This GIADDR is used by the DHCP server to determine what scope and pool to reply back to PCa’s dhcp client request. The broadcast on VLAN 2 is forwarded to the DHCP server 10.1.1.100 via the DHCP relay function with the Gateway IP Address field filled in with the VLAN 2 IP of Switch A, 10.1.2.1. PCc is local to the DHCP server, so there is no need for a DHCP relay. The DHCP relay agent sends the local broadcast, via IP unicast, to the DHCP server address 172.16.1.2 specified by the ip helper-address interface configuration command. In the example, SwitchA has configured VLAN 2 with DHCP relay for DHCP client requests on PCa or PCb (in VLAN 2) to be seen by DHCP server 10.1.1.100 in VLAN 1. This IP address enables the DHCP server to determine which subnet should receive the offer and identify the appropriate IP address range to offer. The address used for PCa or PCb to get to the DHCP server would be the 10.1.2.1 address of the switch forwarding the DHCPdiscover on VLAN 1. This requirement would not be feasible, though you can configure DHCP scopes in some Network OSes, NEXUS OS, Cisco IOS and Comware. Without DHCP relay, there were need to be a DHCP server on every network/subnet/VLAN. The DHCP server requires the forwarding router’s IP address that is forwarding the DHCPDiscover, GIADDR. The broadcast is not forwarded by routers, but configured with DHCP relay, the DHCPDiscover message is repackaged as unicast to the DHCP server. This request forwarding is done by the local router gateway of the DHCP client. Dynamic Host Configuration Protocol (DHCP) relay is used to forward DHCP client’s broadcast request, DHCPDiscover, from one VLAN to the DHCP server’s VLAN.
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