Using the Broadcast Flag Setting to Control Transmission of DHCP Reply Packets

Each DHCP request packet includes a broadcast flag that, if set, specifies how to transmit DHCP Offer reply packets and DHCP ACK and NAK reply packets to DHCP clients during the discovery process. To configure DHCP relay and DHCP relay proxy to use the setting of the broadcast flag to control the transmission of DHCP Offer, DHCP ACK, and DHCP NAK reply packets, use the set dhcp relay broadcast-flag-replies command from Global Configuration mode.

When you issue the set dhcp relay broadcast-flag-replies command, the method that DHCP relay and DHCP relay proxy use to transmit DHCP Offer reply packets and ACK and NAK reply packets depends on whether the broadcast flag in the DHCP request packet is set or not set, as follows:

• If the broadcast flag is set in the DHCP request packet, using the set dhcp relay broadcast-flag-replies command causes DHCP relay and DHCP relay proxy to broadcast DHCP reply packets to clients.
• If the broadcast flag is not set in the DHCP request packet, using the set dhcp relay broadcast-flag-replies command causes DHCP relay and DHCP relay proxy to use the layer 2 unicast transmission method to send DHCP reply packets using the client’s layer 2 (MAC) address and layer 3 (IP) unicast address.

There are exceptions to this behavior for DHCP relay proxy when the DHCP client is already bound to an IP address or is renewing the lease on its IP address. For information, see the Behavior for Bound Clients and Address Renewals section in Managing Host Routes Using DHCP Relay Proxy.

To display whether support for broadcast flag replies is currently on or off on the router, use the show dhcp relay command. For information, see Monitoring and Troubleshooting DHCP.

To troubleshoot applications that use this feature, you can use the dhcpCapture system event log category. For information about how to log system events, see JunosE System Event Logging Reference Guide.