DHCP Relay Cont Shared DHCP Server

CE 91

CE-H

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MPLS v2.0-6-2S

In this example, the corporate DHCP server has been replaced with a shared DHCP server provided by the service provider. Because the server is shared between VPNs, a NAT PE could also be included to provide address translation.

The client broadcasts a DHCP request to the local relay. The local relay converts the broadcast to a unicast request for the shared DHCP server and adds the VPN ID. This request is forwarded to the egress PE router via the NAT PE based upon the DHCP server address. At the NAT PE, an address translation is performed and the request is forwarded to the DHCP server.

The DHCP server assigns the client an address from the VPN pool and replies to the DHCP relay, which in turn forwards the reply to the client.

DHCP Relay (Cont.) Configuration

The figure presents an example of a typical DHCP relay configuration.

The dhcp relay information option vpn command enables the system to insert VPN suboptions into the DHCP relay agent information option in forwarded BOOTREQUEST messages to a DHCP server and sets the gateway address to the outgoing interface toward the DHCP server. The VPN suboptions are also added to the BOOTP broadcast packets when the command is configured.

The DHCP server address is configured using the ip helper-address vrf command.

Note For simplicity, this example uses two separate interfaces for the VPNs. They could also be supported using inter-Switch Link Protocol (ISL) and subinterfaces. Additional DHCP configuration options are available. See the Cisco IOS documentation for further information.

For further information see the following:

■ http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps 1839/products_feature_guide 09186a0080087d3c.html