Chapter 9: Multiarea OSPF
Scaling Networks
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Chapter 9 – Sections & Objectives
§ 9.1 Multiarea OSPF Operation
• Explain how multiarea OSPF operates in a small to medium-sized
business network.
§ 9.2 Implement Multiarea
• Implement multiarea OSPFv2 and OSPFv3.
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9.1 Multiarea OSPF Operation
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Multiarea OSPF Operation
Why Multiarea OSPF?
§ Single-Area OSPF
• If the routes are not summarized, the routing table can become very
large.
• Each router must maintain detailed information about every network in the routing domain.
§ Multiarea OSPF
• Smaller routing tables
• Reduced link-state update overhead
• Reduced frequency of SPF calculations
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Multiarea OSPF Operation
Why Multiarea OSPF?
§ OSPF Two-Layer Area Hierarchy
• Backbone (Transit) area and Regular (Non-backbone) area
§ Types of OSPF Routers
• Internal router
• Backbone router
• Area Border Router (ABR)
• Autonomous System Boundary Router (ASBR)
Backbone
Regular
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Multiarea OSPF Operation
Multiarea OSPF LSA Operation
§ OSPF LSA Types
• Individually, they act as database records and provide specific OSPF network details. In combination, they describe the entire topology of an OSPF network or area.
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Multiarea OSPF Operation
Multiarea OSPF LSA Operation
§ OSPF LSA Types
• Type 1 – Contains a list of the directly connected interfaces, link types,
neighbors, and link states.
• Type2-ContainstherouterIDandIPaddressoftheDR,alongwiththe router ID of all other routers on the multiaccess segment.
• Type 3 – Used by ABRs to advertise networks from other areas.
• Type 4 – Identifies the ASBR and provides a route to it.
• Type 5 – Describe routes to networks outside the OSPF routing domain.
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Multiarea OSPF Operation
OSPF Routing Table and Types of Routes
§ OSPF Routing Table Entries
• O–Indicatestherouteisintra-area.
• OIA–IndicatesasummaryLSA.
• OE1orOE2–IndicatesanexternalLSA.
§ OSPF Route Calculation
• Each router uses the SPF algorithm against the LSDB to build the SPF
tree. The SPF tree is used to determine the best path(s).
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Multiarea OSPF LSA Operation OSPF LSA Type 1
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Multiarea OSPF LSA Operation OSPF LSA Type 2
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Multiarea OSPF LSA Operation OSPF LSA Type 3
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Multiarea OSPF LSA Operation OSPF LSA Type 4
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Multiarea OSPF LSA Operation OSPF LSA Type 5
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9.2 Configuring Multiarea OSPF
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Configuring Multiarea OSPF
Configuring Multiarea OSPF
§ Implementing Multiarea OSPF
• The type of OSPF implementation chosen depends on the specific
network design requirements and existing topology.
• A router simply becomes an ABR when it has two network statements in different areas.
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Configuring Multiarea OSPF
Verifying Multiarea OSPF
§ Verifying Multiarea OSPF v2
• The same verification commands used to verify single-area OSPFv2
also can be used to verify the multiarea OSPF topology.
• For the equivalent OSPFv3 command, simply substitute ip with ipv6.
§ Verify General Multiarea OSPFv2 Settings
• Use the show ip protocols command to verify the OSPFv2 status.
• The Routing for Networks section identifies the networks and their respective areas.
§ Verify the OSPFv2 Routes
• The most common command used to verify a multiarea OSPFv2 configuration is the show ip route command. Add the ospf parameter to display only OSPFv2-related information.
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Configuring Multiarea OSPF
Verifying Multiarea OSPF (Cont.)
§ Verify the Multiarea OSPFv2 LSDB
• Use the show ip ospf database command to verify the contents of the
OSPFv2 LSDB.
• There are many command options available with the show ip ospf database command.
§ Verify Multiarea OSPFv3
• Like OSPFv2, OSPFv3 provides similar OSPFv3 verification commands.
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9.3 Chapter Summary
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Chapter Summary Summary
§ Single-area OSPF is useful in smaller networks but in larger networks multiarea OSPF is a better choice. Multiarea OSPF solves the issues of large routing table, large link-state database, and frequent SPF algorithm calculations.
§ The main area is called the backbone area (area 0) and all other areas must connect to the backbone area. Routing still occurs between the areas while many of the routing operations, such as recalculating the database, are kept within an area.
§ There are four different types of OSPF routers: Internal router, Backbone router, Area Border Router (ABR), and Autonomous System Boundary Router (ASBR). A router can be classified as more than one router type.
§ Link State Advertisements (LSAs) are the building blocks of OSPF. This chapter concentrated on LSA type 1 to LSA type 5. Type 1 LSAs are referred to as the router link entries. Type 2 LSAs are referred to as the network link entries and are flooded by a DR. Type 3 LSAs are referred to as the summary link entries and are created and propagated by ABRs. A type 4 summary LSA is generated by an ABR only when an ASBR exists within an area. Type 5 external LSAs describe routes to networks outside the OSPF autonomous system. Type 5 LSAs are originated by the ASBR and are flooded to the entire autonomous system.
§ OSPFv2 routes in an IPv4 routing table are identified using the following descriptors: O, O IA, O E1 or O E2. Each router uses the SPF algorithm against the LSDB to build the SPF tree. The SPF tree is used to determine the best paths.
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Chapter Summary Summary (Cont.)
§ There are no special commands required to implement a multiarea OSPF network. A router simply becomes an ABR when it has two network statements in different areas.
§ An example of multiarea OSPF configuration:
R1(config)# router ospf 10
R1(config-router)# router-id 1.1.1.1
R1(config-router)# network 10.1.1.1 0.0.0.0 area 1 R1(config-router)# network 10.1.2.1 0.0.0.0 area 1 R1(config-router)# network 192.168.10.1 0.0.0.0 area 0
§ Commands that are used to verify OSPFv2 configuration consist of the following: show ip ospf neighbor
show ip ospf
show ip ospf interface
show ip protocols
show ip ospf interface brief show ip route ospf
show ip ospf database
§ To use the equivalent OSPFv3 command, simply substitute ip with ipv6.
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