OSPF is a widely used and supported routing protocol.
Open standard, meaning any router can support it.
It is an interior gateway protocol designed for a single autonomous system.
OSPF is a link-state protocol, aiming to learn routes through comprehensive network knowledge.
Key Concepts
Link-State Advertisements (LSAs): OSPF routers use LSAs to share information about routers and subnets across the network.
Link-State Database (LSDB): Each router maintains an LSDB to ensure they all have the same network information.
OSPF Process Steps
Becoming OSPF Neighbors
Routers exchange hello messages to establish neighbor relationships.
Each router selects a Router ID (RID), which can be set manually or automatically.
The RID is usually the highest loopback IP address or the highest non-loopback IP if no loopback is available.
Exchanging Database Information
After becoming neighbors, routers exchange their LSDB information.
Designated Routers (DRs) and Backup Designated Routers (BDRs) are elected to manage updates in broadcast networks to prevent excess flooding of updates.
The DR is elected based on the highest OSPF priority or Router ID.
Choosing the Best Routes
OSPF uses a cost metric based on interface bandwidth to determine the best routes.
Cost Calculation: Default reference value is 100,000 kbps. Cost = Reference Value / Interface Bandwidth.
Example Costs:
Serial (1544 kbps) = Cost of 64
Ethernet (10,000 kbps) = Cost of 10
Fast Ethernet (100,000 kbps) = Cost of 1
Any speed greater than Fast Ethernet = Cost of 1.
Route Selection Example
Router 1: Total cost = 138 (two links at 64, one at 10)
Router 2: Total cost = 30 (all links at 10)
Router 5: Total cost = 12 (two links at 1, one at 10)
Best Route: Router 5 to 10.0.0.0 subnet is the lowest cost and is added to the routing table.
Conclusion
OSPF is a complex yet efficient routing protocol.
Many more details can be explored in further discussions.
Remember: "Router" is pronounced as "Rooter" in British English.