Cisco NX-OS requirements for cross subnet NetBooting.

[glutz]

This was not easy to come by directly asking Cisco but below are the details about what is needed to do DHCP-Relay or as called in the past helper address.

When you enable option 82 on the NX-OS device, the following sequence of events occurs:
1. Description: http://www.cisco.com/en/US/i/templates/blank.gifThe host (DHCP client) generates a DHCP request and broadcasts it on the network.
2. Description: http://www.cisco.com/en/US/i/templates/blank.gifWhen the NX-OS device receives the DHCP request, it adds the option-82 information in the packet. The option-82 information contains the device MAC address (the remote ID suboption) and the port identifier, vlan-mod-port, from which the packet is received (the circuit ID suboption).
3. Description: http://www.cisco.com/en/US/i/templates/blank.gifThe device adds the IP address of the relay agent to the DHCP packet.
4. Description: http://www.cisco.com/en/US/i/templates/blank.gifThe device forwards the DHCP request that includes the option-82 field to the DHCP server.
5. Description: http://www.cisco.com/en/US/i/templates/blank.gifThe DHCP server receives the packet. If the server is option-82 capable, it can use the remote ID, the circuit ID, or both to assign IP addresses and implement policies, such as restricting the number of IP addresses that can be assigned to a single remote ID or circuit ID. The DHCP server echoes the option-82 field in the DHCP reply.
6. Description: http://www.cisco.com/en/US/i/templates/blank.gifThe DHCP server unicasts the reply to the NX-OS device if the request was relayed to the server by the device. The NX-OS device verifies that it originally inserted the option-82 data by inspecting the remote ID and possibly the circuit ID fields. The NX-OS device removes the option-82 field and forwards the packet to the interface that connects to the DHCP client that sent the DHCP request.
If the previously described sequence of events occurs, the following values (see Figure 15-2) do not change:
•Description: http://www.cisco.com/en/US/i/templates/blank.gifCircuit ID suboption fields
–Description: http://www.cisco.com/en/US/i/templates/blank.gifSuboption type
–Description: http://www.cisco.com/en/US/i/templates/blank.gifLength of the suboption type
–Description: http://www.cisco.com/en/US/i/templates/blank.gifCircuit ID type
–Description: http://www.cisco.com/en/US/i/templates/blank.gifLength of the circuit ID type
•Description: http://www.cisco.com/en/US/i/templates/blank.gifRemote ID suboption fields
–Description: http://www.cisco.com/en/US/i/templates/blank.gifSuboption type
–Description: http://www.cisco.com/en/US/i/templates/blank.gifLength of the suboption type
–Description: http://www.cisco.com/en/US/i/templates/blank.gifRemote ID type
–Description: http://www.cisco.com/en/US/i/templates/blank.gifLength of the circuit ID type
Figure 15-2 shows the packet formats for the remote ID suboption and the circuit ID suboption. The NX-OS device uses the packet formats when you globally enable DHCP snooping and when you enable option-82 data insertion and removal. For the circuit ID suboption, the module field is the slot number of the module.

[colonelpanic]

Thank you for posting this!