Cisco CML on Proxmox
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While Cisco CML isn’t officially supported to run on Proxmox it is possible to make it work.
I ran into an issue running Cisco CML 2.4.1 on Proxmox 7.2-11 that the virtual device console wasn’t showing any output, and this happened using both the web console provided via CML and while telnetting in using the breakout tool.
After re-installing, searching for answers on the Cisco user community forum and just being annoyed I decided to poke around the CML VM to figure out if the virtual devices actually worked at all. So after using the admin interface provided with CML I was able to look around and inside /var/log I found the following error in the Qemu log files:
KVM: entry failed, hardware error 0x7
The Internet is pointing towards this being an issue with nested virtualization, which I guess makes sense. In this thread on the Proxmox forum there are mentions of using an older kernel and as a luck would have it it was available on my system. After rebooting into kernel version 5.11.22-7-pve the virtual devices in CML would boot up with no issues. Anything newer than this version (5.13.x and 5.15.x) would not work.
I wasn’t able to force Proxmox to always use this kernel after a reboot so my solution is to simply press the shift key on boot, select the “Advanced options” item in grub and pick the kernel version that way.