# interface loopback

#### Syntax

[ no ] interface loopback interfaceSpecifier

#### Release Information

Command introduced before JunosE Release 7.1.0.

#### Description

Defines a loopback interface, which provides a stable address for protocols (for example, BGP, Telnet, or LDP) to use so that they can avoid any impact if a physical interface goes down. The loopback interface sends packets back to the router or access server for local processing. Any packets routed from the loopback interface, but not destined to the loopback interface, are dropped. The no version deletes the loopback interface.

| Note: Do not confuse loopback with the null 0 interface. Traffic routed to null 0 is discarded on the line module. You cannot shut down a loopback interface.

| Best Practice: We recommend that you configure a 32-bit subnet mask for the loopback interface. For example, if you configure a loopback interface with the IP address and mask as 1.1.1.1/16, the 1.1.0.0/16 route entry is entered on the line module and if no specific or longer route entry is found, all traffic destined to the to 1.1.0.0/16 subnet is forwarded to the SRP module by the line module. Although the SRP module responds only to traffic destined to the 1.1.1.1 subnet and discards traffic to all other host IP addresses within that subnet (1.1.1.1/16), if the SRP module receives too much traffic from subnets other than 1.1.1.1, the CPU utilization on the SRP module reaches the saturation level.

#### Options

- interfaceSpecifier—Particular interface; format varies according to interface type; see Interface Types and Specifiers

#### Mode

Global Configuration

#### Related Documentation

- Configuring an MPLS Pseudowire with VCC Cell Relay Encapsulation, Configuring Local ATM Cross-Connects with AAL5 Encapsulation, Configuring Local Cross-Connects Between Ethernet/VLAN Interfaces, Configuring the Loopback Interface and Router ID for BGP for L2VPNs, and Configuring the Loopback Interface and Router ID for VPLS in the JunosE BGP and MPLS Configuration Guide