IP Configuration
IPv4 Management and Interfaces
Cisco Small Business 300 Series Managed Switch Administration Guide 264
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A trusted port is a port that is connected to a DHCP server and is allowed to
assign DHCP addresses. DHCP messages received on trusted ports are allowed
to pass through the device.
An untrusted port is a port that is not allowed to assign DHCP addresses. By
default, all ports are considered untrusted until you declare them trusted (in the
DHCP Snooping Interface Settings page).
DHCPv4 Relay
DHCP Relay relays DHCP packets to the DHCP server.
DHCPv4 in Layer 2 and Layer 3
In Layer 2 system mode, the device relays DHCP messages received from VLANs
on which DHCP Relay has been enabled.
In Layer 3 system mode, the device can also relay DHCP messages received from
VLANs that do not have IP addresses. Whenever DHCP Relay is enabled on a
VLAN wi thout a n IP add ress, Optio n 82 is i nsert ed auto matica lly. This inser tion i s in
the specific VLAN and does not influence the global administration state of Option
82 insertion.
Transparent DHCP Relay
For Transparent DHCP Relay where an external DHCP relay agent is being used,
do the following:
•Enable DHCP Snooping.
•Enable Option 82 insertion.
•Disable DHCP Relay.
For regular DHCP Relay:
•Enable DHCP Relay.
•No need to enable Option 82 insertion.
Option 82
Option 82 (DHCP Relay Agent Information Option) passes port and agent
information to a central DHCP server, indicating where an assigned IP address
physically connects to the network.
The main goal of option 82 is to help to the DHCP server select the best IP subnet
(network pool) from which to obtain an IP address.