This document discusses routing with the OSPF link-state protocol. It describes how to plan and configure a network using OSPF, including dividing the network into areas, establishing adjacencies between routers, and tuning OSPF parameters. It also covers working with multi-protocol environments, summarizing routes, and distributing default routes between areas using ASBR routers.

1. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE I Chapter 6 1
Routing With a
Link-State Protocol

2. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 2
Objectives
 Describe and plan a network using OSPF.
 Design and configure a network using single-area
OSPF.
 Work with multi-protocol environments.

3. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 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

4. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 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

5. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 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

6. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 6
Describe and Plan a Network Using
OSPF
 Adjacencies: neighbor relationships
 Hello protocol
 Router ID
 Role of loopback interfaces

7. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 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)

8. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 9
Design and Configure a Network Using
Single-Area OSPF
OSPF configuration steps:
 Enable OSPF + process ID
 Advertise networks + wildcard mask and area ID

9. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 10
Design and Configure a Network Using
Single-Area OSPF
 Authentication protects integrity of routing information
 Simple password authentication
 Message Digest 5 (MD5)

10. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 11
Design and Configure a Network Using
Single-Area OSPF
Tune OSPF parameters:
 Interface priority
 Router ID
 Loopback and interface addresses
 Bandwidth (cost )

11. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 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

12. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 13
Work with Multi-Protocol Environments
 Default route configured on ASBR
 ASBR distributes the route into the OSPF network

13. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 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

14. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 15
Work with Multi-Protocol Environments
OSPF limitations:
 Increased router memory and processing demands
 Strict design requirements
 Knowledgeable administrator required
 Initial discovery process takes up network bandwidth

15. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 16
Work with Multi-Protocol Environments
 Administrative distance (AD) determines routes that
appear in routing table

16. © 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 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