14. DHCP Relay
ROX™ v2.2 User Guide
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RuggedBackbone™ RX5000
14. DHCP Relay
A DHCP Relay Agent is a device that forwards DHCP packets between clients and servers when they
are not on the same physical LAN segment or IP subnet. The feature is enabled if the DHCP server IP
address and a set of access ports are configured.
DHCP Option 82 provides a mechanism for assigning an IP Address based on the location of the client
device in the network. Information about the client’s location can be sent along with the DHCP request
to the server. Based on this information, the DHCP server makes a decision about an IP Address to
be assigned.
DHCP Relay Agent takes the broadcast DHCP requests from clients received on the configured access
port and inserts the relay agent information option (Option 82) into the packet. Option 82 contains the
VLAN ID (2 bytes) and the port number of the access port (2 bytes: the circuit ID sub-option) and
the switch’s MAC address (the remote ID sub-option). This information uniquely defines the access
port’s position in the network. For example, in ROX™, the Circuit ID for VLAN 2 on LM 4 Port 15 is
00:02:04:0F
.
The DHCP Server supporting DHCP Option 82 sends a unicast reply and echoes Option 82. The DHCP
Relay Agent removes the Option 82 field and broadcasts the packet to the port from which the original
request was received.
The DHCP Relay Agent communicates to the server on a management interface. The agent’s IP
address is the address configured for the management interface.
ROX™ can be configured to act as a DHCP Relay Agent that forwards DHCP and BOOTP requests
from clients on one layer 2 network to one or more configured DHCP servers on other networks. This
allows the implementation of some measure of isolation between DHCP clients and servers.
The DHCP Relay Agent is configured to listen for DHCP and BOOTP requests on particular Ethernet
and VLAN network interfaces, and to relay to a list of one or more DHCP servers. When a request is
received from a client, ROX™ forwards the request to each of the configured DHCP servers. When a
reply is received from a server, ROX™ forwards the reply back to the originating client.
While DHCP Relay and DHCP Server may both be configured to run concurrently, they
may not be configured to run on the same network interface.
Figure 14.1. DHCP Relay Agent Menu
The DHCP Relay Agent menu is available under switch/dhcp-relay-agent.
Figure 14.2. DHCP Relay Agent Form