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Configuring loopback and null interfaces
This chapter describes how to configure a loopback interface and a null interface.
Configuring a loopback interface
A loopback interface is a virtual interface. The physical layer state and link layer protocols of a loopback
interface are 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 highest loopback interface IP address selected 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 Command
Remarks
1.
Enter system view.
system-view
N/A
2.
Create a loopback interface
and enter loopback interface
view.
interface loopback
interface-number
N/A
3.
Set the interface description.
description
text
The default setting is
interface name
Interface
(for example,
LoopBack1
Interface
).
4.
Restore the default settings for
the loopback interface.
default
N/A
5.
Bring up the loopback
interface.
undo shutdown
By default, a loopback interface is up.
Configuring a null interface
A null interface is a virtual interface and is always up, but you can neither use it to forward data packets
nor can you 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
specific network segment, any packets routed to the network segment are dropped.
