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DHCP Relay Overview Configuring DHCP Relay

page 18-6 OmniSwitch 6800/6850/9000 Network Configuration Guide June 2006

DHCP

DHCP (Dynamic Host Configuration Protocol) provides a framework for passing configuration informa-

tion to Internet hosts on a TCP/IP network. It is based on the Bootstrap Protocol (BOOTP), adding the

ability to automatically allocate reusable network addresses and additional configuration options. DHCP

consists of the following two components:

• A protocol for delivering host-specific configuration parameters from a DHCP server to a host.

• A mechanism for allocating network addresses to hosts.

DHCP is built on a client-server model in which a designated DHCP server allocates network addresses

and delivers configuration parameters to dynamically configured hosts. It supports the following three

mechanisms for IP address allocation.

Automatic—DHCP assigns a permanent IP address to a host.

Dynamic—DHCP assigns an IP address to a host for a limited period of time (or until the host explic-

itly relinquishes the address).

Manual—The network administrator assigns a host’s IP address and DHCP simply conveys the

assigned address to the host.

DHCP and the OmniSwitch

The unique characteristics of the DHCP protocol require a good plan before setting up the switch in a

DHCP environment. Since DHCP clients initially have no IP address, placement of these clients in a

VLAN is hard to determine. In simple networks (e.g., one VLAN) rules do not need to be deployed to

support the BOOTP/DHCP relay functionality.

In multiple VLAN network configurations, VLAN rules can be deployed to strategically support the

processing and relay of DHCP packets. The most commonly used rules for this function are IP protocol

rules, IP network address rules, and DHCP rules. All of these classify packets received on mobile ports

based on the packet protocol type, source IP address, or if the packet is a DHCP request. See Chapter 9,

“Defining VLAN Rules,” for more information.

DHCP Relay and Authentication

Authentication clients may use DHCP to get an IP address. For Telnet authentication clients, an IP address

is required for authentication. The DHCP server may be located in the default VLAN, an authenticated

VLAN, or both. If authentication clients will be getting an IP address from a DHCP server located in an

authenticated VLAN, DHCP relay can handle DHCP requests/responses for these clients as well.

There are three relay forwarding options: standard, AVLAN only, and per-VLAN. All three support

DHCP traffic to/from authenticated clients. However, the AVLAN only option specifies that only DHCP

packets received on authenticated ports are processed. See “Setting the Relay Forwarding Option” on

page 18-11 for more information.

Using DHCP Relay with authenticated VLANs and clients also requires relay configuration of the router

port address of the authenticated VLAN. See Chapter 22, “Configuring Authenticated VLANs,” for more

information about this procedure.

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