Cisco 500 Series Administration Manual page 356

IP Configuration
IPv4 Management and Interfaces
Cisco 500 Series Managed Switch Administration Guide
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 without an IP address, Option 82 is inserted automatically. This insertion is 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.
The following Option 82 options are available on the device:
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