nt 2 lecture 2

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NT 2 Lecture 2Multi-Area OSPF

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OSPF Scalability - 1

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OSPF routers in a heavily populated OSPF network can be overwhelmed by: high demand for router processing and memory resources, large routing tables, and large topology tables.

OSPF allows splitting the network into manageable pieces, or areas, thereby allowing very good scalability.

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Hierarchical Routing

OSPF's capability to separate a large internetwork into areas, is referred to as hierarchical routing.Interarea routing allows OSPF to summarize and contain area-specific information so that many of the smaller internal routing operations are restricted to one area.

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Hierarchical Routing Advantages

The hierarchical topology possibilities of OSPF yield several important advantages:

Reduced frequency of SPF calculations

Smaller routing tables

Reduced Link State Updates overhead

Rather than send an LSU about each network to every area, a single route or a few summarized routes can be summarized between areas to reduce the overhead associated with LSUs that cross multiple areas.

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OSPF Router Types - 1

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OSPF Router Types - 2

There are four different types of OSPF Routers:

Internal router - routers that: Have all their interfaces within the same area with identical Link State databases and; running the same routing algorithm.

Backbone router - Routers that are attached to the backbone area of the OSPF

network with at least one interface connected to Area

0.

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OSPF Router Types - 3Area Border Router - routers with interfaces attached to multiple areas that maintain separate link-state databases for each area they are connected to. Areas can have one or more ABRs.

Autonomous System Boundary Router - ASBRs - routers that have at least one interface connected to an external internetwork (another autonomous system), such as a non-OSPF network.

A router can be more than one router type. As shown in the previous slide.

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Quiz

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OSPF LSA and area types – (a)

Multiarea OSPF is scalable because a router's link-state database can include multiple types of LSAs. Each area type use special LSAs to send or summarize routing information.

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OSPF LSA and area types – (b)

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OSPF Area Types - (a)

Backbone area (Area 0) – A transit area, the central entity to which all other areas connect, all other areas must connect to this area to exchange route information. Accepts all LSAs.

Stub area - Area that does not accept information about routes external to the autonomous system (AS), such as routes from non-OSPF sources. If routers need to reach networks outside the AS, they use a default route. Does not accept external LSAs.

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OSPF Area Types - (c)

Totally stubby area - Area that does not Does not accept external or summary LSAs. A totally proprietary Cisco feature that uses a default route to reach networks outside the AS.Not-so-stubby areas - Area that is similar to a stub area but allows for importing external routes as Type 7 LSAs and translation of specific Type 7 LSA routes into Type 5 LSAs.

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LSA types

LSA1s build the Router Link States (O)

LSA2s build the Network Link States (O)

LSA3s build the Summary Net Link States (O IA)

LSA4s build the ASBR Summary Net Link States (O IA)

LSA5s build external route information (E1 or E2)

LSA7s build external route information (N1 or N2)

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Type 1: Router link entry (O-OSPF)

Generated by each router for each area it belongs to.It describes the states of the router's link to the area.These are flooded only within a particular area.The link status and cost are two of the descriptors provided.Routes learned via Type 1 LSAs are denoted by an O in the routing table.

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Type 2: Network link entry (O-OSPF)

Generated by the DR in multi-access networks.A Type 2 LSA describes the set of routers attached to a particular network.Type 2 LSAs are flooded only within the area that contains the network.Routes learned via Type 2 LSAs are denoted by an O in the routing table.

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Type 3: Summary link entry (IA-OSPF inter area)

Originated by ABRs.A Type 3 LSA describes the links between the ABR and in internal routes of a local network.These entries are flooded throughout the backbone area to the other ABRs.Routers learned via Type 3 LSAs are denoted by an IA in the routing table.

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Type 4: ASBR Summary entry

(IA-OSPF inter area)

Originated by ABRs.Type 4 LSAs are flooded throughout the backbone area to other ABRs.Type 4 LSAs describe reachability to ASBRs.Routes learned via Type 4 LSAs are described by an IA in the routing table.

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Type 5: Autonomous system external link entry (E1 - OSPF External Type 1 E2 - OSPF External Type 2 )

Originated by the ASBR.A Type 5 LSA describes routes to destinations external to the autonomous system.These are flooded through out an OSPF autonomous system except for stub and totally stubby areas.Routes learned via Type 5 LSAs are denoted by either an E1 or an E2 in the routing table.

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Type 7: Autonomous system external link entry (N1 - OSPF NSSA External Type 1 N2 - OSPF NSSA External Type 2 )

Originated by the ASBR connected to an NSSA.Type 7 messages can be flooded throughout NSSAs and translated into LSA Type 5 messages by ABRs.Routes learned via Type 7 LSAs are denoted by either an N1 or an N2 in the routing table.

