Reference Guide

VLAN and Port Channels
DHCP client configuration and behavior are the same on Virtual LAN (VLAN) and port-channel (LAG)
interfaces as on a physical interface.
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 Client on a Management Interface
These conditions apply when you enable a management interface to operate as a DHCP client.
• The management default route is added with the gateway as the router IP address received in the
DHCP ACK packet. It is required to send and receive traffic to and from other subnets on the external
network. The route is added irrespective when the DHCP client and server are in the same or different
subnets. The management default route is deleted if the management IP address is released like other
DHCP client management routes.
• ip route for 0.0.0.0 takes precedence if it is present or added later.
• Management routes added by a DHCP client display with Route Source as DHCP in the show ip
management route and show ip management-route dynamic command output.
• Management routes added by DHCP are automatically reinstalled if you configure a static IP route
with the ip route command that replaces a management route added by the DHCP client. If you
remove the statically configured IP route using the
no ip route command, the management route
is reinstalled. Manually delete management routes added by the DHCP client.
• To reinstall management routes added by the DHCP client that is removed or replaced by the same
statically configured management routes, release the DHCP IP address and renew it on the
management interface.
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Dynamic Host Configuration Protocol (DHCP)