Configuring Ip Address Unnumbered For An Interface - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide
Caution:
Because PPP supports IP address negotiation, IP address negotiation of an
interface can be set only when the interface is encapsulated with PPP. When the
PPP link is down, the IP address originated from negotiation will be deleted.
If the interface has original address, then after setting IP address of the interface to
negotiable, the original IP address will be deleted.
After setting IP address of an interface to negotiable, it is unnecessary to configure
IP address for the interface, as negotiation will automatically originate an IP
address.
After setting IP address of an interface to negotiable, if the interface is set to
negotiable again, then the IP address originated from the original negotiation will be
deleted, and the interface obtains IP address through the re-negotiation.
The interface will have no address after the negotiation address is deleted.

1.2.2 Configuring IP Address Unnumbered for an Interface

I. Introduction to IP Address Unnumbered
The main purpose of borrowing IP address is to save IP address resource.
If an interface has no IP address, it can neither generate any route nor forward any
packet. "IP Address Unnumbered" is used when you want to use an interface with no IP
address. In such case, an IP address will be borrowed from another interface. If the
lending interface has multiple IP addresses, then only the main one can be borrowed.
However, if the lending interface has no IP address, then the IP address of the
borrowing interface is 0.0.0.0. This function is implemented through the command ip
address unnumbered.
The following should be noted:
The borrower cannot be an Ethernet interface.
The address of the lending interface cannot be an unnumbered address.
The lending interface can lend its address tomultiple interfaces.
The address of Loopback interface can be borrowed by other interfaces, but it
cannot borrow the addresses of other interfaces.
Because the borrowing interface has no IP address of its own, and cannot route, two
static routes need to be configured manually to connect routers together. Refer to the
configuration examples for the specific configuration procedure.
3Com Corporation
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Chapter 2 IP Address Configuration