Loopback interface is the most widely used virtual interface and is used on almost every router. Common functions are as follows:
1. As the management address of a router
During remote access, an interface of the router may be down due to a fault. However, the loopback interface will never down. And the loopback interface is not needed to interconnect with the peer end, so its address is usually set to a 32-bit mask, which saves address resources.
2. Use the interface address as the router id of the dynamic routing protocol OSPF and BGP.
During the running of the dynamic routing protocol, OSPF and BGP, you need to specify a Router id for the protocol. The Router id is the unique identifier of the router and must be unique in the entire AS. Because router id is a 32-bit unsigned integer, this is very similar to the IP address. In addition, the IP address is unique. Therefore, the router id of the router is usually specified to be the same as the address of an interface on the device. The IP address of the loopback interface is generally regarded as the identifier of the router. Therefore, the IP address of the loopback interface is the best choice for the router id.
3. Use the interface address as the mpls lsr-id.
4. Use the IP address of the interface as the source address for BGP to establish a TCP connection.
5. IP Unnumbered Interfaces
An unnumbered address can borrow a loopback interface address to save network IP addresses.
6. tftp-server/sftp-server/ftp-server address
7. SNMP-SERVER Access
8. TACACS/RADIUS-Server Source Interface
9. NTP Source Interface
10. SYSLOG Source Interface
11、RCMD to the router
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