Studying EVPN to Prepare for a Job Interview
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!
I was fortunate enough never having to apply for a job (I was either a co-owner of a company or working on my own), and the last time I was involved in a hiring decision was over 20 years ago, so I’m not an accurate source of interview questions ideas. However, what I did in those days was try to figure out whether:
- People knew their stuff at the approximate level of what they claimed their knowledge to be;
- They understood the fundamentals;
- They could get a (relatively) simple job done given all the documentation they need (we gave them a few hours for that part).
For that last bit, one could build a lab with preconfigured IP routing, and ask the candidate claiming to be fluent with boxes from vendor X to build EVPN/VXLAN fabric on top of that lab1 given the vendor product documentation2. netlab would be an easy way to set up that lab.
I would always combine such a lab exercise with a followup Q-and-A session similar to what the ancient CCIE lab exams had3. I would ask the candidate why he used particular configuration commands, what those commands did, and what the show printouts mean. Explaining EVPN route types could be an excellent starting point.
Back to fundamentals – here are just a few ideas for the questions you might want to consider:
- Why do we need VXLAN transport? What problem is it solving?
- Why do we need EVPN? What problem is it solving?
- Could we use VXLAN without EVPN? What would be the drawbacks?
- Why do we need proxy ARP?
- How does EVPN propagate ARP requests?
- Can you use IP multicast with EVPN and why would you want to?
- What is the role of anycast gateway?
Want a tougher challenge?
- Why do some vendors advertise host IP addresses as RT5 prefixes?
- How does EVPN solve VM mobility challenges?
- Are different EVPN vendor implementations interoperable?
- What are good interoperability practices for EVPN?
- What are the differences between MLAG and EVPN dual-homing?
If you happen to have too much time, I have another fundamentals idea: we created tons of review questions for the How Networks Really Work webinar. You might want to go through them although they don’t ever get close to VXLAN/EVPN.
I vaguely remember someone telling me that approach might not be legal in some parts of the world. Comments from someone more familiar with such restrictions (should they happen to be real) would be appreciated! ↩︎
In the world of ChatGPT you might want to limit the resources they can access… or you could let them generate the configuration with a large language model to test their troubleshooting skills 😁 ↩︎
That’s how I lost one point on the CCIE lab exam: I got stuck on OSPF type-2 LSA. ↩︎
Why do we need Proxy ARP? Is it used for the ARP surpression? I already built a few VXLAN Fabrics, always with Arista. But I certainly lack a few basics, maybe because I just learned while building the first fabric and sometimes trial and error was faster than searching for the right explanation/documentation. But I can answer almost all questions. I only don't know the two interoperability questions, but I also never build a VXLAN fabric with two vendors. So, is it possible now to build one large fabric consisting of multiple vendors? Would somebody really do that? Our customers want reliability and support in times of need, most of them just grasp the basics of those fabrics. Not one of them wants to build a fabric by themself or deal with multiple vendors.
> Why do we need Proxy ARP? Is it used for the ARP suppression?
> So, is it possible now to build one large fabric consisting of multiple vendors?
As long as it's a single fabric and you don't do VXLAN-to-VXLAN bridging, the answer is a definitive YES.
> Would somebody really do that?
In principle, NO. In practice, life might intervene. It's a great question though worthy of another blog post.