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Chapter 11: OSPF
CCNA Ex loration version 4.0
1
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Objectives
Describe the background and basic features of OSPF Identify and apply the basic OSPF configuration
commands
Describe, modify and calculate the metric used byOSPF
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Describe the Designated Router/Backup DesignatedRouter (DR/BDR) election process in multiaccessnetworks
Describe the uses of additional configurationcommands in OSPF
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Introduction
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Introduction to OSPF
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Background of OSPF Began in 1987
1989 OSPFv1 released in RFC 1131This version was experimental & never deployed 1991 OSPFv2 released in RFC 1247 1998 OSPFv2 updated in RFC 2328
Background of OSPF
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v pu s e n
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Single Area OSPF Concepts: OSPF terminology
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Single Area OSPF Concepts: OSPF terminology
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OSPF area
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OSPF Message Encapsulation
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OSPF Packet Types
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Hello Protocol
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1.Discover OSPF neighbors andestablish neighbor adjacencies.
2.Advertise parameters on which tworouters must agree to become
Hello Protocol
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ne g ors.3.Elect the Designated Router (DR) and
Backup Designated Router (BDR) on
multiaccess networks like Ethernetand Frame Relay.
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OSPF Hello IntervalsUsually multicast (224.0.0.5)
Sent every 30 seconds for NBMAsegments
Hello Protocol
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network
OSPF Dead Intervals
This is the time that must be expiredbefore the neighbor is considered down
Default time is 4 times the hello interval
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Hello Protocol
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Hello Protocol
Electing a DR and BDR
Designated Router (DR): DR isresponsible for updating all other
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Backup Designated Router(BDR): This router takes over
DRs responsibilities if DR fails
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Purpose of a Link State Update(LSU):Used to deliver link state
advertisements
OSPF Link-state Updates
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Purpose of a Link StateAdvertisement (LSA)Contains information about
neighbors & path costs
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OSPF Link-state Updates
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Extra: How routing information is maintained when a failure is occurs
Network 1 isdown
LSAs LSAs LSAs
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1. When a failure occurs in the network, such as a neighbor becomes unreachable
2. Link-state protocols flood LSAs using a special multicast address throughout an area.
3. Each link-state router takes a copy of the LSA, updates its link-state/topological database.
4. The link-state router will then forward the LSA to all neighboring devices.
5. LSAs cause every router within the area to recalculate routes. Because LSAs need to be floodedthroughout an area
Note: all routers within that area need to recalculate their routing tables ==> the number of link-staterouters that can be in an area should be limited
Area x
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OSPF Algorithm
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Administrative Distance
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Purpose is to encrypt & authenticate routinginformation
This is an interface specific configuration Routers will only accept routing information
OSPF Authentication
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from other routers that have beenconfigured with the same password orauthentication information
Note: Authentication does not encrypt therouter's routing table.
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Basic OSPF Configuration
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Lab Topology
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Addressing
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R1(config)#router ospf process-id Process ID:
A locally significant number between 1and 65535
Does not have to match other OSPF
The router ospf command
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routers
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Router(config-router)# network network-address wildcard-maskarea area-id
wildcard mask - the inverse of the subnet mask.The network address along with the wildcard maskis used to specify the interface or range of
The network command
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interfaces that will be enabled for OSPF using thisnetwork command.
area-id
Area-id refers to the OSPF area.OSPF area is a group of routers that share link
state information
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The network command
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Router ID ?
OSPF Router ID
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Commands used to verify current router IDShow ip protocols
Show ip ospf
Show ip ospf interface
OSPF Router ID
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Router ID & Loopback addressesHighest loopback address will be used as router ID if
router-id command isnt used
OSPF Router ID
Host Mask
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Advantage: loopback interface cannot fail OSPFstability
The OSPF router-id command: Introduced in IOS 12.0
Router(config)#router ospf process-id Router(config-router)#router-id ip-address
Modifying the Router ID Router#clear ip ospf process
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OSPF Router ID
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Duplicate Router IDs When two routers have the same router ID in an
OSPF, domain routing may not function properly.If the router ID is the same on two neighboring
OSPF Router ID
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,
occur. When duplicate OSPF router IDs occur, IOS will
display a message similar to:
%OSPF-4-DUP_RTRID1: Detected router withduplicate router ID
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Verifying OSPF
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Verifying OSPF
Neighbor ID - The router ID of the neighboring router. Pri - The OSPF priority of the interface. This is discussed
in a later section.
State - The OSPF state of the interface. FULL state meansthat the router and its neighbor have identical OSPF link-state databases. OSPF states are discussed in CCNP.
