A basic troubleshooting checklist for troubleshooting Ethernet Physical Interfaces on M/MX/T Series devices.

To troubleshoot VLAN or IRB interfaces, refer to KB26487: Troubleshooting Checklist - VLAN & Bridging [juniper.net] and KB26488: Troubleshooting Checklist - IRB Interface [juniper.net].
 

• Troubleshoot Ethernet Physical Interface
• Ethernet interface is down

Use the following checklist to troubleshoot Ethernet interfaces:

1. Check if the correct cable is connected to the correct port (surprisingly, but a common mistake). 

   Note: Most routers require crossover copper cables, but newer chassis like MX have auto-MDI.

2. Verify the physical link status of the interface with the ' show interface <interface-name> media ' command:

   user@router> show interfaces ge-5/0/1 media 
   Physical interface: ge-5/0/1, Enabled, Physical link is Up
     Interface index: 317, SNMP ifIndex: 1602
     Link-level type: Ethernet, MTU: 1514, Speed: 1000mbps, BPDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled,
     Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled, Remote fault: Online, Speed-negotiation: Disabled,
     Auto-MDIX: Enabled
     Device flags   : Present Running
     Interface flags: SNMP-Traps Internal: 0x4000
     Link flags     : None
     CoS queues     : 8 supported, 8 maximum usable queues
     Current address: 2c:6b:f5:xx:xx:xx, Hardware address: 2c:6b:f5:xx:xx:yy
     Last flapped   : 2012-11-30 01:25:37 UTC (03:46:55 ago)
     Input rate     : 880 bps (1 pps)
     Output rate    : 312 bps (0 pps)
     Active alarms  : None
     Active defects : None
     MAC statistics:
       Input bytes: 901296, Input packets: 9799, Output bytes: 976587, Output packets: 10451
     Filter statistics:
       Filtered packets: 68, Padded packets: 0, Output packet errors: 0
     Autonegotiation information:
       Negotiation status: Complete
       Link partner:
           Link mode: Full-duplex, Flow control: Symmetric/Asymmetric, Remote fault: OK
       Local resolution:
           Flow control: Symmetric, Remote fault: Link OK
     Interface transmit statistics: Disabled
   

   If the Ethernet interface has a cable connected and is receiving a valid signal, the physical link shows an “up” status, and there are no active alarms or defects. If the interface is not connected or is not receiving a valid signal, the output shows a physical link down status and an active LINK alarm and defect.

   For more information on the above command output, refer to the technical documentation on show interfaces.

   If the physical link is down, confirm that the cable is connected properly on the local and remote ends. If you suspect that the Ethernet cable is bad, swap the cable for a known good one.

   If this is a fiber interface, make sure you have single-mode fiber cable for a single-mode interface and multimode fiber cable for a multimode interface. Make sure you use the correct SFP on both sides.

3. Review the detailed interface statistics.

   If the Ethernet interface is up, but you are experiencing connectivity problems such as input errors and packet loss, use the ' show interface interface-name extensive ' to display the detailed interface statistics.

   For a detailed explanation of the statistics, refer to the technical documentation on the show interfaces.

