[Day 8] 2. OSPF and its Architecture

OSPF (Open Shortest Path First) is a link-state routing protocol used within an Autonomous System (AS) to find the best path for data transmission. It uses Dijkstra’s algorithm to calculate the shortest path to each destination based on the network topology. OSPF is hierarchical, allowing the network to be divided into areas for better scalability and efficient routing. It is widely used in large enterprise networks due to its fast convergence, support for multiple path options, and ability to handle large networks efficiently.

!2. OSPF Architecture.png

OSPF Router Types

• Area 0 (Backbone Area):

  • Known as the backbone area, with the area identifier 0.
  • This area serves as the central core of OSPF and is essential for communication between other areas.

• Backbone Router:

  • A router that has at least one interface in the backbone area (Area 0).

• Area Border Router (ABR):

  • A router that connects multiple areas, with interfaces in more than one area. It handles routing between different areas.

• Internal Router:

  • A router with all interfaces within a single area, meaning it only operates within that area.

• Autonomous System Boundary Router (ASBR):

  • A router that connects an OSPF network to external networks or other autonomous systems, allowing communication with external routing domains.

OSPF Addressing

OSPF supports route summarization, which occurs when a hierarchical addressing structure is implemented. In OSPF, the network ID (rather than individual subnets) is advertised. This reduces the routing table size and prevents it from being overwhelmed with too many subnets.

OSPF Area Types

1. Backbone Area (Area 0):

   • Core of OSPF: Every OSPF network must have a connection to Area 0.
   • Function: Routes traffic between all other areas in the OSPF domain.

2. Normal Area:

   • Standard OSPF area: Handles internal routes, inter-area routes, and external routes.
   • Function: Supports all types of routes within the OSPF domain.

3. Stub Area:

   • Blocks external routes: Does not accept external routes from other protocols but allows inter-area routes.
   • Purpose: Reduces the size of the routing table by limiting the number of external routes.

4. Totally Stub Area:

   • Highly restricted: Only accepts internal routes and a default route.
   • Purpose: Simplifies routing by blocking external and inter-area routes, minimizing routing table size.

5. Not-So-Stubby Area (NSSA):

   • Stub area with limited external routes: Allows some external routes, but these are converted into special Link-State Advertisements (LSAs).
   • Use case: Useful when limited external routing is required, but full external updates from other areas are unnecessary.