Another area where CLNP is a clear winner when compared to the TCP/IP stack is multi-homed nodes (nodes with multiple interfaces, not site multi-homing, where whole networks are connected to two upstream providers).

Multi-homed TCP/IP nodes must have multiple IP addresses because IP uses address interfaces. There is no well-defined procedure in TCP/IP for how a multi-homed node should behave. In the early days of TCP/IP, they tried to address that in RFC 1122 (Host Requirements RFC), but even then, there were two ideas about dealing with multiple interfaces: the strong and weak end system models (more details).

Today is a perfect day to point out the fantastic Why Quantum Cryptanalysis is Bollocks presentation (HT: A blog post by George Michaelson)

It’s a must-read, even if you’re absolutely uninterested in the topic. Just replace “Quantum mumbo-jumbo” with AI or SDN ;) Have fun!

The resurrected Small Site Multihoming article used Cisco IOS configuration commands from 2007 (when the original article was published), including point-to-point serial interfaces and static routes pointing to interfaces.

I recreated the network topology in netlab, created a new set of router configurations, and updated the article’s configuration and monitoring parts. The lab topology and router configurations for each article section are available on GitHub.

The Dynamic BGP Peers lab exercise gave you the opportunity to build a large-scale environment in which routers having an approved source IP addresses (usually matching an ACL/prefix list) can connect to a BGP route reflector or route server.

In a more controlled environment, you’d want to define BGP neighbors on the BGP RR/RS but not waste CPU cycles trying to establish BGP sessions with unreachable neighbors. Welcome to the world of passive BGP sessions.

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 session/8-passive and execute netlab up.

Bob left a lengthy comment arguing with the (somewhat black-and-white) claims I made in the Rise of NAT podcast. Let’s start with the any-to-any connectivity:

From my young millennial point of view, the logic is reversed: it is because of NATs and firewalls that the internet became so asymmetrical (client/server) just like the Minitel was designed (yes, I am French), whereas the Internet (and later the web, although a client/server protocol, was meant for everyone to be a client and a server) was designed to be more balanced.

Let’s start with the early Internet. It had no peer-to-peer applications. It connected a few large computers (mainframes) that could act as servers but also allowed terminal-based user access and thus ran per-user clients.

One of the most significant problems engineers face when trying to improve their online presence is the “How do I start writing?” roadblock (hint: publishing bland AI-generated slop won’t get you far unless you aim to become a Thought Leader).

Zvi Mowshowitz collected links to over a dozen different writing styles, starting with JRR Tolkien. I’m pretty sure you’ll find something useful in that vast collection.

Last week, I had to push out netlab release 1.9.6 to address a particularly nasty Python dependency hell to make netlab work (again) on Ubuntu 24.04 (more details). The release also brought these goodies (and a bunch of bug fixes):

• Add default gateway (including anycast- and VRRP gateway )as a valid next-hop for static routes
• Rewrite the default gateway processing and add IPv6 default gateways on links without anycast or VRRP gateways
• Set libvirt MTU to 9500 on bridge-based networks to avoid the “transparent fragmentation” on Linux bridges.
• Use device- or node variables to specify the Juniper vMX license file.

This blog post describes another “OMG, this cannot possibly be true” scenario discovered during the netlab VRRP integration testing.

I wanted to test whether we got the nasty nuances of VRRPv3 IPv6 configuration right on all supported platforms and created a simple lab topology in which the device-under-test and an Arista cEOS container would be connected to two IPv6 networks (Arista EOS is a lovely device to use when testing a VRRP cluster because it produces JSON-formatted show vrrp printouts).

Most platforms worked as expected, but Aruba CX, Cumulus Linux with NVUE, and Dell OS10 consistently failed the tests. We were stumped until Jeroen van Bemmel discovered that the Arista container forwards IPv6 router advertisements between the two LAN segments.

In 2007, I wrote a series of articles describing an implementation of small-site (BGP-less) multihoming in the IPv4 world. It seems that this topic is still interesting, as I recently received requests to republish them, and it may (sadly enough) apply equally well to the IPv6 world.

This is the first article in the series. It describes a design with a single router using two uplinks to two upstream ISPs.

In the Router Interfaces and Switch Ports blog post, I described why we have switch ports and routed interfaces on layer-3 switches. Another blog post in the same series described the conceptual architecture of a layer-3 switch:

• All interfaces are connected to a VLAN-aware switch
• The switch interfaces could be access or trunk interfaces¹.
• Each VLAN in a VLAN-aware switch can be connected to an internal router through a VLAN interface.

However, that’s not how we configure layer-3 switches. There’s a significant gap between the conceptual configuration model and the internal architecture:

In the previous blog posts, we discussed how TCP/IP and CLNP reach adjacent nodes and build ARP/ND/ES caches and how they reach off-subnet nodes. Now, let’s move from the network edge into the network core and explore how the two protocol stacks reduce the amount of information they have to propagate in routing protocols.

While I’m not exactly an OSI fan, I must admit they got many things right (and IPv6 copied those ideas), but TCP/IP is a clear winner in this aspect.

I encountered the Escape sequences (named after the first character in the sequence) while programming stuff that would look nicely on the venerable VT100 terminals (not to mention writing one or two VT100 emulators myself).

In the meantime, those sequences got standardized and (par for the course) extended with “proprietary” stuff everyone uses now. Julia Evans did a great job documenting the state of the art. Thanks a million!

This blog post describes yet another bizarre behavior discovered during the netlab integration testing.

It started innocently enough: I was working on the VRRP integration test and wanted to use Arista EOS as the second (probe) device in the VRRP cluster because it produces nice JSON-formatted results that are easy to use in validation tests.

Everything looked great until I ran the test on all platforms on which netlab configures VRRP, and all of them passed apart from Arista EOS (that was before we figured out how Sturgeon’s Law applies to VRRPv3) – a “That’s funny” moment that was directly responsible for me wasting a few hours chasing white rabbits down this trail.

The Queuing Theory webinar by Rachel Traylor is now available without a valid ipSpace.net account. Enjoy!

Like OSPF, IS-IS needs a router to originate the pseudo-node for a LAN segment. IS-IS standards call that router a Designated Intermediate System (DIS), and since it is not responsible for flooding, it does not need a backup.

Want to know more? The Influence the Designated IS Election lab exercise provides the details (and some hands-on work).