Open Shortest-Path First (OSPF) Routing Protocol

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OSPF is like a traffic director for the internet. Imagine a city with many roads - OSPF helps routers (the traffic directors) figure out the best paths for data to travel from one place to another. It’s like a smart GPS for computers, making sure information takes the shortest and fastest routes. OSPF routers talk to each other, share maps of the internet, and decide the best ways to send data. It’s a cool system that keeps the internet running smoothly!

ChatGPT explaining OSPF to a high-school kid

Configuration Tips

This blog started as a collection of (hopefully) helpful configuration tricks, and I documented numerous Cisco IOS configuration tips in the early 2000s.

Implementation Details

Let’s start with the elephant in the room: OSPF areas – a simple concept that got way too convoluted when OSPF started accreting nerd knobs like NSSA areas:

OSPF default routes are another confusing topic. You could have inter-area default routes (used in stub areas) or external default routes that could be conditional or unconditional.

OSPF adjacencies are another fun troubleshooting topic:

The inimitable forwarding address in type-5 LSA will make your head explode when combined with the NSSA areas.

Want even more OSPF details? I documented way too many of them since I started blogging, including:

Deploying OSPF

Creative networking engineers often forget an unpleasant truth: OSPF is a single security domain. You should never run it with less-trusted peers, be it your customers, data center servers, or virtual machines.

OSPF by itself is complex enough, but the real fun starts when you combine it with other protocols (for example, BGP and LDP):

Running OSPF in large hub-and-spoke networks (for example, large DMVPN networks) is another tough challenge:

While you could use OSPF to get unequal-cost multipathing, you might be tripped by numerous caveats; no wonder there are few implementations of this concept.

Finally, you can run OSPF over unnumbered interfaces, be it point-to-point serial links or Ethernet segments:


Now and then, I couldn’t resist writing an OSPF-related rant:

What Others Are Writing About OSPF

Other OSPF Blog Posts