It will be just overview about the Dynamic Host Configuration Protocol (DHCP) and background for post about DHCP snooping feature that will be posted soon. DHCP is the probably the most popular way of dynamic IP address assignment and other options like lease time, gateway and DNS IPs or domain name for the hosts in the PC enviroment.  DHCP works in a client – server architecture and the basic process of address assignments is depicted on the below figure.


We have 4-step process. First clients sends the DHCP Discovery request to the server. Because the client doesn’t have any idea what is a IP address of the server even more client doesn’t have IP address configured yet and it’s what is looking for, it sends layer 2 broadcast address with own MAC address as source and FFFF:FFFF:FFFF as destination MAC address. From the layer 3 perspecctive is placed as IP source and as destination broadcast. Server responds to the client with the DHCP Offer packet that includes client IP address proposition and with other options. Clients responds to the server with DHCP Request (request for IP address that is has been just offered to the client), and server confirm IP address assignment via DHCP ACK packet.

Above example was based on the LAN enviroment where client and server resides on the same subnet. What about if we have clients PC in the branch office and DHCP server in out data center HQ. The solution for this is DHCP Relay Agent feature needs to be configured on the LAN interface of the gateway router. Below diagram shows the process flow for the Relay Agent. 



From the client perspective nothing changed, this process is transparent and client doesn’t know even that traffic is going thru the WAN cloud to the data center on the other side of the world. By default
all subnet broadcast ( is not forwarded unles ip helper-address is confgured.  Once the router gets the broadcast address with bootp on board and ip helper IP address is configured, router will encapsulate the DHCP Discover and sends it as unicast to the configured IP address in the ip helper-address command (it have to be DHCP server IP).

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