H3C Technologies H3C SR8800 - Manual (Page 63)

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Configuring Loopback and null interfaces
Loopback interface
Introduction
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 must assign a loopback interface an IP address with an all-F mask to save 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 simplify 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 unless you manually shut it down, 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.
Configuring a loopback interface
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 The loopback interface number
ranges from 0 to 1023.