Open Shortest Path First, the link-state IGP behind most enterprise networks. Articles tagged OSPF cover neighbor relationships, LSA types, areas, DR/BDR election, and Cisco IOS XE configuration.
Every OSPF adjacency you have ever built started with a Hello packet, and almost every adjacency that refuses to build is a Hello packet being quietly thrown away. The trouble is that show ip ospf neighbor only tells you the
A spine-leaf underlay should be the least interesting part of your fabric. Here is the OSPF build that gets you there, with real Nexus output.
INIT means "I hear you, you do not hear me". One router shows a neighbor in INIT, the other shows an empty table, and that asymmetry is the entire diagnosis. Captured live on IOS XE 17.18.2 in CML with an inbound ACL eating OSPF in one direction.
The full OSPF build on a Cisco ASA: configuration, a verified adjacency with MD5 authentication, the DR/BDR election the firewall loses, the link-state database, and redistribution, all captured from a live ASAv 9.24.
Migrate a live network from EIGRP to OSPF without dropping a packet: run both protocols side by side, then flip administrative distance one command at a time. The ships in the night technique with real lab output.
Two-point redistribution is where networks go to die. This article builds the classic CCIE failure for real in a CML lab, shows a route being re-injected into the protocol it came from, and then fixes it with the exact output at each step.
Five OSPF faults built and broken for real in a CML lab, ticket style: the symptom, the two commands to compare, the actual output, and the fix for each one. OSPF rarely tells you what is wrong; this method finds it anyway.
OSPF compares route types before metrics, so a worse metric routinely wins. Prove the full O, O IA, E1, N1, E2, N2 ordering by injecting the same prefix six different ways into a live CML topology and watching each one take over.
area range, filter-list, distribute-list, summary-address: four OSPF filters that are not interchangeable. Side-by-side lab output shows which ones change the LSDB and which only touch the local RIB.
The OSPF forwarding address can point a Type 5 route at a next hop that never advertised it, or silently drop it from the RIB while the LSA sits in the database. This article covers the five conditions that set it and the failure it causes.
max-metric router-lsa tells OSPF a router is still here but a terrible choice for transit, so traffic drains before you reload it. Covers the RFC 6987 stub router advertisement, its options, and a lab demonstration.
Transit /30s and /31s fill OSPF routing tables with prefixes nobody sends traffic to. Prefix suppression removes them with one command. See it working on Cisco IOS XE, plus the consequence that bites during troubleshooting.