The March 2019 Packet Pushers Virtual Design Clinic had to deal with an interesting question:

Our server team is nervous about full-scale DR testing. So they have asked us to stretch L2 between sites. Is this a good idea?

The design clinic participants were a bit more diplomatic (watch the video) than my TL&DR answer which would be: **** NO!

Let’s step back and try to understand what’s really going on:

The last bits of updated Never-Ending Story of IP Fragmentation were published a few days ago: IP fragmentation and tunnels and summary and related blog posts, RFCs and other articles.

Every now and then I stumble upon a blog post saying “OSI 7-layer model sucks” or “OSI 7-layer model is a lie”, most recent one coming from Robert Graham.

Before going into the details, let’s agree on the fundamentals.

Most everyone who ever tried to build a network spanning more than one transmission technology and including intermediate nodes came to the conclusion that layered approach to networking makes sense.

Whether you have three, four, five, or seven layers in your model doesn’t matter. What really matters is that your model contains all the functionality you need to implement host-to-host networking in target environment.

Last week we started the Autumn 2019 Building Network Automation Solutions online course with an interesting presentation from Matthias Luft focused on open-source supply chain security

TL&DR: Can I download whatever stuff I found as my first Google hit and use it in my automation solution? ****, NO!

Matthias covered these topics:

Imagine you would have a system that would read network device configurations, figure out how those devices might be connected, reverse-engineer the network topology, and be able to answer questions like “what would happen if this link fails” or “do I have fully-redundant network” or even “how will this configuration change impact my network”. Welcome to Batfish.

Interested? You’ll find more in Episode 104 of Software Gone Wild.

As I was preparing the materials for Ansible 2.7 Update webinar sessions I wanted to dive deeper into declarative configuration modules, starting with “I wonder what’s going on behind the scenes”

No problem: configure EEM applet command logging on Cisco IOS and execute an ios_interface module (more about that in another blog post)

Next step: let’s see how multi-platform modules work. Ansible has net_interface module that’s supposed to be used to configure interfaces on many different platforms significantly simplifying Ansible playbooks.

After describing MTU basics and drawbacks of IP fragmentation, it’s time for more details: Path MTU Discovery (PMTUD) and network implications of using ICMP for PMTUD.

Interested in similar topics? Check out How Networks Really Work webinar.

This blog post was initially sent to subscribers of my SDN and Network Automation mailing list. Subscribe here.

Have you ever seen a presentation in which a startup is telling you how awesome their product is because it allows you to simulate your whole network in a virtual environment? Not only that, you can use that capability to build a test suite and a full-blown CI/CD pipeline and test whether your network works every time you make a change to any one box in the network.

Sounds awesome, right? It’s also dead wrong. Let me explain why that’s the case.

Last year when I was creating the first version of VMware NSX Deep Dive content, NSX-V was mainstream and NSX-T was the new kid on the block. A year later NSX-V is mostly sidelined, and all the development efforts are going into NSX-T. Time to adapt the webinar to new reality… taking the usual staged approach:

• The new slide deck covering NSX-V and NSX-T is ready. It includes early information about NSX-T release 2.5; I’ll fill in the details once the documentation becomes public.
• I’ll use the slide deck in day-long workshop in Zurich on September 10th.
• The live webinar sessions (including updated NSX-T 2.5 content) will start on November 14th.

After discussing the challenges one encounters even in the simplest networking scenario connecting two computers with a cable, we took a short diversion into an exciting complication: what if the two computers are far apart and we can’t pull a cable between them?

Trying to answer that question, we entered the wondrous world of transmission technologies. It’s a topic one can spend a whole life exploring and mastering, so we were not able to do more than cover the fundamentals of modulations and multiplexing technologies.

We’re back from the summer break for real - the first autumn 2019 ipSpace.net event takes place today: I’ll talk about the fallacies of distributed computing.

September will be an intensive month:

• We’re starting the autumn 2019 network automation course on September 3^rd;
• A week after that (September 10^th) I’ll run a day-long VMware NSX workshop in Zurich;
• Azure Networking webinar series is starting on September 12^th and continuing on September 24^th;
• Lukas Krattiger will talk about service insertion in leaf-and-spine fabrics on September 17^th.

Of course, we’ll keep going… our event calendar is fully packed till mid-November. More about that in a month.

In mid 2000s I wrote a number of articles describing various TCP/IP features. Most of them are a bit outdated, so I decided to clean up, update and repost the most interesting ones on ipSpace.net, starting with Never-Ending Story of IP Fragmentation.

The first part of that article is already online, covering MTU basics and drawbacks of IP fragmentation.

Andrea Dainese decided to describe a series of mechanisms and protocols you can use in network automation. He started with Zero-Touch Provisioning and continued with screen scraping. Next one on his list: NETCONF and RESTCONF

Remember my rant about the glacial speed of Azure orchestration system? I decided I won’t allow it to derail yet another event and recorded the demos in advance of the first live session. The final videos are just over an hour long; it probably took me at least three hours to record them.

If you plan to attend the live webinar session on September 12th, you might want to watch at least the first few videos before the live session - I will not waste everyone’s time repeating the demos during the live session.

Whenever discussing a complex topic, it’s worth adhering to two principles: (A) identify the challenges you’re trying to solve, and (B) start as simple as you can and add complexity later.

We did precisely that in the Introducing Networking Challenges part of How Networks Really Work webinar. We started with the simplest possible case of two computers connected with a cable… and even there identified a plethora of challenges that had to be solved more than half a century ago (and still have to be solved today no matter what magic software-defined technology someone pulls out of their wizard hat).