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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