Understanding OSPF: Basics and Operations

Aug 25, 2024

OSPF Overview

What is OSPF?

  • OSPF (Open Shortest Path First) is a widely used routing protocol.
  • It is an open standard, meaning it is supported by almost all routers.
  • OSPF is an Interior Gateway Protocol (IGP), designed for use within a single autonomous system.
  • It is a link-state protocol, meaning it learns the network topology and maintains a common database.

Key Concepts

  • Link-State Advertisements (LSAs): OSPF routers use LSAs to share information about the network, including subnets and router information.
  • Link-State Database (LSDB): This database contains all the information learned from LSAs, ensuring each router has the same view of the network.

OSPF Operation Steps

  1. Becoming OSPF Neighbors

    • Routers agree to form neighbor relationships to share routing information.

  2. Exchanging Database Information

    • Neighbors exchange LSDB information after establishing neighbor relationships.

  3. Choosing the Best Routes

    • Each router runs a calculation to determine the best routes to add to the routing table.

Forming Neighbor Relationships

  • Router ID: Each router must select a unique Router ID (RID), which acts like a name for the router. The RID can be set manually or determined automatically based on the highest loopback interface IP or highest non-loopback IP.
  • Hello Messages: Routers send hello messages containing their RID and known neighbors to establish connections.

Requirements for OSPF Neighbor Relationships

  • Area ID must match.
  • Connecting links must be on the same subnet.
  • Hello and dead timers must be the same (default hello timer is 10 seconds).
  • Authentication must match if used.
  • Unique Router IDs are required.

OSPF States

  1. Init State: The initial state where routers exchange hello messages.
  2. Two-Way State: Routers recognize each other as neighbors.
  3. Designated Router (DR) and Backup Designated Router (BDR): In multi-access environments, a DR and BDR are elected to reduce network traffic during updates.

Database Exchange Process

  • ExStart State: Routers determine the master/slave relationship based on Router IDs.
  • Exchange State: Routers send Database Description (DBD) messages to share their LSAs.
  • Loading State: Routers request LSAs they do not have, ensuring they have complete information.
  • Full State: Once all LSAs are exchanged, routers enter the full state.

Route Selection Process

  • OSPF chooses the best routes using a metric called Cost based on bandwidth.
    • Reference value is 100,000 kbps by default.

Cost Examples

  • Serial interface (1544 kbps) = Cost 64
  • Ethernet (10,000 kbps) = Cost 10
  • Fast Ethernet (100,000 kbps) = Cost 1
  • Anything faster than Fast Ethernet also has a cost of 1.

Example Route Calculation

  • Router 1: Total cost to reach 10.0.0.0 = 138
  • Router 2: Total cost = 30
  • Router 5: Total cost = 12
    • Best Route: Router 5 has the lowest cost and thus is selected to be added to the routing table.

Conclusion

  • OSPF is a robust routing protocol that supports efficient routing decisions based on the network topology and link costs.
  • For more details on specific features, further study is encouraged.

Note: In British English, "Router" may be pronounced as "Rooter."