Gary Marcus wrote an interesting essay describing the failure of self-driving cars to face the unknown unknowns. The following gem from his conclusions applies to AI in general:

In a different world, less driven by money, and more by a desire to build AI that we could trust, we might pause and ask a very specific question: have we discovered the right technology to address edge cases that pervade our messy really world? And if we haven’t, shouldn’t we stop hammering a square peg into a round hole, and shift our focus towards developing new methodologies for coping with the endless array of edge cases?

Obviously that’s not going to happen, we’ll keep throwing more GPU power at the problem trying to solve it by brute force.

In the next BGP labs exercise, you’ll build the customer part of an MPLS/VPN solution. You’ll use bidirectional OSPF-to-BGP route redistribution to connect two sites running OSPF over a Service Provider MPLS backbone.

Chinar Trivedi wanted to know what happens when you insert a firewall in the DHCP data path (original question.

TL&DR: Nothing much, but that does not mean you should.

Now for the details:

The OSPF and ARP on Unnumbered IPv4 Interfaces triggered an interesting consideration: does ECMP work across parallel unnumbered links?

TL&DR: Yes, it works flawlessly on Arista EOS and Cisco IOS/XE. Feel free to test it out on any other device on which netlab supports unnumbered interfaces with OSPF.

One of my readers wanted to use EIBGP to load balance outgoing traffic from a pair of WAN edge routers (hint: wrong tool for this particular job¹). He’s using a design very similar to this one with VRRP running between WAN edge routers, and the adjacent firewall cluster using a default route to the VRRP IP address.

The problem: all output traffic goes to the VRRP IP address which is active on one of the switches, and only a single uplink is used for the outgoing traffic.

Talking about BGP routing policy mechanisms is nice, but it’s even better to see how real Internet Service Providers use those tools to implement real-life BGP routing policy.

Getting that information is incredibly hard as everyone considers their setup a secret sauce. Fortunately, there are a few exceptions; Pim van Pelt described the BGP Routing Policy of IPng Networks in great details. The article is even more interesting as he’s using Bird2 configuration language that looks almost like a programming language (as compared to the ancient route-maps used by vendors focused on “industry-standard” CLI).

Have fun!

I got a question from a few of my students regarding the best way to implement end-to-end EVPN across multiple locations. Obviously there’s the multi-pod and multi-site architecture for people believing in the magic powers of stretching VLANs across the globe, but I was looking for something that I could recommend to people who understand that you have to have a L3 boundary if you want to have multiple independent failure domains (or availability zones).

When preparing the materials for the Design Clinic section describing Zero-Trust Network Architecture, I wondered whether I was missing something crucial. After all, I couldn’t find anything new when reading the NIST documents – we’ve seen all they’re describing 30 years ago (remember Kerberos?).

In late August I dropped by the fantastic Roundtable and Barbecue event organized by Gabi Gerber (running Security Interest Group Switzerland) and used the opportunity to join the Zero Trust Architecture roundtable. Most other participants were seasoned IT security professionals with a level of skepticism approaching mine. When I mentioned I failed to see anything new in the now-overhyped topic, they quickly expressed similar doubts.

The first set of BGP labs covered the basics; the next four will help you master simple routing policy tools (BGP weights, AS-path filters, prefix filters) using real-life examples:

• Use BGP weights to prefer one of the upstream providers
• Prevent route leaking between upstream providers with an AS-path filter
• Filter prefixes advertised by your autonomous system with a prefix list
• Minimize the size of your BGP table with inbound filters

The labs are best used with netlab (it supports BGP on almost 20 different devices), but you could use any system you like (including GNS3 and CML/VIRL). For more details, read the Installation and Setup documentation.

More than thirteen years after I started creating vendor-neutral webinars, it’s time for another change¹: the ipSpace.net subscriptions became perpetual. If you have an active ipSpace.net subscription, it will stay valid indefinitely² (and I’ll stop nagging you with renewal notices).

Wow, Free Lunch?

Sadly, that’s not the case.

After figuring out ARP details, describing how routers use ARP to resolve entries in the IP routing table, and considering what we already know about OSPF on unnumbered IPv4 interfaces, we’re finally ready to answer Daniel’s question:

A few days ago, I described how ARP behaves when the source- and destination IP addresses are not on the same subnet (TL&DR: it doesn’t care). Now, let’s see how routers use ARP to get the destination MAC address for various entries in the IP routing table. To keep things simple, we’ll use static routes to insert entries in the IP routing table.

We’ll run our tests in a small virtual lab with two Linux hosts and an Arista vEOS switch. The link between H1 and RTR is a regular subnet. H2 has an IP address on the Ethernet interface, but RTR uses an unnumbered interface.

Bruce Davie published an interesting overview article about Large Language Models. It would be worth reading just for the copious links to in-depth article; I particularly like his conclusions:

We mistake performance (producing realistic text) for competence (understanding the world).

Having a model for language is different from having a model of the world.

And that’s a perfect explanation why it makes no sense to expect ChatGPT and friends to produce picture-perfect device configurations or always-working code.

When figuring out how unnumbered IPv4 interfaces work, Daniel Dib asked an interesting question: How does ARP work when the source and destination IPv4 address are not in the same segment (as is usually the case when using unnumbered interfaces)?

TL&DR: ARP doesn’t care about subnets. If the TCP/IP stack needs to find a MAC address of a node it thinks is adjacent, ARP does its best, no matter what.

The first BGP labs are online. They cover the basic stuff (one has to start with the basics, right?):

• Configuring an EBGP session
• Connecting to multiple upstream ISPs
• Advertise your prefixes
• Configure BGP for IPv6

The labs are supposed to be run on virtual devices, but if you’re stubborn enough it’s possible to make them work with the physical gear. In theory, you could use any system you like to set up the virtual lab (including GNS3 and CML/VIRL), but your life will be way easier if you use netlab – it supports BGP on almost 20 different devices. For more details, read the Installation and Setup documentation.