What’s POAP, seamless switch config! 2025

Back then, configuring a couple of switches was just fine for network engineers. They had to connect console cables for individual switches, configure each switch, load OS images, and provide configuration files. This approach is feasible for a couple of switches, but what if you, as a network engineer, must design a data centre from scratch?

If you are working with Cisco Network devices, then there’s a sigh of relief for you. Cisco has a proprietary technology called POAP, which is short for Power-On Auto Provisioning. This technology is used for automating the deployment of configuration files and Operating Systems for Cisco Nexus series Switches deployed in the network for the first time.

Understanding POAP

When any of the Cisco Nexus series switches boots up for the first time in a network and doesn’t find the configuration files or the operating system files the switch enters the POAP mode, but before we get into technicalities we need to understand the network requirements for PAOP

Network Requirements for POAP

• A DHCP server that assigns IP address, and provides Default Gateway, DNS server and log server.
• A TFTP or an HTTPS server that provides scripts to automate software / OS image installation and setting up configuration
• A server that can accommodate the desired OS image files and configurations

What happens in POAP mode?

So when a switch is turned on for the first time in a network, several processes are running and several decision points occur, out of which one of them is asking the user to enter into POAP mode or continue interactive setup, when we go with the POAP mode we get an option to abort the POAP until the POAP operation is complete.

When the POAP mode starts the entire setup undergoes 5 phases :

• Power up
• USB discovery
• DHCP discovery
• Script execution
• Post-installation reload

Phase-1: Power Up

When you power up a switch, the switch first loads on a pre-installed software image during the manufacturing process, when no matching configuration file is found the switch asks the user to either into POAP mode or get into interactive setup mode.

Note: When entered in POAP mode the interfaces on the switch are set to “layer-2 switching” meaning the data frames are handled in the data-link layer with the help of the MAC address, but at the same time this mode explicitly blocks switching during the time period avoiding other network devices interacting or causing looping.

Phase-2: USB Discovery

When entering the USB discovery phase the Nexus Switch searches the root directories for any of the USB devices for the POAP configuration file in the file formats: Python script file, poap_script.py, or the Tcl script file, poap_script.tcl etc. If found the Switch then runs these configuration files and setup the software or the OS.

If not found the switch goes for DHCP discovery, the switch continuously searches for the configuration files until found or the switch is rebooted or the mode is aborted. The switch can also shift to DHCP discovery because of errors.

Phase-3: DHCP Discovery

The DHCP client in this case the Switch sends out a broadcast DHCP discovery message on all interfaces soliciting all the DHCP servers on the network, the DHCP client identifies itself to the server based on the MAC address. Once identified the DHCP server extends the DHCP offer (details about IP address, subnet, lease period), the client chooses the offer out of the available DHCP offers from various servers.

POAP requires a minimum of 3600 seconds of the lease period (1 hour) if the lease period is less than 3600 seconds the DHCP negotiation is ended by the client. It is also mandated by the DHCP discovery message that certain options must be solicited from the DHCP server, like:

• The DHCP server needs to relay the TFTP server name or its IP address to the DHCP client.
• IP address
• Default gateway
• The DHCP server needs to relay the boot file name and complete path of the TFTP server to the DHCP client.

Once a DHCP offer is randomly chosen, the DHCP server assigns an IP address to the switch, if any error occurs in the subsequent steps the IP address is released back to the DHCP server.

Phase-4 Script Execution

Once the required information is obtained the script files are downloaded from the TFTP or the HTTP server to automate the configuration, then the software image and the configuration files are downloaded from the servers. The configuration files are downloaded based on the Switch model and version specified in the script files.

However, the configuration file is not imposed on the switch during the software installation as the currently installed software may not support the configuration files and end up throwing errors. Once the installation is done the Switch reboots and then the configuration file is applied to the switch.

Phase-5 Post Installation Reload

Once the installation is complete the switch reloads and replays the configuration files, once the necessary checks are done the running configuration file is copied to the startup configuration files.

Conclusion

Hence we can now understand how POAP completely reduces human intervention and automates the configuration of switches at large data centers when switches are deployed for the first time. The POAP is a Cisco proprietary software and hence doesn’t apply to other vendors. PXE (Pre Boot Execution Environment) is a vendor-neutral alternative for you.

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