Cisco 350XG series Administration Manual page 329

IP Configuration
IPv4 Management and Interfaces
Cisco 350XG & 550XG Series 10G Stackable Managed Switches
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.
• 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
DHCP Relay
VLAN without IP Address
Packet arrives Packet arrives
with Option without
82 Option 82
16
Packet arrives
with Option
82
318