Interface lo0 is up, but logical interface lo0.* Flags ''Down'' . This phenomenon only occurs on loopback interface on Junos 14.1X53-D30.3. The physical interfaces shows normal on Junos 14.1X53-D30.3.
*Junos: 14.1X53-D30.3
interface loopback0
-------------------
{master:0}
user@switch> show interfaces terse
Interface Admin Link Proto Local Remote
ge-0/0/0 up up
ge-0/0/0.0 up up inet 1.1.1.1/24
lo0 up up
lo0.0 up up inet 127.0.0.1 --> 0/0
192.168.140.251 --> 0/0
lo0.1 up up inet 127.0.0.1 --> 0/0
218.251.248.15 --> 0/0
user@switch> show interfaces lo0
Physical interface: lo0, Enabled, Physical link is Up
Interface index: 6, SNMP ifIndex: 6
Type: Loopback, MTU: Unlimited
Device flags : Present Running Loopback
Interface flags: SNMP-Traps
Link flags : None
Last flapped : Never
Input packets : 141714
Output packets: 141714
Logical interface lo0.0 (Index 565) (SNMP ifIndex 16)
Description: -> loopback
Flags: Down <<< SNMP-Traps Encapsulation: Unspecified
Logical interface lo0.1 (Index 566) (SNMP ifIndex 514)
Description: -> [global-VR] loopback
Flags: Down <<< SNMP-Traps Encapsulation: Unspecified
From the PFE (ASIC level):
TFXPC0(vty)# show interface lo0
Physical device lo0 (Index 6, context 0x0)
Administration interface
Device flags: (0x0000000000000103) Present Running Loopback
Flags: (0x0000000000000000) Up
Logical interface lo0.0 (Index 565, Alias-Index 0 Peer-Index 0 ifl address 0x246c04d8)
Channel Mode DISABLED (channel1 0 channel2 0)
Flags: (0x0000000000008052) Shutdown <<< Point-To-Point No-Multicast SNMP-Traps
GEN Flags: (0x0000)
Logical interface lo0.1 (Index 566, Alias-Index 0 Peer-Index 0 ifl address 0x246bd700)
Channel Mode DISABLED (channel1 0 channel2 0)
Flags: (0x0000000000008052) Shutdown <<< Point-To-Point No-Multicast SNMP-Traps
GEN Flags: (0x0000)
Compared to physical port:
*Junos: 14.1X53-D30.3
physical interface ge-0/0/0
-------------------
user@switch# run show interfaces ge-0/0/0
Physical interface: ge-0/0/0, Enabled, Physical link is Up
Interface index: 647, SNMP ifIndex: 1874
Link-level type: Ethernet, MTU: 1514, MRU: 0, Speed: Auto, Duplex: Auto,
BPDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled,
Source filtering: Disabled, Flow control: Disabled, Auto-negotiation: Enabled,
Remote fault: Online, Media type: Copper
Device flags : Present Running
Interface flags: SNMP-Traps Internal: 0x4000
Link flags : None
CoS queues : 12 supported, 12 maximum usable queues
Current address: 5c:45:27:9e:5f:c3, Hardware address: 5c:45:27:9e:5f:c3
Last flapped : 2020-02-02 04:43:51 JST (00:49:01 ago)
Input rate : 248 bps (0 pps)
Output rate : 0 bps (0 pps)
Active alarms : None
Active defects : None
Interface transmit statistics: Disabled
Logical interface ge-0/0/0.0 (Index 549) (SNMP ifIndex 507)
Flags: Up <<< SNMP-Traps 0x4004000 Encapsulation: ENET2
Input packets : 4
Output packets: 5
Protocol inet, MTU: 1500
Flags: Sendbcast-pkt-to-re , Is-Primary
Addresses, Flags: Is-Preferred Is-Primary
Destination: 1.1.1/24, Local: 1.1.1.1, Broadcast: 1.1.1.255
From the PFE (ASIC level):
TFXPC0(vty)# show interface ge-0/0/0
Physical device ge-0/0/0 (Index 647, context 0x215fc448)
PFE: Local, PIC: Local, FPC Slot: 0, Global PFE: 0, Chassis ID: 0
Chassis slot = 0, Type: Gig Ethernet, Port: 0, Subport: 0, pic_i2cid: 0xf0ad
Device flags: (0x0000000000000001) Present
Flags: (0x0000000000008000) Up SNMP-Traps
Logical interface ge-0/0/0.0 (Index 567, Alias-Index 0 Peer-Index 0 ifl address 0x24a47d10)
Channel Mode DISABLED (channel1 4095 channel2 0)
Flags: (0x000000000400c000) Up <<< SNMP-Traps Subset-iif
This has no service impact. It is a cosmetic issue in PFE level. To check loopback interface state, use 'show interface terse' instead.