HP ProCurve Secure 7000dl Series Basic Management And Configuration Manual page 294

Configuring the Data Link Layer Protocol for E1, T1, and Serial Interfaces
Configuring the Logical Interface
6-42
You can replace <subnet mask> with the complete subnet mask, or you can
replace </prefix length> with the CIDR notation. For example, you might
enter:
ProCurve(config-hdlc 1)# ip address 10.1.1.1 /24
Configure the HDLC Interface as an Unnumbered Interface. To con-
serve IP addresses on your network, you may want to create the HDLC
interface as an unnumbered interface. When you assign a logical interface an
IP address, that IP address cannot overlap with the IP addresses assigned to
other logical interfaces on your network. As a result, each interface that has
an IP address represents an entire subnet. Depending on the subnetting
scheme you use, this could use more IP addresses than you can spare.
You can configure the HDLC interface (and other interfaces on the ProCurve
Secure Router) as an unnumbered interface. The HDLC interface will then use
the IP address of another interface—the interface you specify. The Secure
Router OS uses the IP address of the specified interface when sending route
updates over the unnumbered interface.
Configuring the HDLC interface as an unnumbered interface has a potential
disadvantage: If the interface to which the IP address is actually assigned goes
down, the HDLC interface will be unavailable as well. For example, suppose
you configure the HDLC 1 interface as an unnumbered interface that takes its
IP address from the Ethernet 0/1 interface. If the Ethernet 0/1 interface goes
down, the HDLC 1 interface will also be unavailable.
To minimize the chances of the interface with the IP address going down, you
can assign the IP address to a loopback interface, which typically does not
go down.
To configure the HDLC interface as an unnumbered interface, enter the
following command from the HDLC interface configuration mode context:
Syntax: ip unnumbered <interface>
Valid interfaces include:
ATM subinterfaces
Ethernet interfaces and subinterfaces
Frame Relay subinterfaces
loopback interfaces
PPP interfaces
demand interfaces