H3C Technologies H3C S12500 Series Switches - Manual (Page 33)

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Configuring a 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.
• Loopback interfaces can 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
switch generates. Because loopback interface addresses are stable unicast addresses, they are
usually used as switch identifications. Therefore, when you configure a rule on an authentication or
security server to permit or deny packets generated by a switch, you can simplify the rule by
configuring it to permit or deny packets carrying the loopback interface address identifying the
switch. 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 as packets sent to the switch itself, so
the switch 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.
Configuration procedure
To configure a loopback interface:
Step Command Remarks
87. Enter system view. system-view N/A
88. Create a loopback interface
and enter loopback interface
view.
interface loopback
interface-number
Optional.
A loopback interface number is in
the range of 0 to 1023.
89. Set the interface description. description text
Optional.
By default, the description of a
loopback interface is interface
name Interface.
90. Shut down the loopback
interface.
shutdown
Optional.
A loopback interface is up on
being created.