CCNP BSCI Official Exam Certification Guide, Fourth Edition

The Operation of Multi-area OSPF 189

Figure 7-2 shows the connectivity and functionality of the different areas. The routers send out

routing updates and other network information through LSAs. The function or type of router

determines the LSAs that are sent.

Figure 7-2 The Different Types of OSPF Areas and LSA Propagation

The Operation of Multi-area OSPF

This section describes how OSPF operates across the various areas to maintain a coherent and

accurate understanding of the autonomous system.

ABR LSA Propagation

When a router is configured as an ABR, it generates summary LSAs and floods them into the

backbone area. Adjacencies within an area are advertised using Type 1 or Type 2 LSAs, and these

prefixes are passed to the backbone using Type 3 summaries. These summaries are then injected

by other ABRs into their own areas (except for totally stubby areas).

External routes and summaries from other areas are received by the ABR and passed back into the

local area.

The flow and propagation of LSAs within and between areas is illustrated in Figure 7-3.

Default

Route

Default

Routes

Summary

LSA

Summary

LSA

Summary

LSA

Summary

& External

Summary

LSA

External (Type 5)

& Summary

External

& Summary

LSAs

External

(Type 7 LSA)

ABR

ASBR

ABR

ABR

ABR

Stub Area

Ordinary Area

Not so Stubby Area

(NSSA)

Backbone Area

Totally Stubby Area

190 Chapter 7: Using OSPF Across Multiple Areas

Figure 7-3 The Propagation of LSAs

OSPF Path Selection Between Areas

The local routing table on a router depends on its position in the network and the type of area it is

in. If there are routes with different routing information sources to the same destination, the router

chooses the path with the lowest administrative distance. If both are OSPF, OSPF will select lower

type advertisements first and choose lower costs to break ties. OSPF, like all IP routing protocols

on Cisco IOS, is capable of load-balancing and will automatically distribute the load over four

equal-cost paths.

Remember the sequence of events:

1. The router receives LSAs.

2. The router builds the topological database.

3. The router runs the Dijkstra algorithm, from which the shortest path is chosen and entered into

the routing table.

Thus, the routing table is the conclusion of the decision-making process. The routing table

displays information on how that decision was made by including the metric for each route. This

enables you to view the operation of the network.

Router &

Network L

SAs

External & Area 4

Summary LSAs

Area 3

Summary LSAs

Area 4

Summary LSAs

Router &

Network

LSAs

Router

LSAs

External

LSAs

Internet

ABR

DR

ABR

Summary

LSAs

Summary

LSA

s

ASBR

External &

Area 3 LSAs

Area 3

Area 0

Area 4