OSPF Multiarea, Metrics & Area Types Quick Reference

Cost = reference bandwidth ÷ interface bandwidth. The default reference bandwidth (100 Mbps) compresses many modern links to cost 1 — many real deployments raise it network-wide, consistently on every router.

OSPF load-balances automatically across equal-cost paths; it doesn't natively support unequal-cost load balancing the way EIGRP's variance feature does.

Standard: full routing detail — intra-area, inter-area, and external routes.

Stub: blocks external (Type 5) routes, replaced by a default route.

Totally stubby (Cisco-specific): blocks external AND inter-area (Type 3) routes too — smallest possible table.

NSSA: a stub-family area that can still originate its own external routes via Type 7 LSAs, translated to Type 5 at the boundary.

Adjacency requires matching area ID, hello/dead timers, area type flags, and authentication (if configured) — plus matching OSPF network type, or DR/BDR election expectations diverge.

Every non-backbone area must connect to area 0. A virtual link repairs a discontiguous backbone by tunneling through a non-stub transit area — it's a stopgap, not a permanent design tool.

Route summarization ('area range') is configured on the ABR at the area boundary — configuring it on an internal router has no effect.