Concept Guide

IP address assignment. If you later enter the no shutdown command and the lease timer for the dynamic IP address has expired, the IP
address is uncongured and the interface tries to acquire a new dynamic address from DHCP server.
If you later enter the no shutdown command and the lease timer for the dynamic IP address has expired, the IP address is released.
When you enter the release dhcp command, although the IP address that was dynamically-acquired from a DHCP server is released
from an interface, the ability to acquire a new DHCP server-assigned address remains in the running conguration for the interface. To
acquire a new IP address, enter either the renew dhcp command at the EXEC privilege level or the ip address dhcp command at
the interface conguration level.
If you enter renew dhcp command on an interface already congured with a dynamic IP address, the lease time of the dynamically
acquired IP address is renewed.
IMPORTANT: To verify the currently congured dynamic IP address on an interface, enter the show ip dhcp lease
command. The show running-configuration command output only displays ip address dhcp; the currently assigned
dynamic IP address is not displayed.
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 of whether the DHCP client
and server are on 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.
DHCP Client on a VLAN
The following conditions apply on a VLAN that operates as a DHCP client:
• The default VLAN 1 with all ports auto-congured as members is the only L3 interface on the Aggregator.
• When the default management VLAN has a DHCP-assigned address and you recongure the default VLAN ID number, the Aggregator:
– Sends a DHCP release to the DHCP server to release the IP address.
– Sends a DHCP request to obtain a new IP address. The IP address assigned by the DHCP server is used for the new default
management VLAN.
DHCP Packet Format and Options
DHCP uses the user datagram protocol (UDP) as its transport protocol.
The server listens on port 67 and transmits to port 68; the client listens on port 68 and transmits to port 67. The conguration parameters
are carried as options in the DHCP packet in Type, Length, Value (TLV) format; many options are specied in RFC 2132. To limit the number
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Dynamic Host Conguration Protocol (DHCP)