13.2.8 When a DHCP relay is connected under the Switch

When you connect a DHCP relay under the Switch, configure the switch so that it can forward the packets.

The following figure shows an example configuration when a DHCP relay is connected under the Switch.

Figure 13-12: Configuration when DHCP relays are connected under the Switch

DHCP snooping settings for the Switch are the same as those in 13.2.2 Basic Settings, 13.2.4 Terminal Filtering, and 13.2.5 Dynamic ARP Checking.

In the example here, the DHCP packets and IPv4 packets from the DHCP client cannot be relayed. In addition, the ARP packets from the Layer 3 switch/router cannot be relayed.

To relay the packets, you need to permit the forwarding of DHCP packets, IPv4 packets, and ARP packets on the Switch.

<Structure of this section>

(1) Setting to allow forwarding of DHCP packets

Points to note: Because the source MAC addresses in the packets sent from the DHCP client are rewritten by the Layer 3 switch/router (DHCP relay), disables the MAC address spoofing check for DHCP packets.

Command examples

(config)# no ip dhcp snooping verify mac-address

Disable the MAC address spoofing check for the DHCP packets received on the untrusted port.

Notes: If this command is not specified, the Switch performs the MAC address spoofing check, in which case the DHCP relay cannot be connected to the untrusted port.

(2) Setting to allow forwarding of IPv4 packets

Points to note: Because the source MAC addresses in the packets sent from the DHCP client are rewritten by the Layer 3 switch/router (DHCP relay), configures a terminal filter on the untrusted port and sets only source IP addresses as the filter conditions.

Command examples

(config)# interface gigabitethernet 1/0/1

(config-if)# ip verify source

(config-if)# exit

Configures a terminal filter on port 1/0/1, and sets only source IP addresses as the filter conditions.

(3) Setting to allow forwarding of ARP packets

The configuration for permitting the forwarding of ARP packets is the same as the configuration when a terminal with a fixed IP address is connected.

For details about the settings, see 13.2.7 When a terminal with a fixed IP is connected.