The system loopback interface is the
lo interface created by the system
in every VRF. It cannot be configured, but it is advertised in the state:
vsr> show state vrf main interface system-loopback system-loopback lo mtu 65536 enabled true ipv4 address 127.0.0.1/8 .. ipv6 address ::1/128 .. oper-status UP counters in-octets 37993 in-unicast-pkts 192 in-discards 0 in-errors 0 out-octets 37993 out-unicast-pkts 192 out-discards 0 out-errors 0 .. ..
The command reference for details.