H3C Technologies H3C S10500 Series Switches - Manual (Page 30)

19
Loopback and null interface configuration
Configuring a loopback interface
Introduction to the loopback interface
A loopback interface is a software-only virtual interface. It delivers the following benefits.
• The physical layer state and link-layer protocols of a loopback interface are always up unless the
loopback interface is manually shut down.
• To save IP address resources, you can assign an IP address with an all-F mask to a loopback
interface. When you assign an IPv4 address whose mask is not 32-bit, the system automatically
changes the mask into a 32-bit mask. When you assign an IPv6 address whose mask is not 128-bit,
the system automatically changes the mask into a 128-bit mask.
• You can enable routing protocols on a loopback interface, and a loopback interface can send and
receive routing protocol packets.
Because of the benefits mentioned above, loopback interfaces are widely used in the following
scenarios.
• You can configure a loopback interface address as the source address of the IP packets that the
device generates. Because loopback interface addresses are stable unicast addresses, they are
usually used as device identifications. When you configure a rule on an authentication or security
server to permit or deny packets generated by a device, you can simplify the rule by configuring it
to permit or deny packets that carry the loopback interface address identifying the device. When
you use a loopback interface address as the source address of IP packets, be sure to perform any
necessary routing configuration to ensure that the route from the loopback interface to the peer is
reachable. All data packets sent to the loopback interface are treated as packets sent to the device
itself, so the device does not forward these packets.
• Because a loopback interface is always up, it can be used in dynamic routing protocols. For
example, if you do not configure a router ID for a dynamic routing protocol, the highest loopback
interface IP address is selected as the router ID. In BGP, to prevent physical port failure from
interrupting BGP sessions, you can use a loopback interface as the source interface of BGP packets.
Configuration procedure
Follow these steps to configure a loopback interface:
To do… Use the command… Remarks
Enter system view system-view —
Create a loopback interface and
enter loopback interface view interface loopback
interface-number
—
Set the interface description description text
Optional
By default, the description of a loopback
interface is interface name Interface.