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ea me - e amount o t me rema n ng t at t e routerwill wait to receive an OSPF Hello packet from theneighbor before declaring the neighbor down. This value isreset when the interface receives a Hello packet.
Address - The IP address of the neighbor's interface towhich this router is directly connected.
Interface - The interface on which this router has formedadjacency with the neighbor.
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Reasons of no adjacency
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Two routers may not form an OSPF adjacency if:The subnet masks do not match, causing the routers to
be on separate networks.
OSPF Hello or Dead Timers do not match.
OSPF Network Types do not match.There is a missing or incorrect OSPF network
command.
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Command Description
Show ip protocols
Displays OSPF process ID,router ID, networks router is
Verifying OSPF - Additional Commands
Verifying OSPF
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a ver s ng a m n s ra ve
distance
Show ip ospf
Displays OSPF process ID,router ID, OSPF area information& the last time SPF algorithm
calculated
Show ip ospf interfaceDisplays hello interval and deadinterval
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show ip protocols
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show ip ospf
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show ip ospf
A network that cycles between an up state and adown state is referred to as a flapping link.
A flapping link can cause OSPF routers in an areato constantly recalculate the SPF algorithm,preventing proper convergence.
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,
seconds (5000 msecs) after receiving an LSUbefore running the SPF algorithm. This is knownas the SPF schedule delay.
In order to prevent a router from constantly
running the SPF algorithm, there is anadditional Hold Time of 10 seconds (10000msecs). The router waits 10 seconds afterrunning the SPF algorithm before rerunning the
algorithm again.
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show ip ospf interface
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show ip ospf database
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E i i h i bl
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Note: OSPF does not automaticallysummarize at major network
boundaries
Examining the routing table
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The OSPF Metric
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OSPF M t i
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OSPF Metric
OSPF uses cost as themetric for determining thebest route
Best route ~ lowest costCost
108/ bandwidth
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Reference bandwidth defaults to 100Mbps
can be modified using
auto-cost reference-
bandwidth command
OSPF Metric
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COST of an OSPF route Is the accumulated value from one router to the
next
OSPF Metric
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OSPF Metric
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Usually the actual speed of a link isdifferent than the default bandwidth
Bandwidth value does not actually affectthe speed of the link; it is used by somerouting protocols to compute the routing
OSPF Metric
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me r c.
The show interface command will displayinterfaces bandwidth
Most serial link default to 1.544Mbps
OSPF Metric
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OSPF Metric
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Modifying the Cost of a link
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Both sides of a serial link should beconfigured with the same bandwidth
Router(config-if)#bandwidthbandwidth kb
Modifying the Cost of a link
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R1(config)#interface serial 0/0/0
R1(config-if)#ip ospf cost 1562
Modifying the Cost of a link
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Modifying the Cost of a link
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Modifying the Cost of a link
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Difference between bandwidthcommand & the ip ospf cost
commandIp ospf cost command
Modifying the Cost of a link
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Sets cost to a specific valueBandwidth command
Link cost is calculated
Modifying the Cost of a link
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Modifying the Cost of a link
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OSPF and Multiaccess Networks
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Challenges in Multi-access Networks
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OSPF defines 5 network types:1. Point-to-point2. Broadcast Multiaccess3. Nonbroadcast Multiaccess (NBMA)4. Point-to-multipoint5. Virtual links
Challenges in Multi access Networks
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OSPF in Multi-access Networks
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OSPF in Multi access Networks
2 challenges presented by multi-access networksMultiple adjacencies
Extensive LSA flooding
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OSPF in Multi-access Networks
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Extensive flooding of LSAsFor every LSA sent out there must be an
acknowledgement of receipt sent back to transmitting
router. lots of bandwidth consumed and chaotic traffic
OSPF in Multi access Networks
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OSPF in Multi-access Networks
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Solution to LSA flooding issue is the use ofDesignated router (DR)
Backup designated router (BDR) DR & BDR selection
OSPF in Multi access Networks
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Sending & Receiving LSADRothers send LSAs via multicast 224.0.0.6 to
DR & BDR
DR forward LSA via multicast address 224.0.0.5to all other routers
OSPF in Multi-access Networks
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DR/BDR Election Process
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DR/BDR elections DO NOT occur in point topoint networks
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DR/BDR Election Process
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DR/BDR elections will take place on multi-accessnetworks as shown below
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DR/BDR Election Process
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Criteria for getting elected DR/BDR
1.DR: Router with the highestOSPF interface priority.
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. : ou er w e seconhighest OSPF interface priority.
