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CCNA 7.3.f: Loopback

Overview:

A loopback interface is a logical, virtual interface that is always up

 

Study Notes:

  • A loopback interface is a logical, virtual interface
  • A loopback interface is always up/up once configured
  • A loopback interface can be used to emulate a physical interface
  • Any IP address can be assigned to a loopback interface
  • Typically, a loopback interface is configured with a /32 subnet mask
  • Loopback interfaces are not configured or enabled by default
  • Common uses for loopback interfaces include:
    • Router-id (for DR/BDR election in OSPF)
    • As an advertised address in routing protocols (it will never go down like a physical interface)
    • Using ip unnumbered on serial links (ip address is shared by serial link and loopback)
    • As a management address (so you can get to it via multiple physical interfaces)
    • For monitoring a device (more accurately shows a device as up or down than monitoring by physical interface IPs)
    • To test routing protocols and/or simulate networks

Configure a loopback interface:

Router(config)#interface loopback 1
Router(config-if)#ip address 1.1.1.1 255.255.255.255

 

PacketTracer Lab: CCNA-7.3.f-Loopback.pkt

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