Cisco Cisco IOS Software Release 12.2(27)SBC

The DHCP relay agent supports the use of unnumbered interfaces. For DHCP clients connected through 
the unnumbered interfaces, the DHCP relay agent automatically adds a static host route once the DHCP 
client obtains an address, specifying the unnumbered interface as the outbound interface. The route is 
automatically removed once the lease time expires or when the client releases the address.

The ip helper-address interface configuration command must be configured on the unnumbered 
interface to enable the Cisco IOS DHCP relay agent on unnumbered interfaces. See the 

 configuration module for more information.

The ISSU and SSO DHCP relay on unnumbered interface functionality adds high availability support 
for host routes to clients connected through unnumbered interfaces. The DHCP relay agent can now 
detect when a router is failing over to the standby RP and keep the states related to unnumbered 
interfaces. 

CPE

DSLAM

BRAS

(DHCP Server

or Relay)

DHCP Offer

DHCP Request

DHCP Ack

DHCP Discover

BNDUPD

Active

Standby

DHCP Request

DHCP Ack

BNDUPD

DHCP Release

BNDUPD

Lease created +

Host route added

Lease updated

Lease destroyed +

Host route deleted

IP

DHCP 

Server

DHCP Offer

DHCP Discover

DHCP Request

DHCP Ack

DHCP Request

DHCP Ack

170467

“ip unnumbered”