DHCP Discovery

After entering ZTP mode, the switch sends a DHCP discovery message on its management interface, requesting DHCP offers from the DHCP servers present on the network. The DHCP server replies with a DHCP offer message. When the switch receives the DHCP offer message, it will look for following information in the offer:

  • An interface IPv4 address

  • A gateway IPv4 address

  • A TFTP or HTTP server IP address (using option 66)

  • Boot file name (using option 67)

The switch completes the DHCP negotiation process (request and acknowledgement) with the DHCP server, which assigns the switch with an IPv4 management address. The switch then uses the acquired TFTP or HTTP server IP address to contact that server to get the boot file, The option 67 contains the complete file path of the boot file on the remote server. The switch then downloads the boot file.

If no DHCP servers reply is received after DHCP discovery message or if the DHCP offer does not meets the ZTP requirements, the switch won't be able to complete the DHCP negotiation and the switch exits ZTP mode and continues the normal boot process.

  • The interface IPv4 address obtained from the DHCP server is kept and used as management address even after the ZTP process completes

  • DHCP servers must be configured with options 66 and 67 to ensure that the switch always obtains the TFTP server hostname and the boot file name during the ZTP process. DHCP options 66 and 67 are enabled by default on the OPBNOS. If either of them is disabled, the ZTP process results in a failure.