Ccna3 Chapter 6

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When a large OSPF area is divided into smaller areas, this is called

Back 1

multiarea OSPF.

Front 2

An OSPF area is a group of routers that share the same

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link-state information in their link-state databases.

Front 3

Multiarea OSPF is useful in larger network deployments to

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reduce processing and memory overhead.

Front 4

Multi-Area OSPF benefits:

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Reduces routing tables

Reduces link-sate database overhead

Reduces SPF algorithm calculation

Front 5

Multiarea OSPF is implemented in a two-layer area hierarchy:

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Backbone (Transit) area

Regular (Non-backbone) area

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Backbone (Transit) area is an area whose primary function is the

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fast and efficient movement of IP packets.

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Regular (Non-backbone) area connects the

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users and resources.

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By default, a regular area does not allow

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traffic from another area.

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All traffic moving from one area to another area must traverse the

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backbone area.

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This traffic is referred to as

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interarea traffic.

Front 11

The OSPF routers are categorized based on the function they perform in the

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routing domain.

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There are four different types of OSPF routers:

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Internal router

Backbone router

Area Border Router (ABR)

Autonomous System Boundary Router (ASBR)

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Internal router is a router that has all of its interfaces

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in the same area.

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Backbone router is a router in the

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backbone area.

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Area Border Router (ABR) is a router that has interfaces attached to

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multiple areas.

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Autonomous System Boundary Router (ASBR) is a router that has at least one interface attached to an

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external internetwork.

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Redistribution in multiarea OSPF occurs when an ASBR connects different routing domains (e.g., EIGRP and OSPF)

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routing domains (e.g., EIGRP and OSPF).

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LSAs are the building blocks of the

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OSPF LSDB.

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LSAs and the LSDB describe the entire

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topology of an OSPF network or area.

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The LSA includes a link ID field that identifies the

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object to which the link connects.

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Each router link is defined as 1 of 5 LSA types:

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Type 1

Type 2

Type 3

Type 4 and type 5

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Type 1 LSAs include a list of directly connected

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network prefixes and link types.

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The type 2 LSA contains

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The router ID and IP address of the DR.

Router IDs of all other routers on the multiaccess segment.

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The purpose of a type 2 LSA is to give other routers information about

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multiaccess networks within the same area.

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Type 3 LSAs are used by ABRs to advertise networks from

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other areas.

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ABRs collect type 1 LSAs in the

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LSDB.

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After an OSPF area has converged, the ABR creates a type 3 LSA for each of its

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learned OSPF networks.

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Type 4 and type 5 LSAs are used collectively to

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Identify an ASBR.

Advertise external networks into an OSPF routing domain.

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A type 4 summary LSA is generated by an ABR only when an ASBR

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exists within an area.

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Type 5 external LSAs describe routes to networks outside the

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OSPF autonomous system.

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Type 5 LSAs are originated by the ASBR and are flooded to the entire

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autonomous system.

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It is strongly recommended that manual route summarization be configured on the

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ASBR.

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OSPF Routing Table Entries: O

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Best paths to destinations within their area (intra-area)

Type 1 and type 2 LSAs.

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OSPF Routing Table Entries: O IA

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Best paths to the other areas within the interarea.

Type 3 and type 4 LSAs.

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OSPF Routing Table Entries: O E1 or O E2

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best paths to the external autonomous system (type 5) destinations.

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When converged, a router can communicate with any network within or outside the

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OSPF autonomous system.

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Summarization helps keep routing tables

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small.

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Summarization involves consolidating multiple routes into a

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single advertisement.

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Summarization also helps increase the network’s stability, because it

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reduces unnecessary LSA flooding.

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Interarea route summarization occurs on ABRs and applies to routes from

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within each area.

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External route summarization is specific to external routes that are injected into OSPF via

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route redistribution.