Why This Cheat Sheet Matters for CCNA
This cheat sheet covers the most important Open Shortest Path First concepts tested on the CCNA (CCNA) certification exam. It contains 3 sections with 15 key points that you should memorize before exam day. Deep dive into OSPF operation: neighbor adjacencies, LSAs, areas, DR/BDR election, cost calculation, route summarization, and troubleshooting OSPF issues. Use this as a quick-reference guide during your final review sessions.
OSPF Fundamentals
- OSPF is a link-state routing protocol. Uses SPF (Dijkstra) algorithm.
- Administrative distance: 110.
- Uses multicast 224.0.0.5 (all OSPF routers) and 224.0.0.6 (DR/BDR).
- Hello timer: 10s on broadcast/point-to-point, 30s on NBMA. Dead timer = 4× hello.
- OSPF metric = cost = reference bandwidth / interface bandwidth (default ref = 100 Mbps).
Neighbor Adjacency
- Neighbors must match: area ID, hello/dead timers, authentication, subnet mask (on broadcast), stub flags.
- OSPF states: Down → Init → 2-Way → ExStart → Exchange → Loading → Full.
- DR/BDR election occurs on broadcast and NBMA networks. Highest priority wins (default 1). Highest router ID breaks ties.
- Point-to-point links do NOT elect DR/BDR.
- Router ID: manually set > highest loopback IP > highest active interface IP.
Configuration
- router ospf <process-id> — starts OSPF process (locally significant).
- network <ip> <wildcard> area <id> — advertises matching interfaces.
- ip ospf <process-id> area <id> — alternative per-interface config.
- passive-interface <intf> — stops sending hellos (use on LAN-facing interfaces).
- default-information originate — advertises a default route into OSPF.
Practice OSPF Questions
Put your knowledge to the test with practice questions.
Cisco Command Quick Reference
Cisco IOS commands follow a hierarchical structure: User EXEC mode (>), Privileged EXEC mode (#), Global Configuration mode (config)#, and Interface Configuration mode (config-if)#. Master the transitions between these modes and the key "show" commands for each technology area — they are heavily tested on every Cisco exam.