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Managing Your DHCP PXE/BOOTP Servers That Do Not Support Option-82

 

Some PXE or BOOTP servers do not support Option-82, that is, their DHCP Offer messages do not include the Option-82 value added by the DHCP Relay. As a result, the DHCP Relay will drop the DHCP Offer and the PXE/BOOTP client will not be able to complete its boot sequence.

Following are the possible solution to resolve this issue:

Solution 1: Upgrade to a PXE Server that supports Option-82

Solution 2: Host the PXE server with a DHCP Server

  • Ensure that the DHCP Server (that supports Option-82) run together with the PXE server.

  • Configure an Option-60 on the DHCP Server.

    • Use the following CLI to configure Option-60 an a Microsoft WS DHCP Server:

      netsh dhcp server dhcp-server-address add optiondef 60 ClientIdentifier STRING 0 PXEClient
    • Activate the option in the user interface of the DHCP server.

This way, the PXE/BOOTP clients will receive proper DHCP Offer with Option-60 “PXEClient” and will reach the PXE server at the same IP address of the DHCP Server.

Solution 3: Include Option-60 and Option-43 DHCP Server Message

If the PXE Server is not hosted together with the DHCP Server, you need the DHCP Server to send an Option-43 also in its DHCP Offer. The Option-43 provides the IP address of the PXE server. Note that, the older PXE or BOOTP clients might ignore Option-43 and will therefore try to get the software from the DHCP Server. Enter the Option-43 in the DHCP Server configuration in a hexadecimal mode.

For is a sample option-43 message:

The above message indicates the following information to the PXE client:

  • Disable broadcast and multicast discovery

  • Accept only the PXE Server provided in this text

  • PXE Server IP is 10.11.12.13 (see the bytes '0A 0B 0C 0D' in the above text)

  • Boot menu on the PXE client (to present to the end user):

    • just one line, “ServerPXE”

    • Autoselect the first Boot option, prompt “S”, no timeout (that is, immediately boot unless you press F8)

DHCP packets on non-configured interfaces are dropped

Once you enable DHCP-Relay on the QFX or EX Switches, the DHCP Snooping feature gets enabled and all DHCP packets incoming through any interface (both configured and unconfigured interface) of the device are analyzed. The interfaces that are not listed under the DHCP configuration are considered ‘unconfigured’.

Depending on the configuration, DHCP packets received on unconfigured interfaces are dropped.

If the DHCP packets are dropped on ‘unconfigured’ interface, you will receive the message like: