OSPF: Inside an Organization

A corporate network with hundreds of routers can't use RIP (15-hop limit) or manual statics (too many changes). OSPF scales through areas, converges quickly, and works on any equipment.

• **Enterprise:** standard IGP for campus/WAN
• **Datacenter:** OSPF or BGP for underlay
• **Service Provider:** often IS-IS, but OSPF is also used

Предварительные знания

• Dynamic Routing

OSPF Areas

**OSPF** (Open Shortest Path First) - a link-state IGP protocol. It divides the network into **areas** for scalability: each area has its own LSDB. Area 0 (backbone) is central - all others must connect to it. This reduces SPF load.

**Rule:** All non-backbone areas must connect to Area 0. If physically impossible - use a virtual link through an intermediate area.

LSA Types

**LSA** (Link State Advertisement) - OSPF messages about network state. Different LSA types carry different information: about routers, networks, external routes. LSAs are stored in the LSDB and used for SPF calculation.

**LSA Aging:** Every LSA has a max age (3600 sec). A router refreshes its LSAs every 30 minutes. If an LSA is not refreshed - it is removed from the LSDB.

SPF Algorithm

**SPF (Shortest Path First)** - Dijkstra's algorithm for computing shortest paths. Each router builds a tree from itself as the root to all networks. When an LSA changes, SPF is re-run (with optimizations to minimize load).

**SPF Throttling:** OSPF doesn't run SPF immediately on every change. There is a hold-time (to accumulate changes) to avoid overloading the CPU with constant recalculations.

OSPF Neighbors and DR/BDR

**OSPF Neighbors** are established via Hello packets. On multi-access networks (Ethernet), a **DR** (Designated Router) and **BDR** (Backup DR) are elected for optimization - all LSAs go through the DR, not N×N.

**Non-broadcast:** On NBMA networks (Frame Relay), neighbors must be specified manually since there is no broadcast for Hello. Or use point-to-multipoint mode.

Summary

• **Areas** scale OSPF; Area 0 is the backbone, all areas connect to it
• **LSA Types:** 1 (Router), 2 (Network), 3 (Summary), 5 (External)
• **SPF** (Dijkstra) builds a shortest path tree from the LSDB
• **DR/BDR** optimize multi-access networks (N adjacencies vs N²)

Related Topics

OSPF often works alongside other technologies:

• BGP — BGP for external routing, OSPF for internal
• Dynamic Routing — OSPF vs RIP vs EIGRP - IGP comparison
• MPLS — OSPF is often used with MPLS in SP networks

Вопросы для размышления

• Why must all areas connect to Area 0?
• How does DR election affect segment fault tolerance?
• When should you increase OSPF reference bandwidth?

Связанные уроки

• alg-14-dijkstra