Dhcp Relay - Cisco SF500-24 Administration Manual

Configuring DHCP

DHCP Relay

DHCP Relay
Option 82
Cisco 500 Series Stackable Managed Switch Administration Guide
DHCP Relay relays DHCP packets to the DHCP server.
In Layer 2 mode, the switch relays DHCP messages received from VLANs on
which DHCP Relay has been enabled.
In Layer 3 mode, the switch 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.
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 switch:
• 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.
"Figure 1" 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.
• DHCP client and DHCP server are connected to different VLANs. In the
case, only DHCP Relay can and does broadcast DHCP messages between
DHCP client and DHCP server. Unicast DHCP messages are passed by
regular routers and therefore if DHCP Relay is enabled on a VLAN without
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