Replacing type-3summary LSAs and type-7 default external LSAs with a type-3 default route LSA

By default, a routing switch operating as an ABR for a stub area or NSSA injects non-default, summary routes (LSA type 3) into the stub areas and NSSAs. For NSSAs, the routing switch also injects a type-7 default external route. You can further reduce LSA traffic into these areas by using no-summary.This command option configures the routing switch to:

  • Replace type-3 summary LSA injection into a stub area or NSSA with a type-3 default summary route (0.0.0.0/0.)

  • Disable injection of the type-7 default external route into an NSSA.

You can enable this behavior when you first configure the stub area or NSSA, or at a later time. For the full command to use, see Configuring a stub or NSSA area.

The no-summary command does not affect intra-area advertisements, meaning the switch still accepts summary LSAs from OSPF neighbors within its area and floods them to other neighbors. The switch can form adjacencies with other routers regardless of whether summarization is enabled or disabled for areas on each switch.

When you use no-summary, the change takes effect immediately. If you apply the option to a previously configured area, the switch flushes all of the summary LSAs it has generated (as an ABR) from the area.

NOTE:

This feature applies only when the switch is configured as an ABR for a stub area or NSSA. To completely prevent summary LSAs from injection into the area, use no-summary to disable the summary LSAs on each OSPF router that is an ABR for the area.

To implement the above operation for a stub area or NSSA, enter a command such as the following: 

switch(ospf)# area 40 stub 3 no-summary