Starting with  releases 13.2X51-D27, 13.2R7, 12.3R9, 14.1X53-D15, 14.1R4, and 14.2R2, a new configuration statement set forwarding-options dhcp-relay overrides delete-binding-on-renegotiation is required when configuring DHCP relay for environments using multiple DHCP servers. 

Starting with releases 13.2X51-D27, 13.2R7, 12.3R9, 14.1X53-D15, 14.1R4, and 14.2R2, a new configuration statement set forwarding-options dhcp-relay overrides delete-binding-on-renegotiation" is required when configuring DHCP relay for environments using multiple DHCP servers. PXE boot environments are one such example. Not configuring set forwarding-options dhcp-relay overrides delete-binding-on-renegotiation will lead to a second DHCP offer from the secondary server (e.g., PXE boot server) being dropped when the first establishes a DHCP relay binding.

This configuration statement was introduced as part of PR1031605.

When the set forwarding-options dhcp-relay overrides delete-binding-on-renegotiation statement is not configured, DHCP offers from servers other than the server that first establishes the DHCP relay binding will be dropped. This will lead to a continuous loop of DHCP Offer/Request packets.

The solution is to configure set forwarding-options dhcp-relay overrides delete-binding-on-renegotiation. When this statement is configured, the switch will not drop secondary DHCP offers. For reference, the following is a sample PXE boot DHCP exchange sequence:

1 12:05:59 0.0.0.0 255.255.255.255 DHCP 590 DHCP Discover - Transaction ID 0xe3180bea
2 12:05:59 10.250.250.138 255.255.255.255 DHCP 397 DHCP Offer - Transaction ID 0xe3180bea
3 12:05:59 10.250.250.20 255.255.255.255 DHCP 1066 DHCP Offer - Transaction ID 0xe3180bea
4 12:06:03 0.0.0.0 255.255.255.255 DHCP 590 DHCP Request - Transaction ID 0xe3180bea
5 12:06:03 10.250.250.138 255.255.255.255 DHCP 397 DHCP ACK - Transaction ID 0xe3180bea
6 12:06:03 10.14.6.198 10.250.250.20 DHCP 590 DHCP Request - Transaction ID 0xe3180bea
7 12:06:03 10.250.250.20 10.14.6.198 DHCP 384 DHCP ACK - Transaction ID 0xe3180bea
8 12:06:03 10.14.6.198 10.250.250.20 DHCP 345 DHCP Request - Transaction ID 0xe3180bea
9 12:06:03 10.250.250.20 10.14.6.198 DHCP 490 DHCP ACK - Transaction ID 0xe3180bea


In above packet capture, 10.250.250.138 is the DHCP server, 10.250.250.20 is the PXE boot server, and 10.14.6.198 is the IP address that the client receives during this process. Please note that this capture was taken without DHCP relay in place to better illustrate the exchange process. If DHCP relay was configured, certain source/destination IPs would be replaced by the IP address that is relaying the DHCP messages.