Mastering OSPF
A working series from my CCIE Enterprise Infrastructure study. Each chapter takes one part of OSPF, from area design to summarization to the overlays it runs on, and explains why it behaves the way it does, not just how to configure it.
Design
OSPF Area Types & Hierarchy
Normal, stub, totally stubby and NSSA areas; ABR and ASBR roles; and which LSAs each area type filters out.
Areas · LSA FilteringOSPF vs BGP: Summarization & Architecture
Inter-area Type-3 summarization hides topology; BGP aggregation hides routes. Why the difference matters, and how IGP and EGP split the work.
Summarization · IGP vs EGPTransport
DMVPN: GRE, mGRE & NHRP
Overlay versus underlay, point-to-point versus multipoint tunnels, and how NHRP maps tunnel addresses to transport, the network OSPF runs across.
Overlays · NHRPLSA Mechanics
OSPF Type-5 LSAs: Intra-Area vs Inter-Area Reachability
A Type-5 LSA describes the destination, not the path. How routers reach the ASBR through Type-1/2 and Type-4 LSAs, and why E1 makes that path count.
External Routes · Type-4 / Type-5OSPF Cost Calculation: How a Summary LSA Becomes a Routing Table Entry
Why the Type-3 metric and the O IA metric differ: the router adds its own cost to the ABR. Two-ABR selection, verification, and why OSPF is "distance vector between areas".
Inter-Area Routing · Type-3 MetricsConvergence
OSPF Micro-loops: Transient Loops During SPF Convergence
Same LSDB, same SPF result, different moment. How flooding and SPF timing create local and remote cross-point loops, how to report them, and how TI-LFA and SR remove them.
SPF · Micro-loops · TI-LFANext
OSPFv3: OSPF for IPv6 and Beyond
Link-local adjacencies, the new LSA types, address families, and what changed from v2.
In Progress