Bootp/Dhcp Relay - Planet Networking & Communication WGS3-2620 User Manual

6.6 BOOTP/DHCP Relay

Dynamic Host Configuration Protocol (DHCP), described in RFC 1541, is an extension of the Bootstrap
Protocol (BOOTP). DHCP allows hosts on a TCP/IP network to dynamically obtain basic configuration
information. When a DHCP client starts, it broadcasts a DHCP Request packet, looking for DHCP
servers. DHCP servers respond to this packet with a DHCP Response packet. The client then chooses a
server to obtain TCP/IP configuration information, such as its own IP address.
Since DHCP uses a broadcast mechanism, a DHCP server and its client must physically reside on the
same subnet. However, it is not practical to have one DHCP server on every subnet; in fact in many
cases, DHCP/BOOTP clients and their associated DHCP/BOOTP server(s) do not reside on the same IP
network or subnet. In such cases, a third-party agent is required to transfer BOOTP messages between
clients and servers.
BOOTP/DHCP Relay, described in RFC 1542, enables a host to use a BOOTP or DHCP server to obtain
basic TCP/IP configuration information, even if the servers do not reside on the local subnet. When an
Switch BOOTP/DHCP Relay Agent receives a DHCP Request packet destined for a BOOTP/DHCP
server, it inserts its own IP address into the DHCP Request packet so the server knows the subnet
where the client is located. Then, depending on the configuration setup, the switch either:
• Forwards the packet to a specific server as defined in the switch's configuration using unicast routing,
or
• Broadcasts the DHCP Request again to another directly attached IP subnet specified in the switch
configuration for the receiving IP subnet.
When the DHCP server receives the DHCP request, it allocates a free IP address for the DHCP client
from its scope in the DHCP client's subnet, and sends a DHCP Response back to the DHCP Relay
Agent. The DHCP Relay Agent then broadcasts this DHCP Response packet received from the DHCP
server to the appropriate client.
WGS3 Layer 3 Switch User's Manual
- 246 -