Configuring OSPF Route Summarization

Context

Route summarization on a large-scale OSPF network reduces the number of routing entries. This function minimizes system resource consumption while maintaining system performance. In addition, if a link frequently alternates between Up and Down, the links not involved in route summarization are not affected. This prevents route flapping and improves network stability.

OSPF supports two types of route summarization:

Inter-area route summarization

Inter-area route summarization is performed on ABRs and applies to routes from within an AS. An ABR generates Network-summary (Type 3) LSAs by network segment while advertising routing information to other areas. If network segments are contiguous, the ABR can summarize them into a single segment. This allows the ABR to send only the summary LSA for all the specified network segments.
External route summarization

External route summarization is performed on ASBRs and is specific to external routes that are imported into OSPF. ASBRs can summarize imported AS-external (Type 5) LSAs within a summary address range. After an NSSA is configured, an ASBR must summarize imported NSSA (Type 7) LSAs with the summary address range.

Devices that are both ABRs and ASBRs summarize the Type 5 LSAs translated from Type 7 LSAs.

Perform the following steps on a switch running OSPF.

Procedure

Configure inter-area route summarization on an ABR.
1. Run system-view
  
  The system view is displayed.
2. Run ospf [ process-id ]
  
  The OSPF process view is displayed.
3. Run area area-id
  
  The OSPF area view is displayed.
4. Run abr-summary ip-address mask [ [ cost { cost | inherit-minimum } | [ advertise [ generate-null0-route ] | not-advertise | generate-null0-route [ advertise ] ] ] ^* ]
  
  ABR route summarization is configured.
Configure external route summarization on an ASBR.
1. Run system-view
  
  The system view is displayed.
2. Run ospf [ process-id ]
  
  The OSPF process view is displayed.
3. (Optional) Run asbr-summary type nssa-trans-type-reference [ cost nssa-trans-cost-reference ]
  
  OSPF is configured to consider Type 5 LSAs translated from Type 7 LSAs when setting types and costs for summarized routes.
  
  By default, OSPF does not consider Type 5 LSAs translated from Type 7 LSAs when setting types and costs for summarized routes.
4. Run asbr-summary ip-address mask [ not-advertise | tag tag | cost cost | distribute-delay interval ] *
  
  ASBR route summarization is configured.
  
  After route summarization is configured, the routing table on the local OSPF switch remains unchanged, but the routing table on a neighboring OSPF switch contains only one summarized route. This summarized route is removed only when all specific routes involved in the summarization are interrupted.