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

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