Users 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 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 trac 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 dierent 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 congure a static IP route with the ip route command that
replaces a management route added by the DHCP client. If you remove the statically congured 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 congured management
routes, release the DHCP IP address and renew it on the management interface.
• Management routes added by the DHCP client have higher precedence over the same statically congured management route. Static
routes are not removed from the running conguration if a dynamically acquired management route added by the DHCP client
overwrites a static management route.
• Management routes added by the DHCP client are not added to the running conguration.
NOTE
: Management routes added by the DHCP client include the specic routes to reach a DHCP server in a dierent subnet
and the management route.
Dynamic Host Conguration Protocol (DHCP) 291