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.

So, getting a broad vision helps me remember why I chose something and maybe solve my bad choices. There are many things that I may never use, but with the movement of all things in the cloud it’s great to know, or at least understand, how things really work.

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?

We run two types of integration tests before shipping a netlab release: device integration tests that check whether we correctly implemented netlab features on all supported devices, and platform integration tests that check whether rarely-used core functionality works as expected.

I want to have some validation included in the platform integration tests to ensure the lab devices are started, and that the links and the management network work as expected. The simplest way to get that done is to start OSPF with short hello intervals (to get adjacency up in no time), for example:

My first encounter with Ansible release 12 wasn’t exactly encouraging. We were using a few Ansible Jinja2 filters (ipaddr and hwaddr) in internal netlab templates, and all of a sudden those templates started crashing due to some weird behavior of attributes starting with underscore.

We implemented don’t use Ansible release 12 as a quick workaround, but postponing painful things is never a good solution(see also: visiting a dentist), so I decided to try to make netlab work with Ansible release 12. What a mistake to make.

After launching the BGP labs in 2023 and IS-IS labs in 2024, it was time to start another project that was quietly sitting on the back burner for ages: open-source (and free) VXLAN/EVPN labs.

The first lab exercise is already online and expects you to extend a single VLAN segment across an IP underlay network using VXLAN encapsulation with static ingress replication.

Pat Allen wrote an interesting guide for managers of networking teams dealing with the onslaught of AI (HT: PacketPushers newsletter).

The leitmotif: use AI to generate a rough solution, then review and improve it. That makes perfect sense and works as long as we don’t forget we can’t trust AI, assuming you save time doing it this way.

Found this incredible gem¹ hidden in the Usage Guidelines for the OSPFv3 router-id configuration command part of the Cisco IOS IPv6 reference guide.

The whole paragraph seems hallucinated², but that couldn’t be because the page was supposedly last updated in 2019, and LLMs weren’t good enough to write well-structured nonsense at that time:

OSPFv3 is backward-compatible with OSPF version 2.

No, it is not.

netlab does not support a Syslog server (yet), but it’s really easy to add one to your lab topology, primarily thanks to the Rsyslog team publishing a ready-to-run container. Let’s do it ;)

Adding a Syslog Server

Rsyslog is an open-source implementation of a Syslog server (with many bells and whistles, most of which we won’t use) that can (among other things) log incoming messages to a file. Even better (for our use case), the Rsyslog team regularly publishes Rsyslog containers; we’ll use the rsyslog/rsyslog-collector container because it can “receive logs via UDP, TCP, and optionally RELP, and can send them to storage backends or files.”

I love well-organized small conferences, so it wasn’t hard to persuade me to have another talk at the DEEP Conference in Zadar, Croatia. This time, I talked about the role of digital twins in disaster recovery/avoidance testing. You might know my take on networking digital twins; after that, I only had enough time to focus on bandwidth and latency matter, and this is how you emulate limited bandwidth and add latency bit.

Here’s a cool feature every routing protocol should have: a flag that tells everyone a node is going down, giving them time to adjust their routing tables before disrupting traffic flow.

OSPF never had such a feature; common implementations set the cost of all interfaces to a very high value to emulate it. BGP got it (the Graceful BGP Session Shutdown) almost 30 years after it was created. IS-IS had the overload bit from day one, and it’s just what an IS-IS router needs to tell everyone else they should stop using it for transit traffic. You can try it out in the Drain Traffic Before Node Maintenance lab exercise.

Click here to start the lab in your browser using GitHub Codespaces (or set up your own lab infrastructure). After starting the lab environment, change the directory to feature/5-drain and execute netlab up.

The first half of the Graph Algorithms in Networks webinar by Rachel Traylor is now available without a valid ipSpace.net account; it discusses algorithms dealing with trees, paths, and finding centers of graphs. Enjoy!