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Implementing a Scalable Multiarea Network OSPF-Based Solution

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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  1. Configuring and Verifying OSPF Route Summarization Implementing a Scalable Multiarea Network OSPF-Based Solution

  2. 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.

  3. 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

  4. 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.

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

  6. Configure Interarea Route Summarization • Configures type 3 summarization on ABRs

  7. Route Summarization Configuration Example at the ABR

  8. 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

  9. Configure External Route Summarization • Configures type 5 summarization of redistributed routes on ASBRs

  10. Route Summarization Configuration Example at ASBR

  11. 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

  12. 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).

  13. Default Route Configuration Example

  14. 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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