Loopback interface, Introduction to loopback interface – H3C Technologies H3C S7500E Series Switches User Manual
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Loopback Interface and Null Interface
Configuration
When configuring loopback interfaces and null interfaces, go to these sections for information you are
interested in:
Displaying and Maintaining Loopback and Null Interfaces
Loopback Interface
Introduction to Loopback Interface
A loopback interface is a software-only virtual interface. It delivers the following benefits:
The physical layer state and link layer protocols of a loopback interface are always up unless the
loopback interface is manually shut down.
You can assign a loopback interface an IP address with an all-F mask, thus saving the IP address
resources. When you assign an IPv4 address whose mask is not 32-bit, the system automatically
changes the mask into a 32-bit mask. When you assign an IPv6 address whose mask is not
128-bit, the system automatically changes the mask into a 128-bit mask.
You can enable routing protocols on a loopback interface, and a loopback interface can send and
receive routing protocol packets.
Because of the benefits mentioned above, loopback interfaces are widely used in the following
scenarios:
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. Therefore, when you configure a rule on an authentication
or security server to permit or deny packets generated by a device, you can streamline the rule by
configuring it to permit or deny packets carrying the loopback interface address identifying the
device. Note that, when you use a loopback interface address as the source address of IP packets,
make sure that 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 as packets
sent to the device itself, so the device does not forward these packets.
Because a loopback interface is always up, it can be used in dynamic routing protocols. 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. In BGP, 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.