Blog Posts in July 2025
Cisco IOS/XE Hates Redistributed Static IPv6 Routes
Writing tests that check the correctness of network device configurations is hard (overview, more details). It’s also an interesting exercise in getting the timing just right:
- Routing protocols are an eventually-consistent distributed system, and things eventually appear in the right place (if you got the configurations right), but you never know when exactly that will happen.
- You can therefore set some reasonable upper bounds on when things should happen, and declare failure if the timeouts are exceeded. Even then, you’ll get false positives (as in: the test is telling you the configurations are incorrect, when it’s just a device having a bad hair day).
And just when you think you nailed it, you encounter a device that blows your assumptions out of the water.
netlab 25.07: Summaries and Confederations
netlab release 25.07 was published yesterday. The major new features include:
- The ospf.areas plugin supports OSPFv2 and OSPFv3 stub areas, NSSA areas, and area ranges.
- The BGP routing policies plugin supports aggregate BGP routes
- The BGP configuration module supports BGP confederations
But wait, there’s much more:
Dual-Stack Common-Services VRF Confuses Aruba CX
As I was running the netlab pre-release integration tests, I noticed that ArubaCX failed the IPv6 Common Services test (it worked before). Here’s the gist of what that test does:
- It creates three VRFs (red, blue, and common)
- It imports routes from red and blue VRF into the common VRF and routes from the common VRF into the red and blue VRF (the schoolbook example of common services VRF)
- Just to be on the safe side, it imports red routes into the red VRF and so on.
Here’s the relevant part of the netlab lab topology:
Worth Reading: The Secret Rules of the Terminal
Did you ever wonder why pressing an up-arrow in a (Linux) terminal window sometimes recalls the previous command but other times creates ^[[A?
Julia Evans did, and spent months exploring the quirks of the Linux terminal (and writing blog posts describing what she found), finally resulting in The Secret Rules of the Terminal (including the various shells, terminal emulators, escape codes, and TTY driver). A must-read if you’re a newbie who wants to understand why things happen the way they do.
Expanding a Running Netlab Topology
One of the happy netlab users sent me an interesting challenge:
- He’s built a large lab and added tons of extra configuration to the lab devices.
- Afterwards, he realized he’d like to add a few more devices to the lab and was worried about losing all the changes he had made.
Unfortunately, you cannot add new devices to an already-running lab. You must shut down the lab, change the topology description, and start a new lab. However, there are things you can do to preserve the extra work you already did:
Worth Reading: Expert Generalists
Martin Fowler published an interesting article about Expert Generalists. Straight from the abstract:
As computer systems get more sophisticated we’ve seen a growing trend to value deep specialists. But we’ve found that our most effective colleagues have a skill in spanning many specialties.
Also:
There are two sides to real expertise. The first is the familiar depth: a detailed command of one domain’s inner workings. The second, crucial in our fast-moving field is the ability to learn quickly, spot the fundamentals that run beneath shifting tools and trends, and apply them wherever we land.
Remember how I told you to focus on the fundamentals? 😎
Molly-Guard: a Lifesaver on a Ubuntu Server
Have you ever managed to type reload in the wrong terminal window and brought down a core switch (I probably did)? I managed to do the Ubuntu equivalent of that stupidity: I told my main Ubuntu server to sudo poweroff instead of doing that to a Vagrant VM.
Fortunately, the open-source world doesn’t have to rely on the roadmaps created by networking vendors’ product managers; if there’s a big enough pain, someone will solve it.
IS-IS 3-Way Handshake and the Power of SHOULD
Yesterday, I mentioned that a Cisco router running pre-standard IS-IS 3-way handshake (this is why you need it) interoperates with multiple implementations of RFC 5303. How’s that possible, and does it matter whether you configure the ancient Cisco routers (release 15.x) to use IETF 3-way handshake instead of the “proprietary” one?
I took a trip to the Wireshark land to figure out the details (you can download the capture file):