Administrator Guide

1158 Layer-2 and Layer-3 Relay Features
configured number, the agent discards the packet. If the giaddr field is zero,
the agent must fill in this field with the IP address of the interface on which
the request was received. The agent unicasts the valid packets to all
configured DHCP servers. Each server responds with a unicast BOOTREPLY
addressed to the relay agent closest to the client as indicated by
giaddr field.
Upon reception of the BOOTREPLY from the server, the agent forwards this
reply as broadcast or unicast on the interface where the BOOTREQUEST
arrived. This interface can be identified by the
giaddr field or option 82.
The Dell Networking N1500, N2000, N3000, and N4000 Series switches
DHCP component also supports DHCP relay agent options to identify the
client interface. If configured, the relay agent inserts these options when
forwarding the request to the server and removes them when sending the
reply to the clients.
If an interface has more than one IP address, the relay agent uses the primary
IP address configured as its relay agent IP address.
What Is L2 DHCP Relay?
In layer-2 switched networks, hosts (DHCP clients) may be connected
directly to a switch which is connected to a router configured as a DHCP relay
agent, or they may be connected directly to a DHCP server (unusual). In this
instance, some of the client device information required by the DHCP server
may not be included in the DHCP packets sent by the DHCP client. An L2
relay agent can be used to add the information that the DHCP server needs
to perform its role in address and configuration and assignment. The
information added by the L2 relay agent can include location and
identification information that can assist the DHCP server in applying
policies such as service offerings or address assignment.
Before it relays DHCP requests from clients, the switch can add a Circuit ID
and a Remote ID. These IDs provide information about the circuit and port
number connected to the client. This information is added as suboptions in
the DHCP Option 82 packets as defined in sections 3.1 and 3.2 of RFC3046.
The switch removes 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 be used by the relay agent to limit broadcast replies to the
specific circuit or attachment point of the client.