Cisco 350 Series Administration Manual page 380

IP Configuration
IPv4 Management and Interfaces
Cisco 350, 350X and 550X Series Managed Switches, Firmware Release 2.4, ver 0.4
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.
• 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 an IP address, an external router is
needed.
DHCP Relay and only DHCP Relay relays DHCP messages to a DHCP server
Interactions Between DHCPv4 Snooping, DHCPv4 Relay and Option 82
The following tables describe how the device behaves with various combinations of DHCP
Snooping, DHCP Relay and Option 82.
The following describes how DHCP request packets are handled when DHCP Snooping is not
enabled and DHCP Relay is enabled.
DHCP Relay
VLAN with IP Address
Packet arrives
without Option 82
Option 82 Packet is sent
Insertion without Option 82
Disabled
DHCP Relay
VLAN without IP Address
Packet arrives Packet arrives
with Option 82 without Option
82
Packet is sent Relay – inserts
with the original Option 82
Option 82
Bridge – no
Option 82 is
inserted
16
Packet arrives
with Option 82
Relay – discards
the packet
Bridge – Packet
is sent with the
original Option
82
293