H3C S6850 Series, Configuration Manual

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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 in the following scenarios:
•
Configuring 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.
•
Using a loopback interface in dynamic routing protocols —With no router ID configured for
a dynamic routing protocol, the system selects the highest loopback interface IP address as
the router ID. In BGP, to avoid interruption of BGP sessions due to physical port failure, you
can use a loopback interface as the source interface of BGP 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.