Cisco Systems 3750E manual 22-4, Dhcp Relay Agent in a Metropolitan Ethernet Network

Chapter 22 Configuring DHCP Features and IP Source Guard

Understanding DHCP Features

Figure 22-1is an example of a metropolitan Ethernet network in which a centralized DHCP server assigns IP addresses to subscribers connected to the switch at the access layer. Because the DHCP clients and their associated DHCP server do not reside on the same IP network or subnet, a DHCP relay agent (the Catalyst switch) is configured with a helper address to enable broadcast forwarding and to transfer DHCP messages between the clients and the server.

Figure 22-1 DHCP Relay Agent in a Metropolitan Ethernet Network

DHCP server

Catalyst switch Access layer (DHCP relay agent)

Host A (DHCP client)

VLAN 10

Subscribers	Host B
	(DHCP client)

98813

When you enable the DHCP snooping information option 82 on the switch, this sequence of events occurs:

•The host (DHCP client) generates a DHCP request and broadcasts it on the network.

•When the switch receives the DHCP request, it adds the option-82 information in the packet. By default, the remote-ID suboption is the switch MAC address, and the circuit-ID suboption is the port identifier, vlan-mod-port, from which the packet is received.You can configure the remote ID and circuit ID. For information on configuring these suboptions, see the “Enabling DHCP Snooping and Option 82” section on page 22-12.

•If the IP address of the relay agent is configured, the switch adds this IP address in the DHCP packet.

•The switch forwards the DHCP request that includes the option-82 field to the DHCP server.

•The DHCP server receives the packet. If the server is option-82-capable, it can use the remote ID, the circuit ID, or both to assign IP addresses and implement policies, such as restricting the number of IP addresses that can be assigned to a single remote ID or circuit ID. Then the DHCP server echoes the option-82 field in the DHCP reply.

•The DHCP server unicasts the reply to the switch if the request was relayed to the server by the switch. The switch verifies that it originally inserted the option-82 data by inspecting the remote ID and possibly the circuit ID fields. The switch removes the option-82 field and forwards the packet to the switch port that connects to the DHCP client that sent the DHCP request.

In the default suboption configuration, when the described sequence of events occurs, the values in these fields in Figure 22-2do not change:

•Circuit-ID suboption fields

–Suboption type

–Length of the suboption type

–Circuit-ID type

–Length of the circuit-ID type

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

22-4 OL-9775-02