DHCP Relaying Details
Chinar Trivedi asked an interesting question about DHCP relaying in VXLAN/EVPN world on Twitter and my first thought was “that shouldn’t be hard” but when I read the first answer that turned into “wait a minute, how exactly does DHCP relaying works?”
I’m positive there’s a tutorial out there somewhere, but I decided to go back to the sources of wisdom: the RFCs. It turned out to be a long walk down the IETF history lane.
Test DHCP Relaying with netlab
After figuring out how DHCP relaying works, I decided to test it out in a lab. netlab has no DHCP configuration module (at the moment); the easiest way forward seemed to be custom configuration templates combined with a few extra attributes.
This is how I set up the lab:
Test VRF-Aware DHCP Relaying with netlab
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.
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.