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4.6.6 DHCP Relay
DHCP Relay Agent Overview
The DHCP Relay Agent makes it possible for DHCP broadcast messages to be sent over routers that do not support
forwarding of these types of messages. The DHCP Relay Agent is therefore the routing protocol that enables
DHCP clients to obtain IP addresses from a DHCP server on a remote subnet, or which is not located on the local
subnet. To enable clients to obtain IP addresses from a DHCP server on a remote subnet, you have to configure the
DHCP Relay Agent on the subnet that contains the remote clients, so that it can relay DHCP broadcast messages to
your DHCP server.
Data forwarding of DHCP relay agent is different from general routing forwarding. General routing forwarding is
relatively transparent and usually the transmitted IP packets won’t be modified. However, if DHCP relay agent
receives a DHCP packet, it will generate a new one and forward it out.
To the DHCP client, DHCP relay agent is the DHCP server; to the DHCP server, DHCP relay agent is DHCP
client.
DHCP relay forwarding process:
DHCP relay working process:
When network devices with DHCP relay feature receive DHCP-DISCOVER or DHCP-REQUEST packets
broadcast transmitted by DHCP clients, the giaddr field will be filled with DHCP relay IP and packets will be
forwarded, using unicast, to the designated DHCP server according to configurations.
According to the giaddr field, the DHCP server assigns IPs to clients and forwards configuration info to clients via
DHCP relay, and thus clients are dynamically configured.
Option 82
Option 82 records the location of the DHCP Client. Administrator can be acquainted with the location of the DHCP
Client via Option 82 so as to locate the DHCP Client for fulfilling the security control and account management of
Client.
When the DHCP relay receives DHCP request packets, the device will process them according to process strategies
of user configuration and whether option 82 is included or not.
This switch supports two sub-options: Circuit ID and Remote ID:
Sub-option 1(Circuit ID): the number of the port which receives the DHCP Request packets and its VLAN number.
Sub-option 2(Remote ID): the MAC address of DHCP Snooping device which receives the DHCP Request packets
from DHCP Clients.
Summary of Contents for G3224P
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