OSPF Scaling, Stub Areas, and Summarization
Domain 6 - OSPF Scaling, Stub Areas, and Summarization
This domain tests techniques that keep large OSPF domains stable: area design, summarization, and control-plane tuning.
Summarization
- Inter-area summarization: on an ABR, area-range under the source area aggregates Type 3 LSAs.
area-range 10.0.0.0/16with advertise (default) or not-advertise. - External summarization: on an ASBR, aggregate Type 5/7 with the export policy or aggregate commands.
- A summary creates a discard route on the summarizing router to prevent loops toward the aggregate. Summarization only works at area boundaries; a router inside an area cannot summarize its own Type 1/2 LSAs.
Stub and NSSA design
Stub and totally stubby areas reduce LSDB size and SPF work at the edge. In SR OS, area 1 stub creates a stub area; adding summaries allows Type 3 LSAs in, otherwise it behaves as totally stubby. area 1 nssa allows Type 7 externals; summaries and default-originate control what the ABR injects.
Scaling controls
Virtual links extend area 0 across a transit area. SPF and LSA timers (throttle), export-limit, and max-paths limit resource use. Keep areas contiguous and keep the backbone free of external LSAs where possible.
Worked example
Area 1 contains 10.1.0.0/24 through 10.1.15.0/24. On the ABR, area 1 area-range 10.1.0.0/20 replaces 16 Type 3 LSAs with one. Routers in area 0 see 10.1.0.0/20 with the ABR's cost. If one /24 goes down, the summary remains as long as any component is active; a discard route on the ABR prevents blackholing to the missing component.
Traps
Trying to summarize on a non-ABR. Expecting a stub area to receive Type 5 LSAs. Forgetting that SR OS stub areas suppress summaries unless summaries is set. Using a virtual link through a stub area - not allowed.