NSAP/NET Structure and Area Design
What the exam tests
Area design questions test whether you can plan contiguous L1 areas, a contiguous L2 backbone, summarization points, and stable NETs without creating black holes.
Core rules
- L1 areas should be contiguous; the L2 backbone must be contiguous across all L1/L2 routers.
- Area boundaries sit on L1/L2 routers. An L1/L2 router belongs to one or more L1 areas plus Level 2.
- Summarization is configured on L1/L2 routers to aggregate L1 prefixes into L2 and, where route leaking is used, L2 prefixes into L1.
- Keep L1 areas small enough for fast SPF and easy troubleshooting; avoid overloading a single router with many areas.
- Plan NETs so the area ID reflects topology and system IDs are stable, commonly derived from loopback addresses.
Worked example
Area 49.0001 contains 10.1.0.0/24 through 10.1.3.0/24. On the L1/L2 router, a summary covering 10.1.0.0/22 reduces the number of prefixes injected into L2. If a link inside the area fails, the summary remains as long as at least one more-specific route is present; if the last component fails, the summary is withdrawn. That is the intended trade-off between stability and granularity.
Traps
Designing non-contiguous L1 areas; leaving the L2 backbone discontiguous; overlapping area IDs; summarizing so aggressively that traffic is drawn toward a router that cannot reach the more-specific destination.