Each router along a route is dependent on the router before it to perform its calculations correctly and then correctly pass along the results. This is in contrast to distance vector protocols, in which route information is passed hop-by-hop through the network and a calculation is performed at each hop – a distributed calculation using local information. One of the advantages of link state protocols is that the link state database provides a “view” of the entire network, preventing most routing loops. This tree is a sort of map of the shortest path to every other router. Each router then independently runs a shortest-path-first calculation on its database – a local calculation using distributed information – to derive a shortest-path tree. From this flooded information each router builds an identical link state database. Why does OSPF require all traffic between non-backbone areas to pass through a backbone area (area 0)?Ĭomparing three fundamental concepts of link state protocols, concepts that even most OSPF beginners understand, easily derives the answer to the question.Įvery link state router floods information about itself, its links, and its neighbors to every other router. And I am still surprised how often people stumble on it. It’s a question that reveals whether a person has some understanding of how the protocol works, rather than just how to configure it. That leads to my favorite technical interview question, one that I asked almost every time. I did, however, expect every candidate to have extensive experience with OSPF and that’s where I focused most of my questions. So I usually couldn’t dig very deep there, and only tried to determine their capacity for quickly coming up to speed on those protocols. Unless the candidate came from a service provider background they probably had no practical MPLS and IS-IS, and only basic BGP, experience. A CCIE and experience commensurate with that certification was typically a gateway to just being considered for a technical interview, so these job candidates were no slouches in what they knew. I was often called upon, in past jobs, to conduct technical interviews with applicants to network engineering and professional services positions. Today’s post is about one of the most basic OSPF rules.

Mpls interview questions