Configuring ospf, Information about ospf, C h a p t e r – Cisco ASA 5505 User Manual

Page 463

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C H A P T E R

24-1

Cisco ASA 5500 Series Configuration Guide using the CLI

24

Configuring OSPF

This chapter describes how to configure the ASA to route data, perform authentication, and redistribute
routing information using the Open Shortest Path First (OSPF) routing protocol.

The chapter includes the following sections:

Information About OSPF, page 24-1

Licensing Requirements for OSPF, page 24-2

Guidelines and Limitations, page 24-3

Configuring OSPF, page 24-3

Customizing OSPF, page 24-4

Restarting the OSPF Process, page 24-14

Configuration Example for OSPF, page 24-14

Monitoring OSPF, page 24-16

Feature History for OSPF, page 24-17

Information About OSPF

OSPF is an interior gateway routing protocol that uses link states rather than distance vectors for path
selection. OSPF propagates link-state advertisements rather than routing table updates. Because only
LSAs are exchanged instead of the entire routing tables, OSPF networks converge more quickly than RIP
networks.

OSPF uses a link-state algorithm to build and calculate the shortest path to all known destinations. Each
router in an OSPF area contains an identical link-state database, which is a list of each of the router
usable interfaces and reachable neighbors.

The advantages of OSPF over RIP include the following:

OSPF link-state database updates are sent less frequently than RIP updates, and the link-state
database is updated instantly, rather than gradually, as stale information is timed out.

Routing decisions are based on cost, which is an indication of the overhead required to send packets
across a certain interface. The ASA calculates the cost of an interface based on link bandwidth rather
than the number of hops to the destination. The cost can be configured to specify preferred paths.

The disadvantage of shortest path first algorithms is that they require a lot of CPU cycles and memory.

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