Transparent Dhcp Relay - Cisco SF500-24 Administration Manual

IP Configuration
IPv4 Management and Interfaces
Cisco 500 Series Stackable Managed Switch Administration Guide Release 1.3

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:
• DHCP Insertion - Add Option 82 information to packets that do not have
foreign Option 82 information.
• DHCP Passthrough - Forward or reject DHCP packets that contain Option
82 information from untrusted ports. On trusted ports, DHCP packets
containing Option 82 information are always forwarded.
The following table shows the packet flow through the DHCP Relay, DHCP
Snooping, and Option 82 modules:
The following cases are possible:
• DHCP client and DHCP server are connected to the same VLAN. In this
case, a regular bridging passes the DHCP messages between DHCP client
and DHCP server.
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