OSPF: Multi-Area Design, Area Types, Metrics, and Convergence
Areas and area types
OSPF scales by splitting the topology into areas, and every non-backbone area must touch area 0.0.0.0 (directly or through a virtual link). A router with interfaces in two areas is an ABR and condenses topology into Type 3 summary LSAs. Only an ASBR originates Type 5 externals; Type 4 tells the domain how to reach that ASBR.
Area types control which LSAs are flooded:
- Stub - no Type 4/5; the ABR injects a default route.
- Stub no-summaries (totally stubby) - no Type 3/4/5, only a default.
- NSSA - no Type 5, but Type 7 carries externals inside the area and the ABR translates 7 to 5. On Junos an NSSA injects no default route unless you add
nssa default-lsa.
Trap: the stub/NSSA flag must match on every router in the area, or hellos are rejected on option mismatch and the adjacency never comes up.
Metrics
Cost = reference bandwidth ÷ interface bandwidth, minimum 1. Junos defaults the reference bandwidth to 100 Mbps, so a Gigabit link costs 1 and metrics stop differentiating. Change it with set protocols ospf reference-bandwidth 10g - on every router. A mismatch does not break adjacencies, but it produces asymmetric paths and broken traffic engineering.
Convergence and authentication
Hello/dead are 10/40 on broadcast and point-to-point, 30/120 on NBMA; fast-hello, BFD, SPF throttling, LFA and graceful restart (with helper mode on neighbors) all shorten or hide downtime. Authentication is MD5 or a simple password, set per interface or per area; a key mismatch silently drops hellos.
Rule of thumb
With reference bandwidth 10 Gbps: 1 Gbps interface → cost 10, 10 Gbps → cost 1, 100 Mbps → cost 100.