Interaction of DHCP Relay Broadcast Flag with the Layer 2 Unicast Transmission Method

As described in Configuring the Layer 2 Unicast Transmission Method for Reply Packets to DHCP Clients, you can use the set dhcp relay layer2-unicast-replies command to configure DHCP relay and DHCP relay proxy to use the layer 2 unicast and layer3 broadcast transmission method to send DHCP Offer reply packets and DHCP ACK and NAK reply packets to clients.

The set dhcp relay broadcast-flag-replies command and the set dhcp relay layer2-unicast-replies command are mutually exclusive. If you attempt to issue the set dhcp relay broadcast-flag-replies command when the set dhcp relay layer2-unicast-replies command is already in effect, the operation fails and the router displays the following message:

% layer2-unicast-replies and broadcast-flag-replies are mutually exclusive

If this message appears, you must first issue the no set dhcp relay layer2-unicast-replies command to disable layer 2 unicast replies, and then issue the set dhcp relay broadcast-flag-replies command again to enable broadcast flag replies.

Table 4 summarizes how the configuration of the set dhcp relay broadcast-flag-replies command and the set dhcp relay layer2-unicast-replies command interacts with the setting of the broadcast flag in DHCP request packets to control how the router transmits DHCP reply packets to clients during the discovery process. Because these commands are mutually exclusive, broadcast flag replies and layer 2 unicast replies cannot both be enabled on the router at the same time.

Table 4: Router Configuration and Transmission of DHCP Reply Packets

Broadcast Flag Replies

Layer 2 Unicast Replies

Router Behavior if Broadcast Flag Set

Router Behavior if Broadcast Flag Not Set

Enabled (on)

Disabled (off)

DHCP relay and DHCP relay proxy broadcast DHCP reply packets to clients.

DHCP relay and DHCP relay proxy use layer 2 unicast and layer 3 unicast transmission to send DHCP reply packets to clients.

Disabled (off)

Enabled (on)

DHCP relay and DHCP relay proxy use layer 2 unicast and layer 3 broadcast transmission to send DHCP reply packets to clients.

DHCP relay and DHCP relay proxy use layer 2 unicast and layer 3 broadcast transmission to send DHCP reply packets to clients.

Disabled (off)

Disabled (off)

DHCP relay and DHCP relay proxy broadcast DHCP reply packets to clients. For information about exceptions to this behavior for DHCP relay proxy, see the Behavior for Bound Clients and Address Renewals section in Managing Host Routes Using DHCP Relay Proxy.

DHCP relay and DHCP relay proxy broadcast DHCP reply packets to clients. For information about exceptions to this behavior for DHCP relay proxy, see see the Behavior for Bound Clients and Address Renewals section in Managing Host Routes Using DHCP Relay Proxy.

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