Blog Posts in June 2025
Static Routes in netlab Lab Topologies
As much as we’d love everything in our networks to be dynamic, auto-configured, or software-defined, reality often intervenes and forces us to use static routes, so we needed a mechanism to specify them in netlab lab topologies.
A static route has two components: the destination prefix and the next hop – the device that we hope knows how to reach that destination. The next hop is usually specified as an IPv4 or IPv6 address, but may also include outgoing interface information1.
Network Digital Twins: Between PowerPoint and Reality
A Network Artist left an interesting remark on one of my blog posts:
It’s kind of confusing sometimes to see the digital twin (being a really good idea) never really take off.
His remark prompted me to resurface a two-year-old draft listing a bunch of minor annoyances that make Networking Digital Twins more of a PowerPoint project than a reality.
Dear Vendors, EVPN Route Attributes Matter
Another scary tale from the Archives of Sloppy Code: we can’t decide whether some attributes are mandatory or optional.
When I was fixing the errors in netlab SR-OS configuration templates, I couldn’t get the EBGP-based EVPN with overlapping leaf AS numbers to work. I could see the EVPN routes in the SR-OS BGP table, but the device refused to use them. I concluded (incorrectly) that there must be a quirk in the SR-OS EVPN code and moved on.
Public Videos: Whole IPv6 Curriculum
Based on the feedback I received on LinkedIn and in private messages, I made all my IPv6 content public; you can watch those videos without an ipSpace.net account.
Want to spend more time watching free ipSpace.net videos? The complete list is here.
netlab 25.06: Fixing Nokia SR-OS Configuration Templates
TL&DR: netlab release 25.06 was published last week.
Before discussing the new features, let’s walk the elephant out of the room: I changed the release versions to YY.MM scheme, so I will never again have to waste my time on the existential question of which number in the release specification to increase.
Now for the new features:
Finding Source Routing Paths
In the previous blog post, we discussed the generic steps that network devices (or a centralized controller) must take to discover paths across a network. Today, we’ll see how these principles are applied in source routing, one of the three main ways to move packets across a network.
Brief recap: In source routing, the sender has to specify the (loose or strict) path a packet should take across the network. The sender thus needs a mechanism to determine that path, and as always, there are numerous solutions to this challenge. We’ll explore a few of them, using the sample topology shown in the following diagram.
ArubaCX Cannot Count When Dealing with VXLAN
This blog post describes yet another bizarre example of how reliable digital twins are, but don’t worry; they all work great in PowerPoint.
After “fixing” the integration tests to deal with ArubaCX’s notion of VXLAN VNI having 16 bits, the bridging test worked, but the IRB tests kept failing.
In the IRB test, the lab has two layer-3 switches. Each of them should be able to bridge within a VLAN/VXLAN segment and route across the segments.
netlab 2.0: Routers, Hosts, Gateways and Bridges
In a previous blog post, I explained how you can use bridges in a netlab topology to create custom LAN segments. Netlab supports two other node roles (host and router), and we’ll eventually add gateways.
netlab assumes that most network devices are routers (it considers a firewall to be a router in disguise), apart from Linux hosts, but you can always change what a node is with the role node attribute:
Interesting: Juniper MX and Jumbo Frames
Did you know that there’s an Ethernet link between the Packet Forwarding Engine (PFE – data plane) and Routing Engine (RE – control plane) in every Juniper MX? That’s why you have to run two VMs to emulate it (sometimes conveniently packed into one larger VM, proving RFC 1925 rule 6a).
That Ethernet link happens to have the MTU fixed at 1500 bytes. Guess what happens in the world where everyone uses jumbo frames? Did you say fragmentation? Bingo! And what do you think happens when one of those fragments gets dropped due to control-plane policing, and the rest of them are stuck in the reassembly queue? You’ll find the gory details in a lengthy blog post by Nitzan Tzelniker.
Publishing Content as an Introvert
I got an interesting question from a reader. He listened to my podcast with Eric Chou and decided to try to learn in public:
Currently, I’m studying for the CCNP ENARSI exam, and would like to start posting my labs to LinkedIn, and perhaps even upload my lab topologies and configs to Git.
That’s a great idea. I would minimize the LinkedIn part1 and focus on Git:
Finding End-to-End Paths: Topology and Endpoints
We know there are three main ways to move packets across a network. However, before we can start forwarding packets, someone has to populate the forwarding tables in the intermediate devices or build the sequence of nodes to traverse in source routing.
Usually, whoever is responsible for the contents of the forwarding tables must first discover the network topology. Let’s start there, using the following network diagram to illustrate the discussion.
Weird: Ports on Linux Bridge Are Stuck
Just when you thought you got used to the weirdnesses in the networking implementations, you get a curveball like this one. Life is never dull if you test network devices.
Before releasing netlab release 2.0, I ran the full suite of integration tests for all devices for which I have the images. Interestingly, most VXLAN tests failed for Cumulus Linux 4.x even though we haven’t touched that code for ages.
Next step: trying to figure out what changed. The configuration changes were minimal. Even worse, the failure was non-deterministic. Somehow, we managed to transform a Cumulus Linux 4.x VM into a Heisenberg switch.
Where Are the NETCONF/YANG Tools?
Jo attempted to follow the vendor Kool-Aid recommendations and use NETCONF/YANG to configure network devices. Here’s what he found (slightly edited):
IMHO, the whole NETCONF ecosystem primarily suffers from a tooling problem. Or I haven’t found the right tools yet.
ncclient is (as you mentioned somewhere else) an underdocumented mess. And that undocumented part is not even up to date. The commit hash at the bottom of the docs page is from 2020… I am amazed how so many people got it working well enough to depend on it in their applications.
Interesting: Bootstrapping HTTPS
Jan Schaumann published an interesting blog post describing the circuitous journey a browser might take to figure out that it can use QUIC with a web server.
Now, if only there were a record in a distributed database telling the browser what the web server supports. Oh, wait… Not surprisingly, browser vendors don’t trust that data and have implemented a happy eyeballs-like protocol to decide between HTTPS over TCP and QUIC.
Cumulus Linux (As We Know It) Is Gone
A reader of my blog pointed out the following minutiae hidden at the very bottom of the Cumulus Linux 5.13 What’s New document:
NVIDIA no longer releases Cumulus VX as a standalone image. To simulate a Cumulus Linux switch, use NVIDIA AIR.
And what is NVIDIA AIR?