216 Chapter 7: Using OSPF Across Multiple Areas
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
Foundation Summary 217
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.

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