H3C WA Series, Configuration Manual

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5
Loopback and Null Interface Configuration
This chapter includes these sections:
z
Loopback Interface
z
Null Interface
z
Displaying and Maintaining Loopback and Null Interfaces
Loopback Interface
Introduction to Loopback Interface
A loopback interface is a software-only 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. A loopback
interface can be configured with an IP address. For the purpose of saving IP address resources, the IP
address is automatically configured with a 32-bit mask. Routing protocols can be enabled on a loopback
interface, and a loopback interface is capable of sending and receiving routing protocol packets.
Loopback interfaces are widely used. For example, you can configure a loopback interface address as
the source address of all the IP packets that the AP generates. Because loopback interface addresses
are stable unicast addresses, they are usually used as device identifications. Therefore, when you
configure a rule on an authentication or security server to permit or deny packets generated by an AP,
you can streamline the rule by configuring it to permit or deny packets carrying the loopback interface
address identifying the AP. Note that, when a loopback interface is used for source address binding
(that is, assigning an IP address to this loopback interface), make sure that the route from the loopback
interface to the peer is reachable; all data packets sent to the loopback interface are considered as
packets sent to the AP itself, so the AP does not forward these packets.
Because a loopback interface is always up, it can be used for some other special purposes. For
example, if no router ID is configured for a dynamic routing protocol, the highest loopback interface IP
address is selected as the router ID. For another example, to avoid BGP sessions being interrupted by
physical port failure, you can use a loopback interface as the source interface of BGP packets. However,
you must ensure that the IP address of the loopback interface on the BGP peer is reachable. If EBGP
connection is involved, you need to allow the establishment of EBGP connections to
non-directly-connected peers.
Configuring a Loopback Interface
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
—