Dell POWEREDGE M1000E - Manual (Page 920)

920 Configuring L2 and L3 Relay Features
The PowerConnect DHCP Relay Agent enables DHCP clients and servers to
exchange DHCP messages across different subnets. The relay agent receives
the requests from the clients, and checks the valid hops and giaddr fields in
the DHCP request. If the number of hops is greater than the 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 PowerConnect M6220/M6348/M8024/M8024-k switch 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, 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 device information required by the
L3 Relay agent may not be visible to it. In this case, an L2 Relay agent can be
used to add the information that the L3 Relay Agent and DHCP server need
to perform their roles in address and configuration and 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.