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Configuring loopback, null, and
inloopback interfaces
This chapter describes how to configure a loopback interface, a null interface, and an inloopback
interface.
About loopback, null, and inloopback interfaces
About loopback interfaces
A loopback interface is a virtual interface. The physical layer state of a loopback interface is always
up unless the loopback interface is manually shut down. Because of this benefit, loopback
interfaces are widely used.
Typically 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, you can configure it to permit or deny packets carrying the loopback interface
address of a device. This simplifies your configuration and achieves the effect of permitting or
denying packets that the device generates. To use a loopback interface address as the source
address of IP packets, make sure the loopback interface is reachable from the peer by performing
routing configuration. All data packets sent to the loopback interface are considered packets sent to
the device itself, so the device does not forward these packets.
About null interfaces
A null interface is a virtual interface and is always up, but you cannot use it to forward data packets
or configure it with an IP address or link layer protocol. The null interface provides a simpler way to
filter packets than ACL. You can filter undesired traffic by transmitting it to a null interface instead of
applying an ACL. For example, if you specify a null interface as the next hop of a static route to a
network segment, any packets routed to the network segment are dropped.
About inloopback interfaces
An inloopback interface is a virtual interface created by the system, which cannot be configured or
deleted. The physical layer and link layer protocol states of an inloopback interface are always up.
All IP packets sent to an inloopback interface are considered packets sent to the device itself and
are not forwarded.
Configuring a loopback interface
1.
Enter system view.
system-view
2.
Create a loopback interface and enter loopback interface view.
interface loopback interface-number
3.
Configure the interface description.
description text
The default setting is interface name Interface (for example, LoopBack0 Interface).
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