Foundation Summary

The Foundation Summary provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary might help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final prep before the exam, the following lists and tables are a convenient way to review the day before the exam.

OSPF is an interior gateway link-state routing protocol. It has a number of advantages:

  • OSPF is classless and allows summarization.

  • It converges very quickly.

  • OSPF is a standard, and ubiquitous support can be expected in a heterogeneous environment.

  • It conserves network bandwidth.

  • It uses multicasts instead of broadcasts.

  • It sends incremental change-based updates.

  • It uses cost as the metric.

OSPF uses three tables:

  • Neighbor table

  • Topology table

  • Routing table

The OSPF metric is calculated based on interface speed:

There are five types of OSPF packets:

  • Hello

  • DBD

  • LSR

  • LSU

  • LSAck

All OSPF packets have a common format that contains nine fields:

  • Version

  • Type

  • Packet Length

  • Router ID

  • Area ID

  • Checksum

  • Authentication Type

  • Authentication Data

  • Data

The following two commands are required for configuring OSPF on a single internal router:

  • router ospf process-number

  • network network wildcard-mask area area

The OSPF router ID will be one of the following:

  • Address defined by the router-id command

  • Highest IP address of a loopback interface

  • Highest IP address of an interface configured to run OSPF

The set of commands shown in Table 5-12 is invaluable in both configuration and maintenance of a live network.

Table 5-12. The show Command Options for OSPF
Command OptionDescription
show ip ospfShows OSPF process details—for example, how many times the router has recalculated its routing table.
show ip ospf databaseShows contents of the topological database.
show ip ospf interfaceGives information about how OSPF has been configured on each interface.
show ip ospf neighborDisplays neighbor information. Used to verify that all neighbors are present.
show ip protocolsShows IP routing protocol configuration on the router.
show ip routeShows networks the router is aware of and preferred paths. Also gives next hop for each path.

The debug options available for monitoring OSPF are listed in Table 5-13.

Table 5-13. The debug Command Options for OSPF
Command OptionDescription
debug ip ospf eventsDisplays information about OSPF-related events, such as adjacencies, flooding information, designated router selection, and SPF calculation.
debug ip packetIP debugging information includes packets received, generated, and forwarded. Fast-switched packets do not generate messages.