HP ProCurve 7000dl Series Basic Management And Configuration Manual page 305

Configuring the Data Link Layer Protocol for E1, T1, and Serial Interfaces
Configure the PPP Interface as an Unnumbered Interface. To con-
serve IP addresses on your network, you may want to create the PPP interface
as an unnumbered interface. When you assign a logical interface on the router
an IP address, that IP address cannot overlap with the IP addresses that are
assigned to other logical interfaces. 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 PPP interface (and other interfaces on the ProCurve
Secure Router) as an unnumbered interface. The PPP 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.
Before configuring the PPP interface as an unnumbered interface, you should
be aware of a potential disadvantage: If the interface to which the IP address is
actually assigned goes down, the PPP interface will be unavailable as well. For
example, suppose you configure the PPP 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 PPP 1 interface will also be unavailable.
To minimize the chances that the interface with the IP address will go down,
you can assign the IP address to a loopback interface, which typically does
not go down.
To configure the PPP interface as an unnumbered interface, enter the follow-
ing command from the PPP interface configuration mode context:
Syntax: ip unnumbered <interface>
Valid interfaces include:
Ethernet interfaces and subinterfaces
Frame Relay subinterfaces
other PPP interfaces
HDLC interfaces
loopback interfaces
Asynchronous Transfer Mode (ATM) subinterfaces
demand interfaces
Configuring the Logical Interface
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