Interactions Between Dhcp Snooping, Dhcp Relay And Option - Cisco 500 series Administration Manual

Configuring DHCP

Interactions Between DHCP Snooping, DHCP Relay and Option 82

Interactions Between DHCP Snooping, DHCP Relay and
Option 82
Cisco 500 Series Stackable Managed Switch Administration Guide
• 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 or if the switch is not a router (Layer 2 switch) then an external
router is needed.
DHCP Relay and only DHCP Relay relays DHCP messages to a DHCP server
The following tables describe how the switch 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
Disabled 82
DHCP Relay
VLAN without IP Address
Packet arrives Packet arrives
with Option without
82 Option 82
Packet is sent Relay –
with the inserts Option
original 82
Option 82
Bridge – no
Option 82 is
inserted
23
Packet arrives
with Option
82
Relay –
discards the
packet
Bridge –
Packet is sent
with the
original
Option 82
385