3com S7906E Manuel De Montage

 

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When configuring loopback interfaces and null interfaces, go to these sections for information you are 

interested in: 

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Loopback Interface 

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Displaying and Maintaining Loopback and Null Interfaces 

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 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 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 device itself, so the device 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.   

Follow these steps to configure a loopback interface: 

Enter system view 

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