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Determining IP Routes
Enabling OSPF
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Outline
Overview
OSPF Features
OSPF and Distance Vector Routing ProtocolComparison
Shortest Path Algorithm
Single Area OSPF Configuration
Loopback Interfaces
OSPF Configuration Verification OSPF Configuration Troubleshooting
Summary
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Introducing OSPF
Open standard
Shortest path first (SPF) algorithm
Link-state routing protocol (vs. distance vector)
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OSPF as a Link-State Protocol
OSPF propagates link-state advertisements ratherthan routing table updates.
Link = router interface
State = description of an interface and itsrelationship to neighboring routers
LSAs are flooded to all OSPF routers in the area.
The OSPF link-state database is pieced together
from the LSAs generated by the OSPF routers. OSPF uses the SPF algorithm to calculate the
shortest path to a destination.
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OSPF Hierarchical Routing
Consists of areas and autonomous systems
Minimizes routing update traffic
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Shortest Path First Algorithm
Places each router at the root of a tree and calculates theshortest path to each destination based on the cumulative cost
Cost = 108/bandwidth (bps)
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Configuring Single-Area OSPF
Router(config-router)#network wildcard-maskarea area-id
Assigns networks to a specific OSPF area
Router(config)#router ospfprocess-id Defines OSPF as the IP routing protocol
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OSPF Configuration Example
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Configuring Loopback Interfaces
Router ID
Number by which the router is known to OSPF Default: The highest IP address on an active interface at the moment
of OSPF process startup
Can be overridden by a loopback interface: Highest IP address of anyactive loopback interface
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Router#show ip ospf interface
Verifying the OSPF Configuration
Displays area ID and adjacency information
Router#show ip protocols
Verifies that OSPF is configured
Router#show ip route
Displays all the routes learned by the router
Router#show ip ospf neighbor
Displays OSPF neighbor information on a per-interface basis
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Router#debug ip ospf events
OSPF:hello with invalid timers on interface Ethernet0
hello interval received 10 configured 10
net mask received 255.255.255.0 configured 255.255.255.0dead interval received 40 configured 30
Router#debug ip ospf packet
OSPF: rcv. v:2 t:1 l:48 rid:200.0.0.117
aid:0.0.0.0 chk:6AB2 aut:0 auk:
Router#debug ip ospf packet
OSPF: rcv. v:2 t:1 l:48 rid:200.0.0.116
aid:0.0.0.0 chk:0 aut:2 keyid:1 seq:0x0
OSPF debug Commands
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Summary
OSPF is an interior gateway protocol similar toIGRP, but based on link states rather than distancevectors.
OSPF advertises information about each of its links
rather than sending routing table updates like adistance vector protocol does.
Hierarchical routing enables separation of a largeinternetwork into smaller internetworks, calledareas.
The SPF algorithm places each router at the root ofa tree and calculates the shortest path to eachdestination based on the cumulative cost requiredto reach that destination.
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Summary (Cont.)
The router ospfcommand starts an OSPF routingprocess. The network command is used to associateaddresses to an OSPF area.
The interface loopback command is used to modify
the OSPF router ID to a loopback address. Any one of a number ofshow commands can be
used to display information about an OSPFconfiguration.
The debug ip ospf events privileged EXEC commandcan be used to display information on OSPF-relatedevents, such as adjacencies, flooding information,designated router selection, and SPF calculation.
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