OSPF Fundamentals

🌐 OSPF Fundamentals

OSPF (Open Shortest Path First) is a dynamic link-state routing protocol widely used in enterprise networks. It provides fast convergence, hierarchical network design, and scalability, making it ideal for medium to large-scale deployments.

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🔁 OSPF Overview

• Type: Link-State Routing Protocol
• Administrative Distance (AD): 110 (Cisco)
• Open Standard: Supports multivendor environments

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🧠 OSPF Operation

1. Link-State Database (LSDB): Contains a complete map of the network.
2. LSA (Link-State Advertisement): Carries route information.
3. LSU (Link-State Update): Transmits LSAs.
4. LSAck: Acknowledges received LSUs.

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🧩 OSPF Adjacency & Neighborship

🔗 Neighborship Requirements

Routers must match the following parameters to form a neighbor relationship:

1. Area ID
2. Authentication settings
3. Subnet
4. Hello and Dead timers
5. Stub flag
6. MTU (Maximum Transmission Unit)

🤝 Neighborship Process

• Routers exchange Hello packets to form neighborship.
• Exchange of DBD (Database Description) and LSU packets establishes adjacency.
• Only adjacent routers exchange full routing information.

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🏆 DR/BDR Election

In broadcast/multi-access networks (e.g., Ethernet), a Designated Router (DR) and Backup Designated Router (BDR) are elected to reduce overhead.

🗳️ Election Process

1. Router Priority (Highest)
   • Default: 1 (carried in Hello packet)
   • Configuration: ip ospf priority <number>
2. Router ID (Highest) (Tie-breaker)
   • Configuration: router-id <ID>

Routers with priority 0 are ineligible for DR/BDR election.

➕ Why DR/BDR?

Without DR/BDR:

• Number of adjacencies = n(n-1)/2
• Example: 5 routers → 10 adjacencies

With DR/BDR:

• Each router forms adjacency only with DR and BDR → Reduces overhead

📡 Multicast Addresses

• 224.0.0.5 → All OSPF Routers
• 224.0.0.6 → DR/BDR Only

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🗺️ OSPF Areas & Design

🔄 Hierarchical Design

• Backbone Area (Area 0): Central required area.
• All other areas must connect to Area 0 (Backbone).

🧱 Router Types

Type	Description
AR	Area Router – exists within a single area
ABR	Area Border Router – connects multiple areas
ASBR	Autonomous System Boundary Router – connects to external networks

📈 Scalability

• Old recommendation (Router Series 2500): Max 50 routers per area
• Current hardware: Supports unlimited routers per area (hardware-dependent)

🧮 Dijkstra’s Algorithm in OSPF

OSPF uses Dijkstra’s Shortest Path First (SPF) algorithm to calculate the best paths.

🔍 What Is It?

Dijkstra’s algorithm finds the shortest path from the router to all other nodes in the network, building a Shortest Path Tree (SPT) using the LSDB.

⚙️ How OSPF Uses It:

1. OSPF routers collect LSAs from others in the area.
2. Create the Link-State Database (LSDB).
3. Run Dijkstra’s algorithm on the LSDB to build the SPT.
4. Update the routing table with optimal paths.

📌 Why Use It?

• Fast convergence
• Accurate and loop-free
• Responds quickly to topology changes

📚 Summary

Concept	Description
Protocol Type	Link-State
SPF Algorithm	Dijkstra’s Algorithm
Areas	Hierarchical with mandatory Backbone (Area 0)
Adjacencies	Requires matching neighbor parameters
DR/BDR	Reduces link overhead in broadcast networks
Scalability	Modern OSPF allows large (virtually unlimited) areas
Packet Types	Hello, DBD, LSU, LSA, LSAck

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