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 Router roles are

  • Internal router

  • Backbone router

  • ABR

  • ASBR

The six most common LSA types are

  • Router link LSA (Type 1)

  • Network link LSA (Type 2)

  • Network-summary link LSA (Type 3)

  • AS external ASBR summary link LSA (Type 4)

  • External link LSA (Type 5)

  • NSSA external LSA (Type 7)

OSPF networks use several types of areas:

  • Standard

  • Stub

  • Totally stubby

  • Not-so-stubby

LSAs learned by OSPF can have the cost of the path calculated in one of two ways:

  • E1— Cost of the path to ASBR added to external cost

  • E2— Only external cost of path from ASBR considered in calculation

Two types of summarization exist:

  • Interarea summarization— Performed at the ABR, creates Type 3 and Type 4 LSAs.

  • External summarization— Performed at the ASBR, creates Type 5 LSAs.

Configuration commands for a multi-area OSPF network are as follows:

  • area area-id range address mask

  • summary-address address mask [not-advertise][tag tag]

  • area area-id stub

  • area area-id stub no-summary

  • area area-id default-cost cost

  • area area-id virtual-link

  • area area-id virtual-link router-id

Commands to verify the configuration and monitor the network are

  • show ip ospf border-routers

  • show ip route

  • show ip ospf virtual-links

  • show ip ospf database

Useful debug commands are as follows:

  • log-adjacency-changes

  • debug ip packet

  • debug ip ospf events

Use the following guidelines when troubleshooting adjacencies:

  • Make sure that both routers are configured with the same IP mask, MTU, interface Hello timer, OSPF Hello interval, and OSPF dead interval.

  • Make sure that both neighbors are the same area type and part of the same area.

  • Use the debug and show commands to trace the problem.