Administrator Guide

DHCP Snooping
A DHCP client can run on a switch simultaneously with the DHCP snooping feature as follows:
● If you enable DHCP snooping globally on a switch and you enable a DHCP client on an interface, the trust port, source MAC
address, and snooping table validations are not performed on the interface by DHCP snooping for packets destined to the
DHCP client daemon.
The following criteria determine packets destined for the DHCP client:
○ DHCP is enabled on the interface.
○ The user data protocol (UDP) destination port in the packet is 68.
○ The chaddr (change address) in the DHCP header of the packet is the same as the interface’s MAC address.
● An entry in the DHCP snooping table is not added for a DHCP client interface.
DHCP Server
A switch can operate as a DHCP client and a DHCP server. DHCP client interfaces cannot acquire a dynamic IP address from
the DHCP server running on the switch. Acquire a dynamic IP address from another DHCP server.
Virtual Router Redundancy Protocol (VRRP)
Do not enable the DHCP client on an interface and set the priority to 255 or assign the same DHCP interface IP address to a
VRRP virtual group. Doing so guarantees that this router becomes the VRRP group owner.
To use the router as the VRRP owner, if you enable a DHCP client on an interface that is added to a VRRP group, assign a
priority less than 255 but higher than any other priority assigned in the group.
DHCP Relay When DHCP Server and Client are in
Different VRFs
When the DHCP server and DHCP clients belong to different VRFs on the relay agent, you can configure the system to leak
routes across VRFs.
You can configure the system to leak the following routes across VRFs:
● Connected routes
● The complete routing table
● Selective routes
The following illustration depicts the topology in which routes are leaked between VRFs in the relay agent.
VRF_1 VRF_2
DHCP Server --------------------- DHCP relay agent --------------------------- Client
(10.0.0.1) (10.0.0.2) (20.0.0.2) (20.0.0.4)
Configuring Route Leaking between VRFs on DHCP Relay Agent
To configure route leaking between VRFs on DHCP relay agent, include the configuration similar to the following along with your
DHCP relay configuration on your system.
Route Leaking for Connected Routes
!
ip vrf VRF_1
ip route-import 1:1 rmap1
ip route-export 2:2 rmap2
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Dynamic Host Configuration Protocol (DHCP)