DHCP Snooping and the Information Option

Hi all,

I don't understand the following sentence in the sample solution:

Now try simulating a DHCP request with a non-zero “giaddr” field across SW1. To accomplish this, configure DHCP snooping on SW2 for VLAN 10 and configure SW4 as DHCP client on VLAN 10, before activating ip dhcp snooping information option allow-untrusted on SW1.

giaddr will always remain 0.0.0.0 in a pure L2 switching domain. This sentence doesn't make any sense to me.

SW1#

DHCP_SNOOPING: process new DHCP packet, message type: DHCPDISCOVER, input interface: Fa0/13, MAC da: ffff.ffff.ffff, MAC sa: 2c3f.38dc.a1c1, IP da: 255.255.255.255, IP sa: 0.0.0.0, DHCP ciaddr: 0.0.0.0, DHCP yiaddr: 0.0.0.0, DHCP siaddr: 0.0.0.0, DHCP giaddr: 0.0.0.0, DHCP chaddr: 2c3f.38dc.a1c1

%DHCP_SNOOPING-5-DHCP_SNOOPING_NONZERO_GIADDR: DHCP_SNOOPING drop message with non-zero giaddr or option82 value on untrusted port, message type: DHCPDISCOVER, MAC sa: 2c3f.38dc.a1c1

SW3#

DHCPD: incoming interface name is Vlan10

DHCPD: DHCPREQUEST received from client 0063.6973.636f.2d32.6333.662e.3338.6463.2e61.3163.312d.566c.3130.

DHCPD: Sending notification of ASSIGNMENT:

DHCPD: address 160.1.10.7 mask 255.255.255.0

DHCPD: htype 1 chaddr 2c3f.38dc.a1c1
DHCPD: remote id 020a0000a0010a030a000000

DHCPD: lease time remaining (secs) = 86400

DHCPD: giaddr = 0.0.0.0


DHCPD: interface = Vlan10

DHCPD: Sending DHCPACK to client 0063.6973.636f.2d32.6333.662e.3338.6463.2e61.3163.312d.566c.3130 (160.1.10.7).
DHCPD: broadcasting BOOTREPLY to client 2c3f.38dc.a1c1.

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