Configuring and verifying ospf route summarization
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Implementing a Scalable Multiarea Network OSPF-Based Solution PowerPoint PPT Presentation


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Configuring and Verifying OSPF Route Summarization. Implementing a Scalable Multiarea Network OSPF-Based Solution. Summarization. Networks are normally translated into type 3 LSAs in other areas. Route summarization is the consolidation of advertised addresses.

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Implementing a Scalable Multiarea Network OSPF-Based Solution

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Configuring and Verifying OSPF Route Summarization

Implementing a Scalable Multiarea Network OSPF-Based Solution


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.


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


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.


Using Route Summarization

  • Interarea summary link carries a mask

  • One or more entries can represent several subnets


Configure Interarea Route Summarization

  • Configures type 3 summarization on ABRs


Route Summarization Configuration Example at the ABR


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


Configure External Route Summarization

  • Configures type 5 summarization of redistributed routes on ASBRs


Route Summarization Configuration Example at ASBR


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


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).


Default Route Configuration Example


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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