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Alteon OS Application Guide
Chapter 1: Accessing the Switch
3142C4911, January 2007
DHCP Relay Agent
DHCP is described in RFC 2131, and the DHCP relay agent supported on the GbESM 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 switch acts as a relay agent. The DHCP relay feature
(/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 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.
DHCP Relay Agent Configuration
To enable the GbESM to be the BOOTP forwarder, you need to configure the DHCP/BOOTP
server IP addresses on the switch. Generally, you should configure the command on the switch
IP interface closest to the client so that the DHCP server knows from which IP subnet the
newly allocated IP address should come.
The following figure shows a basic DHCP network example:
Figure 1-3 DHCP Relay Agent Configuration
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