Dhcp Relay Agent - Lenovo RackSwitch G8264 Application Manual

DHCP Relay Agent

© Copyright Lenovo 2016
DHCP is described in RFC 2131, and the DHCP relay agent supported on the
G8264 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 G8264 acts as a relay agent. The DHCP relay feature
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 destination IP (DIP) addresses. The request is then forwarded as a UDP
Unicast MAC layer message 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.
To enable the G8264 to be the BOOTP forwarder, you need to configure the
DHCP/BOOTP server IP addresses on the switch. Generally, it is best to configure
the switch IP interface on the client side to match the client's subnet, and configure
VLANs to separate client and server subnets. The DHCP server knows from which
IP subnet the newly allocated IP address will come.
In G8264 implementation, there is no need for primary or secondary servers. The
client request is forwarded to the BOOTP servers configured on the switch. The use
of five servers provide failover redundancy. However, no health checking is
supported.
Use the following commands to configure the switch as a DHCP relay agent:
RS G8264(config)# ip bootp­relay server 1 <IP address>
RS G8264(config)# ip bootp­relay server 2 <IP address>
RS G8264(config)# ip bootp­relay server 3 <IP address>
RS G8264(config)# ip bootp­relay server 4 <IP address>
RS G8264(config)# ip bootp­relay server 5 <IP address>
RS G8264(config)# ip bootp­relay enable
RS G8264(config)# show ip bootp­relay
Additionally, DHCP Relay functionality can be assigned on a per interface basis.
Use the following commands to enable the Relay functionality:
RS G8264(config)# interface ip <Interface number>
RS G8264(config­ip­if)# relay
Chapter 26: Basic IP Routing
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