2.1 Explain characteristics of routing technologies

Dynamic routing

📘CompTIA Network+ (N10-009)

Open Shortest Path First (OSPF)

OSPF is a dynamic routing protocol used in IP networks to help routers figure out the best path for data to travel. Unlike static routing, where an admin manually sets paths, OSPF automatically discovers and maintains routes.

1. Type of Protocol

OSPF is a link-state routing protocol.
- This means it knows the entire network topology, not just which neighboring routers exist.
- Each router builds a map of the network and uses that map to calculate the shortest path to every network.
Contrast with distance-vector protocols (like RIP):
- Distance-vector routers only know the distance (hop count) to a destination, not the full topology.
- OSPF is faster and more efficient in large networks.

2. How OSPF Works

OSPF uses Link-State Advertisements (LSAs) to share information about the network.

Step-by-step process:

Neighbor Discovery
- Routers find other OSPF routers on the same network using Hello packets.
- This helps routers establish adjacencies (trusted relationships with neighbors).
Topology Sharing
- Each router sends LSAs describing its directly connected networks and links.
- All routers share this information so every router has a complete network map.
Shortest Path Calculation
- OSPF uses Dijkstra’s algorithm (Shortest Path First, SPF) to calculate the best path to each network.
- The best path is based on cost, which usually reflects link speed (faster links = lower cost).
Routing Table Update
- After calculation, routers update their routing tables with the best paths.
- OSPF automatically adjusts if the network changes (e.g., a link goes down).

3. Key OSPF Concepts

a) Areas

OSPF divides a network into areas to improve efficiency and scalability.
Area 0 is called the backbone area; all other areas connect through it.
Benefits:
- Reduces unnecessary routing updates.
- Limits the scope of changes to a smaller area.

b) Router Types

Internal Router – All interfaces are in the same area.
Backbone Router – Connected to Area 0.
Area Border Router (ABR) – Connects one area to another.
Autonomous System Boundary Router (ASBR) – Connects OSPF to external networks (like the internet or another routing protocol).

c) Metric (Cost)

OSPF calculates path cost using bandwidth: Cost = Reference Bandwidth / Interface Bandwidth
- Example: If reference bandwidth = 100 Mbps and the interface is 10 Mbps:
Cost = 100 / 10 = 10
- Lower cost = preferred path.

d) Hello and Dead Intervals

Routers send Hello packets to detect neighbors.
If no Hello is received within the Dead Interval (default 4x Hello), the router assumes the neighbor is down and recalculates routes.

4. OSPF Packet Types

OSPF uses 5 main packet types to communicate:

Packet Type	Purpose
Hello	Discover neighbors
Database Description (DBD)	Summarize LSAs to neighbors
Link-State Request (LSR)	Request more details about LSAs
Link-State Update (LSU)	Share LSAs with neighbors
Link-State Acknowledgment (LSAck)	Confirm receipt of LSAs

5. OSPF Advantages

Fast convergence when network changes.
Scales well to large networks.
Loop-free due to SPF algorithm.
Supports VLSM (Variable Length Subnet Masking) and CIDR.
Reduces unnecessary traffic using areas.

6. OSPF Limitations

More complex to configure than RIP.
Consumes more CPU and memory because it stores the full network topology.
Needs careful design in very large networks.

7. OSPF in Real IT Networks

Used in enterprise networks and data centers.
Connects multiple offices or VLANs efficiently.
Handles redundant links well:
- If a link fails, OSPF recalculates the best alternative automatically.
Integrates with IPv4 and IPv6 (OSPFv2 for IPv4, OSPFv3 for IPv6).

8. Key Exam Points to Remember

OSPF = link-state, dynamic routing protocol.
Uses LSAs and Dijkstra’s SPF algorithm.
Divides networks into areas, with Area 0 as backbone.
Cost is the metric, based on bandwidth.
Uses Hello packets to discover neighbors.
Types of routers: Internal, ABR, ASBR, Backbone.
Scales well, fast convergence, supports VLSM/CIDR.
5 packet types: Hello, DBD, LSR, LSU, LSAck.

This covers everything you need to pass the CompTIA Network+ exam regarding OSPF, in simple IT-focused language without real-world analogies outside networking.