The early October 2021 Facebook outage generated a predictable phenomenon – couch epidemiologists became experts in little-known Bridging the Gap Protocol (BGP), including its Introvert and Extrovert variants. Unfortunately, I also witnessed several unexpected trips to Mount Stupid by people who should have known better.

To set the record straight: everyone’s been there, and the more vocal you tend to be on social media (including mailing lists), the more probable it is that you’ll take a wrong turn and end there. What matters is how gracefully you descend and what you’ve learned on the way back.

When I started creating the How Networks Really Work series, I wondered whether our subscribers (mostly seasoned networking engineers) would find it useful. Turns out at least some of them do; this is what a long-time subscriber sent me:

How Networks Really Work is great, it’s like looking from a plane and seeing how all the roads are connected to each other. I know networking just enough to design and manage a corporate network, but there are many things I have learned, used and forgotten along the way.

Two weeks ago I replied to a battle-scar reaction to 7-layer OSI model, this time I’ll address a much more nuanced view from Russ White. Please read his article first (as always, it’s well worth reading) and when you come back we’ll focus on this claim:

The OSI Model does not accurately describe networks.

Like with any tool in your toolbox, you can view the 7-layer OSI model in a number of ways. In the case of OSI model, it can be used:

I should have known better, but I got pulled into another stretched VLANs for disaster recovery tweetfest. Surprisingly, most of the tweets were along the lines of you really shouldn’t be doing that and that would never work well, but then I guess I was only exposed to a small curated bubble of common sense… until this gem appeared in my timeline:

Interestingly, that’s exactly how IP works:

One of my readers sent me this interesting question:

It begs the question in how far graduated students with a degree in computer science or applied IT infrastructure courses (on university or college level or equivalent) are actually aware of networking fundamentals. I work for a vendor independent networking firm and a lot of my new colleagues are college graduates. Positively, they are very well versed in automation, scripting and other programming skills, but I never asked them what actually happens when a packet traverses a network. I wonder what the result would be…

I can tell you what the result would be in my days: blank stares and confusion. I “enjoyed” a half-year course in computer networking that focused exclusively on history of networking and academic view of layering, and whatever I know about networking I learned after finishing my studies.

I was speaking with a participant of an SDN event in Zurich after the presentations, and he made an interesting comment: whenever he experienced serious troubleshooting problems in his career, it was due to lack of understanding of networking fundamentals.

Let me give you a few examples: Do you know how ARP works? What is proxy ARP? How does TCP offload work and why is it useful? What is an Ethernet collision and when would you see one? Why do we need MLD in IPv6 neighbor discovery?

In late June, we added GRE tunnels to netlab, including a Junos implementation. It looked great (as in “everything worked”) until I changed the integration tests to have GRE tunnels between a tested device and a pair of FRR containers. All other implementations worked as before, but Junos failed to establish an OSPFv3 adjacency over the GRE tunnel with FRR.

Stefano Sasso quickly identified the culprit: Junos OSPFv3 process thinks it should send the DBD packets over GRE tunnels with MTU set to zero (the behavior reserved for virtual links)¹.

Summer is almost over, and it’s time to resume regular programming with the next example from the Segment Routing workshop I had at ITNOG10: multi-vendor SR-MPLS. I used the same lab topology as in the previous examples but deployed Arista EOS on PE1, FRRouting on P, and SR Linux on PE2¹

When someone starts singing the Use Digital Twins to Test Your Network hymn (or, more recently, tells you how AI agents can do that to validate their ideas), ask them about these minor details. If they persist, point them (not that it would help) to this long list of gotchas.

That list just got longer: Arista cEOS container does not apply inbound ACLs to control-plane traffic (Arista vEOS VM does).

Tony Mattke continues publishing excellent “Git for Networking Engineers” content. In his latest article, he described how to find who made what changes (and why) in a Git repository.

Absolutely worth reading if you’re new to Git, but even oldtimers might find a few useful tidbits (Git is one of those things one never truly masters).