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How does a router handle a LSU received – (a)

Type-3Type-3

Type-1 links are

summarized and

passed as LSA-3

(passed to all ABRs)

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LSA-5 comes from ASBR

•LSA-5 starts from ASBR and is flooded through out areas •At stub (ABR2) it is changed to default route.

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Cost of LSA-5 routesAn external route, e.g. from RIP, can be configured as

E1-OSPF external type 1 or E2-OSPF external type 2External routes is sent from an ASBR to an OSPF ASLSA-5 not allowed in stub, totally stubby, or NSSA areas. E1 is for multiple routes to an external entity (multihomed AS) and the cost = external cost + internal cost. E2 is for only one external route (singlehomed AS) and the

cost = external cost. (no change when go thru’ many areas) E2 is the default.

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Stub and Totally Stub Areas – (a)

You can configure an OSPF router interface to either operate in a stub area (does not accept information about routes external to the AS (or Lsa-5)) or as a totally stubby area (does not accept LSA-5 and LSA-3) from other areas internal to the AS).

By configuring an area as stub, you can greatly reduce the size of the link-state database inside that area and, as a result, reduce the memory requirements of area routers.

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Stub and Totally Stub Areas - (b)

Stub areas are typically created when you have a hub-and-spoke topology, with the spokes (such as branch offices) configured as stub areas.

Branch offices may not need to know about every network at the headquarters site but can instead use default routes to get there.

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Stub and Totally Stub Areas - (c)

HUB

SPOKE

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Stub and Totally Stub Areas - (d)

An area can be configures as stub or totally stubby when it meets the following criteria:There is a single exit point from that area. The area is not needed as a transit area for virtual links.No ASBR is internal to the stub area. The area is not the backbone area (Area 0).

All these criteria are important because a stub/totally stubby area is configured primarily to exclude external routes.

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Stub and Totally Stub Areas - (e)

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Stub and Totally Stub Areas - (f)

Stub areas do not accept Type 5 (that is, external) LSAs.A totally stubby area is a stub area that blocks external Type 5 LSAs and summary (that is, Type 3 and Type 4) LSAs from entering the area. ABRs inject the default summary link 0.0.0.0/0 into the totally stubby area. Thus, totally stubby areas further minimize routing information and increase stability and scalability of OSPF internetworks.

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Configuring Stub/Totally Stub Areas To configure a stub or totally stubby area, use

the following on all router interfaces in that area:

Router(config-router)#area area-id stub An optional no-summary keyword is added

only on ABRs. This configures the ABR to block interarea routes (Type 3 and Type 4 LSAs). The no-summary creates a totally stubby area.

The area stub command is configured on each router in the stub location and is essential for the routers to become neighbors and exchange routing information.

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OSPF stub area configuration example

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OSPF totally stubby area configuration example

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Not-so-stubby Areas (NSSAs) -1

NSSAs are a relatively new, standards-based OSPF enhancement.Generally, NSSAs are the area located between a RIP/IGRP network and Area 0 of an OSPF network.This area allows LSA5s in, in a limited fashion. Data updates about the RIP or IGRP networks come in as LSA7s and are then switched into Area 0 via an ABR as LSA5s.

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Configuring NSSAs

To configure an area as a NSSA, you must configure all OSPF router interfaces that belong to the area using the following command syntax:

Router(config-router)#area area-id nssa [no-summary]

The no-summary keyword is used on the ABR and typically makes the NSSA totally stubby.

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Virtual Links - 1

If a new area is added after the OSPF internetwork has been designed, and it is not possible to provide that new area with direct access to the backbone, a virtual link can be defined to provide the needed connectivity to the backbone area. Because all areas must be connected to Area 0, the virtual link provides the disconnected area a logical path to the backbone.

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Virtual Links - 2

The virtual link has the two requirements: It must be established between two routers that share a common area.

One of these two routers must be connected to the backbone.

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Virtual Links - 3

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Configuring Virtual Links - 1

On each router that will use the virtual link, create the "virtual link" configuration. The routers that make the links are the ABR that connects the remote area to the transit area and the ABR that connects the transit area to the backbone area:

router(config-router)#area area-id virtual-link router-id

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Configuring Virtual Links - 2

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Summarization Summarization is the consolidation of multiple routes into one single, supernet advertisement to conserve bandwidth, CPU, and memory resources .

Summarization is done only on the ABR and ASBR.

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Summarization commands:

On the ABR – (config-router)#area id range address maskOn the ASBR –

(config-router)#summary-address address mask

nt 2 lecture 2

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