1. Routing With a
Link-State Protocol
Introducing Routing and Switching in the Enterprise –
Chapter 6
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2. Objectives
 Describe and plan a network using OSPF.
 Design and configure a network using single-area
OSPF.
 Work with multi-protocol environments.
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3. Describe and Plan a Network Using
OSPF
 Link-state routing protocol
 Open standard developed by IETF
 Classless interior gateway protocol
 Divides the network into areas for scalability
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4. Describe and Plan a Network Using
OSPF
 Bandwidth determines the cost of a link
 Higher bandwidth = lower cost = desirable route
• Cost = 100,000,000 / bandwidth of link in bps
• Manual cost configuration at an interface: ip ospf cost
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5. Describe and Plan a Network Using OSPF
 SPF algorithm (Diijkstra’s Algorithm)
 Each router has a topological tree
 SPF tree information stored in topology database
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6. Describe and Plan a Network Using
OSPF
 Adjacencies: neighbor relationships
 Hello protocol
 Router ID
 Role of loopback interfaces
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7. Describe and Plan a Network Using OSPF
 Designated router (DR)
 Backup designated router (BDR)
 DROther
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8. Describe and Plan a Network Using
OSPF
 Area 0
 Roles of routers in an OSPF Autonomous System:
Area Border Router (ABR)
Autonomous System Border Router (ASBR)
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9. Design and Configure a Network Using
Single-Area OSPF
OSPF configuration steps:
 Enable OSPF + process ID
 Advertise networks + wildcard mask and area ID
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10. Design and Configure a Network Using
Single-Area OSPF
 Authentication protects integrity of routing information
 Simple password authentication
 Message Digest 5 (MD5)
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11. Design and Configure a Network Using
Single-Area OSPF
Tune OSPF parameters:
 Interface priority
 Router ID
 Loopback and interface addresses
 Bandwidth (cost )
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12. Design and Configure a Network Using
Single-Area OSPF
Verification and troubleshooting commands:
 show ip ospf neighbor
 show ip protocols, show ip route
 show ip ospf, show ip ospf interface
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13. Work with Multi-Protocol Environments
 Default route configured on ASBR
 ASBR distributes the route into the OSPF network
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14. Work with Multi-Protocol Environments
Benefits of OSPF summarization:
 Reduces number of networks advertised
 Reduces memory requirements
 Reduces number of entries in router updates
 Isolates flapping and other problems to their location
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15. Work with Multi-Protocol Environments
OSPF limitations:
 Increased router memory and processing demands
 Strict design requirements
 Knowledgable administrator required
 Initial discovery process takes up network bandwidth
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16. Work with Multi-Protocol Environments
 Administrative distance (AD) determines routes that
appear in routing table
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17. Summary
 OSPF is a classless interior link-state routing protocol
used in enterprise networks
 OSPF uses bandwidth to generate the cost metric
 OSPF routers elect a DR and BDR on multi-access
networks
 Router ID or router priority can be used to dictate the
selection of DR and BDR
 The OSPF network command uses a wildcard mask
 Default route distribution and inter-area route
summarization are used in OSPF networks
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18. © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 18