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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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