© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-1 Implementing a Scalable Multiarea Network OSPF- Based Solution Configuring and Verifying OSPF Route Summarization

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-2 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.

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-3 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

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-4 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.

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-5 Using Route Summarization  Interarea summary link carries a mask  One or more entries can represent several subnets

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-6 Configure Interarea Route Summarization  Configures type 3 summarization on ABRs

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-7 Route Summarization Configuration Example at the ABR

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-8 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

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-9 Configure External Route Summarization  Configures type 5 summarization of redistributed routes on ASBRs

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-10 Route Summarization Configuration Example at ASBR

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-11 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

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-12 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).

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-13 Default Route Configuration Example

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-14 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.

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-15