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Implementing a Scalable Multiarea Network OSPF-Based Solution

Configuring and Verifying OSPF Route Summarization

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Summarization Networks are normally translated into type 3 LSAs in other areas. Route summarization is the consolidation of advertised

addresses. – On ABR, summarize type 3 LSAs– On ASBR, summarize type 5 LSAs

A good addressing plan is required. A drawback is the possibility of suboptimal routing.

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Benefits of Route Summarization Minimizes the number of routing table entries Localizes the impact of a topology change Reduces LSA flooding and saves CPU resources

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Interarea Route Summarization A summary route will be generated if at least one subnet within

the area falls in the summary address range. A summarized route metric will be equal to the lowest cost of all

subnets within the summary address range. Only for the summary routes of connected areas:

– The ABR creates a route to Null 0 to avoid loops.

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Using Route Summarization Interarea summary link carries a mask One or more entries can represent several subnets

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Configure Interarea Route Summarization Configures type 3 summarization on ABRs

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Route Summarization Configuration Example at the ABR

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External Route Summarization Summarization can be used for external routes:

– on an AS boundary for type 5 LSAs (redistributed routes)– on an NSSA ABR for type 5 translated from type 7

A summary route to Null 0 will be created for each summary range

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Configure External Route Summarization Configures type 5 summarization of redistributed routes on

ASBRs

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Route Summarization Configuration Example at ASBR

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Default Routes in OSPF A default route is injected into OSPF as an external type 5 LSA. Default route distribution is not on by default. Benefits of default routes include:

– A smaller routing table– Fewer resources used in the router

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Configure OSPF Default Route The first command allows the ASBR to originate a type 5 default

route if it has the gateway of last resort. The second command allows the ASBR to originate a type 5

default route even if there is no gateway of last resort (optional). Use the route map to define a dependency on any condition

inside the route map (optional).

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Default Route Configuration Example

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Summary Route summarization improves CPU utilization, reduces LSA

flooding, and reduces routing table sizes. The area range command is used to summarize at

the ABR. The summary-address command is used to summarize at the ASBR.

Default routes can be used in OSPF to prevent the need for a specific route to each destination network. The benefits include a much smaller routing table and an LSDB with complete reachability.

OSPF uses the default-information originate command to inject a default route.

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