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Alteon OS Application Guide
Chapter 8: Basic IP Routing
15342C4911, January 2007
DHCP Relay Agent
DHCP is described in RFC 2131, and the DHCP relay agent supported on GbE Switch Mod-
ules is described in RFC 1542. DHCP uses UDP as its transport protocol. The client sends
messages to the server on port 67 and the server sends messages to the client on port 68.
DHCP defines the methods through which clients can be assigned an IP address for a finite
lease period and allowing reassignment of the IP address to another client later. Additionally,
DHCP provides the mechanism for a client to gather other IP configuration parameters it needs
to operate in the TCP/IP network.
In the DHCP environment, the GbE Switch Module acts as a relay agent. The DHCP relay fea-
ture (/cfg/l3/bootp) enables the switch to forward a client request for an IP address to
two BOOTP servers with IP addresses that have been configured on the switch.
When a switch receives a UDP broadcast on port 67 from a DHCP client requesting an IP
address, the switch acts as a proxy for the client, replacing the client source IP (SIP) and desti-
nation IP (DIP) addresses. The request is then forwarded as a UDP Unicast MAC layer mes-
sage to two BOOTP servers whose IP addresses are configured on the switch. The servers
respond as a UDP Unicast message back to the switch, with the default gateway and IP address
for the client. The destination IP address in the server response represents the interface address
on the switch that received the client request. This interface address tells the switch on which
VLAN to send the server response to the client.