Configuring Switching Information 459DHCP RelayWhen a DHCP client and server are in the same IP subnet, they can directly connect to exchange IPaddress requests and replies. However, having a DHCP server on each subnet can be expensive and isoften impractical. Alternatively, network infrastructure devices can be used to relay packets between aDHCP client and server on different subnets. Such a device, a Layer 3 Relay agent, is generally a routerthat has IP interfaces on both the client and server subnets and can route between them. However, inLayer 2 switched networks, there may be one or more infrastructure devices (for example, a switch)between the client and the L3 Relay agent/DHCP server. In this instance, some of the client deviceinformation required by the L3 Relay agent may not be visible to it. In this case, an L2 Relay agent can beused to add the information that the L3 Relay Agent and DHCP server need to perform their roles inaddress and configuration and assignment.Before it relays DHCP requests from clients, the switch can add a Circuit ID and a Remote ID. Theseprovide information about the circuit and port number connected to the client. This information isadded as suboptions in the DHCP Option 82 packets (see sections 3.1 and 3.2 of RFC3046). The switchremoves this option from packets that it relays from L3 Relay agents/DHCP servers to clients.These sub-options may be used by the DHCP server to affect how it treats the client, and also may beused by the relay agent to limit broadcast replies to the specific circuit or attachment point of the client.The Switching →DHCP Relay page provides links to the following pages:• DHCP Relay Global Configuration• DHCP Relay Interface Configuration• DHCP Relay Interface Statistics• DHCP Relay VLAN ConfigurationDHCP Relay Global ConfigurationUse this page to enable or disable the switch to act as a DHCP Relay agent. This functionality must alsobe enabled on each port you want this service to operate on (see DHCP Relay Interface Configuration).The switch can also be configured to relay requests only when the VLAN of the requesting clientcorresponds to a service provider’s VLAN ID that has been enabled with the L2 DHCP relayfunctionality (see DHCP Relay VLAN Configuration).To access this page, click Switching →DHCP Relay →Global Configuration in the tree view.
