An ipSpace.net subscriber sent me this question:

I am on job hunting. I have secured an interview and they will probably ask me about VxLAN BGP EVPN fabrics. If you have some time, it would be a great help for me if you could tell me 1 or 2 questions that you would ask in such interviews.

TL&DR: He got the job. Congratulations!

The last time I spent days poring over vendor datasheets collecting information for the overview part of Data Center Fabrics webinar a lot of 1RU data center leaf switches came in two form factors:

• 48 low-speed server-facing ports and 4 high-speed uplinks
• 32 high-speed ports that you could break out into four times as many low-speed ports (but not all of them)

I expected the ratios to stay the same when the industry moved from 10/40 GE to 25/100 GE switches. I was wrong – most 1RU leaf data center switches based on recent Broadcom silicon (Trident-3 or Trident-4) have between eight and twelve uplinks.

After I got the testing infrastructure in place (simple DHCP relay, VRF-aware DHCP relay), I was ready for the real fun: DHCP relaying in VXLAN (and later EVPN) segments.

TL&DR: It works exactly as expected. Even though I had anycast gateway configured on the VLAN, the Arista vEOS switches used their unicast IP addresses in the DHCP relaying process. The DHCP server had absolutely no problem dealing with multiple copies of the same DHCP broadcast relayed by different switches attached to the same VLAN. One could only wish things were always as easy in the networking land.

I have blog post ideas sitting in my to-write queue for over a decade. One of them is why would you need a VRF (and associated router) between virtual servers and a firewall?

Andrea Dainese answered at least part of that question in his Off-Path firewall with Traffic Engineering blog post. Enjoy!

Another interesting take on ChatGPT in networking, this time by Tom Hollingsworth in The Dangers of Knowing Everything:

In a way, ChatGPT is like a salesperson. No matter what you ask it the answer is always yes, even if it has to make something up to answer the question.

To paraphrase an old joke: It’s not that ChatGPT is lying. It’s just that what it knows isn’t necessarily true. See also: the difference between bullshit and lies.

Did you know most chassis switches look like leaf-and-spine fabrics¹ from the inside? If you didn’t, you might want to watch the short Chassis Architectures video by Pete Lumbis (author of ASICs for Networking Engineers part of the Data Center Fabric Architectures webinar).

TL&DR: No. You can move on.

NANOG87 summary by John Kristoff prompted me to look at NANOG87 presentations, and one of them discussed ChatGPT and Network Engineering (video). I couldn’t resist the clickbait ;)

Like most using ChatGPT for something articles we’re seeing these days, the presentation is a bit too positive for my taste. After all, it’s all fine and dandy to claim ChatGPT generates working router configurations and related Jinja2 templates if you know what the correct configurations should look like and can confidently say “and this is where it made a mistake” afterwards.

Over the years I wrote a dozen blog posts describing various aspects of using CI/CD in network automation. These blog posts are now collected in the new CI/CD in Networking page that also includes links to related podcasts, webinars, and sample network automation solutions.

A networking engineer attending the Building Next-Generation Data Center online course asked this question:

What is the best practice to connect DC fabric to outside world assuming there are 2 spine switches in the fabric and EVPN VXLAN is used as overlay? Is it a good idea to introduce edge (border) switches, or it is better to connect outside world directly to the spine?

As always, the answer is “it depends,” this time based on:

After figuring out how DHCP relaying works and testing it in a simple lab, I went a step further and tested VRF-aware DHCP relaying.

Lab Topology

I had to make just a few changes to the DHCP relaying lab topology:

• DHCP server is running on CSR 1000v. IOSv DHCP server does not support subnet selection DHCP option and thus doesn’t work with relays that do inter-VRF DHCP relaying.
• I put the link between the DHCP client and DHCP relay into a VRF.

Just in case you wondered why we have eight bits per byte: after Julia Evans investigated this mystery, Steven Bellovin published an excellent overview of the early years of bytes and words.

Vadim Semenov created an interesting solution out of open-source tools (and some glue): a system that tracks, logs, and displays OSPF changes in your network.

It might not be exactly what you’re looking for (and purists would argue it should use BGP-LS), but that’s the beauty of open-source solutions: go and adapt it to your needs, generalizes your fixes, and submit a pull request.

After implementing MLAG functionality with EVPN and having a VXLAN-like fabric transport path between MLAG members, it becomes possible to get rid of the MLAG peer link.

Not surprisingly, most implementations of virtual MLAG peer link remain proprietary. Lukas Krattiger described the details of Cisco’s vPC Fabric Peering implementation in the EVPN Deep Dive webinar.

A few weeks ago I described why EBGP TCP packets have TTL set to one (unless you configured EBGP multihop). Although some people claim that (like NAT) it could be a security feature, it’s not a good one. Generalized TTL Security Mechanism (GTSM, described in RFC 5082) is much better.

Most BGP implementations set TTL field in outgoing EBGP packets to one. That prevents a remote intruder that manages to hijack a host route to an adjacent EBGP peer from forming a BGP session as the TCP replies get lost the moment they hit the first router in the path.

Even though IPv6 could buy its own beer (in US, let alone rest of the world), networking engineers still struggle with its deployment – one of the first questions I got in the ipSpace.net Design Clinic was:

We have been tasked to start IPv6 planning. Can we discuss (for enterprises like us who all of the sudden want IPv6) which design paths to take?

I did my best to answer this question and describe the basics of creating an IPv6 addressing plan. For even more details, watch the IPv6 webinars (most of them at least a few years old, but nothing changed in the IPv6 world in the meantime apart from the SRv6 madness).