf database
OSPF Router with ID (192.168.0.12) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count192.168.0.10 192.168.0.10 817 0x80000003 0xFF56 1192.168.0.11 192.168.0.11 817 0x80000003 0xFD55 1192.168.0.12 192.168.0.12 816 0x80000003 0xFB54 1192.168.0.13 192.168.0.13 816 0x80000003 0xF953 1192.168.0.14 192.168.0.14 817 0x80000003 0xD990 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum192.168.0.14 192.168.0.14 812 0x80000002 0x4AC8
show ip ospf database
SS
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SummarySummary
Link-state routing protocols collect routing information fromall other router in the network or within a defined area of the network. Link-state routing protocols perform thefollowing functions:
Respond quickly to network changes
Send triggered update only when a network change hasoccurred
Send periodic updates know as link-state refreshes
Use a hello mechanism to determine the reachability of neighbors
OSPF is link-state routing protocol based on open standards. OSPF routing uses the concept of area. Each router contains
a complete database of link-states in specific area
Q&AQ&A
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8/6/2019 CCNAV3.3 302
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