H3C S10500X Series Configuration Manual page 38

Configuring loopback, null, and
inloopback interfaces
This chapter describes how to configure a loopback interface, a null interface, and an inloopback
interface.
Configuring a loopback interface
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 to permit or deny

packets that a device generates, you can simplify the rule by configuring it to permit or
deny packets carrying the loopback interface address that identifies the device.
When you use a loopback interface address as the source address of IP packets, make

sure the route from the loopback interface to the peer is reachable 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.
To configure a loopback interface:
Step
1. Enter system view.
2. Create a loopback interface
and enter loopback interface
view.
3. Configure the interface
description.
4. Configure the expected
bandwidth of the loopback
interface.
5. Restore the default settings
for the loopback interface.
6. Bring up the loopback
interface.
Configuring a null interface
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
Command
system-view
interface loopback
interface-number
description text
bandwidth bandwidth-value
default
undo shutdown
1
Remarks
N/A
N/A
The default setting is interface name
Interface (for example, LoopBack1
Interface).
By default, the expected bandwidth
of a loopback interface is 0 kbps.
N/A
By default, a loopback interface is
up.