3.If OSPF interface priorities areequal, the highest router ID isused to break the tie.
DR/BDR Election Process
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DR/BDR Election Process
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DR/BDR Election Process
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Timing of DR/BDR ElectionOccurs as soon as 1st router has its
interface enabled on multi-access networkWhen a DR is elected it remains as the
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DR until one of the following occurs
1. The DR fails.
2. The OSPF process on the DR fails.
3. The multiaccess interface on theDR fails.
DR/BDR Election Process
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DR/BDR Election Process
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DR/BDR Election Process
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DR/BDR Election Process
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DR/BDR Election Process
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OSPF in Multi-access Networks
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Manipulating the election process
1. Boot up the DR first, followed by theBDR, and then boot all other routers,
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2. Shut down the interface on allrouters, followed by a no shutdown
on the DR, then the BDR, and thenall other routers.
OSPF Interface Priority
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Manipulating the DR/BDR electionprocess continued
Router(config-if)#ip ospf priority {0- 255
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Priority number range 0 to 255 0 means the router cannot
become the DR or BDR 1 is the default priority value
OSPF Interface Priority
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More
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OSPF neighbor relationships progress through seven states:
not exchanged information
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not exchanged information
Send Hello packets to establish arelationship with neighbor routers.
There are 2 kinds of relationship
OSPF interfacescan be in one ofseven states.
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Each router keeps a list ofadjacent neighbor, called theAdjacency databese.
Steps in the operation of OSPF
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OSPF routers progress through the following 5
distinct steps of operation:
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.
2.Elect a DR and BDR, if necessary3.Discover routes4.Select the appropriate routes to use
5.Maintain routing information
1.Establish router adjacencies
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2.Elect a DR and BDR, if necessary
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3.Discover routes
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4.Select the appropriate routes to use
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5.Maintain Routing Information
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11
22
33
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More OSPF Configuration
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Redistributing an OSPF Default Route
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Router connected to ISP Called an autonomous system border router
Used to propagate a default route
Example of static default route
R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1
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Requires the use of the default-information
originate command Example of default-information originate command
R1(config-router)#default-information
originate
Example default route
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Example default route
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Example default route
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Example default route
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Fine-Tuning OSPF
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Since link speeds are getting faster it maybe necessary to change referencebandwidth values
- -
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bandwidth commandExample:
R1(config-router)#auto-costreference-bandwidth 10000
Fine-Tuning OSPF
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Modifying OSPF timers
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Reason to modify timers Faster detection of network failures
Manually modifying Hello & Dead intervals Router(config-if)#ip ospf hello-
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n erva secon s
Router(config-if)#ip ospf dead-interval seconds
Point to be made Hello & Dead intervals must be the same
between neighbors
Configuring Simple OSPF authentication
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The clear-text authentication Create a plain text password:
Router(config-if)#ip ospf authentication-key{password}
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Router(config-router)#area {area-number}authentication
Or
Enable authentication on an interface:Router(config-if)# ip ospf authentication
Configuring Simple OSPF authentication
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The encrypted authentication Create a plain text password:
Router(config-if)#ip ospf message-digest-key {key-
id}md5 {password} Enable authentication on all interfaces:
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ou er con g-rou er area area-num er
authentication message-digest Or Enable authentication on an interface:
Router(config-if)# ip ospf authentication message-digest
Note: key-idand passwordmust to match on all routers onthe same link
Configuring Simple OSPF authentication
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The encrypted authentication
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OSPF redistribution
Planning Redistribution
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Planning Redistribution
Locate the boundary router between two routing processes.
Determine which routing process is the core or backboneprocess
Determine which routing process is the edge or migration
process Select a method for injecting the required edge protocol routes
into the core.
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Configuring Redistribution into OSPF
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Use this command to redistribute routes into OSPF:
Router(config-router)# redistribute
protocol [process-id] [metric metric-value]
[metric-type type-value] [route-map map-
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-
Default metric is 20 or 1.
Default metric type is 2.
Subnets do not redistribute by default.
Configuring Redistribution into OSPF
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RtrA(config)# router ospf 1
RtrA(config-router)# redistribute eigrp ?
Autonomous system numberRtrA(config-router)# redistribute eigrp 100 ?
metric Metric for redistributed routes
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me r c- ype - ex er or me r c ype or
redistributed routes
route-map Route map referencesubnets Consider subnets for redistribution into OSPF
tag Set tag for routes redistributed into OSPF
Configuring Redistribution into OSPF
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Extra: E1-E2 routes
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Extra: RIPv1 versus OSPF
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Summary
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