Link State theory (LSA – LSDB – SPF Algorithm)

In this lesson, I am going to explore how Link State routing protocols function. Understanding how these protocols operate is crucial because it lays the groundwork for comprehending OSPF, one of the most commonly used Link State protocols. By knowing how Link State protocols think and work, you’ll be better equipped to navigate OSPF configurations, troubleshooting, and optimizations. I will start with some theory in this lesson, and later, I will dive into lab exercises to solidify these concepts with hands-on experience.

So, let’s visualize a basic network setup: imagine three routers connected in a triangular fashion, and OSPF is enabled on all of them. Each router is configured to advertise its directly connected networks to OSPF. Now, what happens when OSPF is activated? Each router sends out what is known as a Link State Advertisement (LSA). This is a vital component of OSPF communication and forms the backbone of how routers share information. Think of an LSA as a message that says, “I am Router 1, and I have these networks connected on these interfaces.” This LSA is sent to all neighboring routers.