Ospf

# OSPF (Open Shortest Path First) — OSPFv2 for the CCNA Exam OSPF is the single most important routing protocol topic on the CCNA 200-301 v1.1 exam, appearing in simulation questions, multiple-choice questions, and troubleshooting scenarios within the IP Connectivity domain (20% of the exam). Mastering OSPF configuration and neighbor adjacency rules is not optional — it is the core of this domain. > Domain context (sourced from Cisco 200-301 v1.1 PDF): IP Connectivity carries 20% weight on the 120-minute exam. OSPF configuration and verification is a primary topic within this domain. Note: the exam blueprint does not subdivide domain weights below the domain level — the 20% figure is authoritative; per-topic percentages within domains are not published by Cisco. --- ## What Is OSPF and Why Does It Matter? OSPF (Open Shortest Path First) is a link-state interior gateway protocol (IGP) that uses the Dijkstra Shortest Path First (SPF) algorithm to build a complete map of the network and calculate the best path to each destination. Unlike distance-vector protocols, every OSPF router builds an identical Link State Database (LSDB) for its area, giving it a full picture of the topology. On the CCNA exam, you are tested on OSPFv2 (IPv4). OSPFv3 (IPv6) exists but is not heavily tested. --- ## Core OSPF Concepts ### Administrative Distance and Cost | Protocol | Administrative Distance | |----------|------------------------| | Connected | 0 | | Static | 1 | | OSPF | 110 | | RIP | 120 | OSPF selects paths based on cost, not hop count. Cost is calculated as: > **Cost =…