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.  This request forwarding is done by the local router gateway of the DHCP client. The broadcast is not forwarded by routers, but configured with DHCP relay, the DHCPDiscover message is repackaged as unicast to the DHCP server.  The DHCP server requires the forwarding router’s IP address that is forwarding the DHCPDiscover, GIADDR.

Without DHCP relay, there were need to be a DHCP server on every network/subnet/VLAN.  This requirement would not be feasible, though you can configure DHCP scopes in some Network OSes, NEXUS OS, Cisco IOS and Comware.

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.

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.  PCc is local to the DHCP server, so there is no need for a DHCP relay. 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.  This GIADDR is used by the DHCP server to determine what scope and pool to reply back to PCa’s dhcp client request.

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LINKS

1. Cisco IOS IP Addressing: DHCP Configuration Guide, Cisco IOS Release
2. HP Comware DHCP Technology White Paper
3. Duke’s DHCP Snooping for Procurve and Comware