I found myself in a situation at work where after a downgrade, my Cisco Nexus 3000 switches were sitting at a loader> prompt with no readable images on the bootflash. Here are some things I noticed while trying to get unstuck.

Network Connectivity for Loader and Kickstart

If you end up in a jam where your switch cannot boot to a system image, you will need one of the following to get full network access:

• a bootable kickstart/system image on the boot flash (assuming it is readable... mine was not)
  • a bootable kickstart and system image on a USB drive which has been connected to the USB port of the switch
  • a TFTP server that can be reached via the Management Ethernet port (MGMT) on the switch if you apply an IP address and, if needed, a default gateway (no dynamic routing).

In this last option, I stress that The MGMT port is the only port you can configure with an IP address in loader or kickstart.

We don’t normally have the MGMT port cabled up, so I needed to reach out to the Data Center engineers to get them move a cable from a normal switch port to the MGMT port. Once I did that, I was able to from the loader to kickstart.

Examples of interaction for loader and kickstart

TBD... I will update this when I get my log files off my work computer for these interactions.

The Case For An Out of Band Management Ethernet

All of this jockeying to try to get network access when the switch is down hard... This makes it very clear that the management interface is special.

There is generally value to being able to reach the switch independent of the routing protocols running on the switch. And if hands-off recovery of a switch that is not able to boot is a requirement, having a management ethernet network built out is a prerequisite.

Alternatively, If you have a solid way of ensuring that a spare switch can be swapped in for a failed one, and that switch will either have or will be configured with a valid configuration, then the case for building out a management ethernet network is less.

Either of these will address the problem for this scenario. You probably don't need both. It's important to remember that having an out of band management ethernet network is a solution to one or more problems. And if you have alternate solutions, it may make sense not to have one.

That being said, a management ethernet network is also useful for stats, logging, and controlling the switch in a manner which is independent of the switch's routing state and access-control-lists applied to the normal switch ports. So there are other scenarios for which being able to swap in doesn't get you the same level of functionality.