On Theory and Practice

I recently read a must-read blog post by Russ White in which he argued that you need to understand both theory and practice (see also Knowledge or Recipes and my other certification rants) and got a painful flashback of a discussion I had with a corner-cutting SE (fortunately he was an exception) almost two decades ago when I was teaching my Advanced OSPF course at Cisco.

Him: How many OSPF routers can you have in an area? (asked during the first morning of a 3-day course)

Me: As you’ll see in these three days, OSPF is a pretty complex protocol, so there’s no right answer. It depends on the area topology, the frequency of topology changes, the type of routers… For example, areas with low-end routers have to be smaller due to their limited CPU power, for example.

Him: You don’t understand. I need a simple answer that I can use in my desings.

Me: How about 50?

Him: And how many OSPF areas can you have on a router?

Me: Obviously that depends on the area size (and all other things I already mentioned), the CPU power of the router, what other things that same router is doing, the desired convergence speed…

Him: That’s too complex. I need a simpler answer.

Me: Well, the official answer is three. Is that good enough for you?


  1. In theory there is no difference between theory and practice.
  2. Also of note, theories can be slain by practical evidence:

  3. Me: "Get out of my class ans stop wasting my oxygen"
  4. Ahaha Awesome Ivan :)
  5. "The difference between theory and practice is even larger in practice than it is in theory."
  6. Theory and practice

    Theory is when you think you know something but it doesn't work.
    Practice is when something works but you don't know why.
    Usually we combine theory and practice: nothing works and we don't know why.
  7. Answer would be as many interface on that router and if you create a loopback and assign them into a different area for fun ! then you can have thousands. But it doesn't mean you should of course :) Even in theory there is no numeric answer for " what you should " depends of topology,change rate , other control plane interactions , type of devices etc but , numeric answer would be estimated based on Router LSA size and desire to prevent fragmentation..
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