   user@router> show interfaces ge-5/0/1 extensive        
   Physical interface: ge-5/0/1, Enabled, Physical link is Up
     Interface index: 317, SNMP ifIndex: 1602, Generation: 322
     Link-level type: Ethernet, MTU: 1514, Speed: 1000mbps, BPDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled,
     Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled, Remote fault: Online, Speed-negotiation: Disabled,
     Auto-MDIX: Enabled
     Device flags   : Present Running
     Interface flags: SNMP-Traps Internal: 0x4000
     Link flags     : None
     CoS queues     : 8 supported, 8 maximum usable queues
     Hold-times     : Up 0 ms, Down 0 ms
     Current address: 2c:6b:f5:xx:xx:xx, Hardware address: 2c:6b:f5:xx:xx:xy
     Last flapped   : 2012-11-30 01:25:37 UTC (04:38:32 ago)
     Statistics last cleared: Never
     Traffic statistics:
      Input  bytes  :               806283                    0 bps
      Output bytes  :              1153215                  424 bps
      Input  packets:                10818                    0 pps
      Output packets:                11536                    0 pps
      IPv6 transit statistics:
       Input  bytes  :                   0
       Output bytes  :                   0
       Input  packets:                   0
       Output packets:                   0
     Label-switched interface (LSI) traffic statistics:
      Input  bytes  :                    0                    0 bps
      Input  packets:                    0                    0 pps
     Dropped traffic statistics due to STP State:
      Input  bytes  :                    0
      Output bytes  :                    0
      Input  packets:                    0
      Output packets:                    0
     Input errors:
       Errors: 0, Drops: 0, Framing errors: 0, Runts: 0, Policed discards: 233060, L3 incompletes: 0, L2 channel errors: 0,
       L2 mismatch timeouts: 0, FIFO errors: 0, Resource errors: 0
     Output errors:
       Carrier transitions: 11, Errors: 0, Drops: 0, Collisions: 0, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0,
       MTU errors: 0, Resource errors: 0
     Egress queues: 8 supported, 4 in use
     Queue counters:       Queued packets, Transmitted packets      Dropped packets
       0 best-effort                 3216                 3216                    0
       1 expedited-fo                   0                    0                    0
       2 assured-forw                   0                    0                    0
       3 network-cont                8320                 8320                    0
     Queue number:         Mapped forwarding classes
       0                   best-effort
       1                   expedited-forwarding
       2                   assured-forwarding
       3                   network-control
     Active alarms  : None
     Active defects : None
     MAC statistics:                      Receive         Transmit
       Total octets                       1007655          1082219
       Total packets                        10886            11536
       Unicast packets                       4350             4184
       Broadcast packets                       32               77
       Multicast packets                     6504             7275
       CRC/Align errors                         0                0
       FIFO errors                              0                0
       MAC control frames                       0                0
       MAC pause frames                         0                0
       Oversized frames                         0
       Jabber frames                            0
       Fragment frames                          0
       VLAN tagged frames                       0
       Code violations                          0
     Filter statistics:
       Input packet count                   10886
       Input packet rejects                    68
       Input DA rejects                        68
       Input SA rejects                         0
       Output packet count                                   11536
       Output packet pad count                                   0
       Output packet error count                                 0
       CAM destination filters: 0, CAM source filters: 0
     Autonegotiation information:
       Negotiation status: Complete
       Link partner:
           Link mode: Full-duplex, Flow control: Symmetric/Asymmetric, Remote fault: OK
       Local resolution:
           Flow control: Symmetric, Remote fault: Link OK
     Packet Forwarding Engine configuration:
       Destination slot: 5
     CoS information:
       Direction : Output
       CoS transmit queue               Bandwidth               Buffer Priority   Limit
                                 %            bps     %           usec
       0 best-effort            95      950000000    95              0      low    none
       3 network-control         5       50000000     5              0      low    none
     Interface transmit statistics: Disabled

   The following statistics are worth mentioning:

   Usually, the error statistics do not pinpoint exactly the source of the connectivity problem.

   Important: Isolating the source of the problem typically requires you to verify the local Ethernet port (local loopback test), the cabling, and the remote Ethernet port.  These are explained in steps 4-6.

   • Policed discards: Frames discarded that were not recognized or not of interest. For example, Cisco CDP packets.
   • L2 channel errors: Packets for which the router could not find a valid logical interface.  For example, when the packet is for a VLAN that is not configured on the interface.
   • Input DA rejects: Number of packets with a destination MAC address that is not on the accept list.  It is normal to see this number increment.
   • Input SA rejects: Number of packets with a source MAC address that is not on the accept list.  You should see this increment only when source MAC address filtering is configured.
   • Drops: Number of packets dropped by the input queue of the I/O Manager ASIC. If the interface is saturated, this number increments once for every packet that is dropped by the ASIC's RED mechanism.
   • Framing errors: Number of packets received with an invalid frame checksum (FCS).
   • Runts: Number of received frames that are smaller than the runt threshold.
   • L3 incompletes: Number of incoming packets discarded, as they have failed Layer 3 sanity checks for the headers. For example, a frame with less than 20 bytes of available IP header is discarded.
   • L2 mismatch timeouts: Number of malformed or short packets that caused the incoming packet handler to discard the frame as unreadable.
   • FIFO errors: Number of FIFO errors in the receive direction that are reported by the ASIC on the PIC. If this value is ever non-zero, the PIC is probably malfunctioning.
   • Resource errors: Sum of transmit drops.
   • Carrier transitions: Number of times that the interface has gone from down to up. This number does not normally increment quickly; increasing only when the cable is unplugged, the far-end system is powered down and then up, or another issue occurs. If the number of carrier transitions increments quickly (perhaps once every 10 seconds), the cable, far-end system, PIC  (Physical Interface Cards), or PIM (Physical Interface Module) is malfunctioning.
   • Collisions: The number of Ethernet collisions. The Gigabit Ethernet PIC supports only full-duplex operation, so for Gigabit Ethernet PICs, this number should always remain 0. If it is non-zero, there is a software bug.
   • Aged packets: The number of packets that remained in the shared packet SDRAM for so long that the system automatically purged them. The value in this field should never increment. If it does, it is most likely a software bug or possibly malfunctioning hardware.
   • FIFO errors: The number of FIFO errors in the send direction, as reported by the ASIC on the PIC. If this value is ever non-zero, the PIC is probably malfunctioning.
   • HS link CRC errors: The number of errors on the high-speed links between the ASICs, which are responsible for handling the router interfaces.
   • MTU errors: The number of packets whose size exceeded the interface MTU.
   • Resource errors: Sum of transmit drops.

4. In order to test if the Ethernet port or PIC is faulty, perform a local loopback test (software and/or hardware).

   Software loopback test:

   Include the loopback statement at the [gigether-options] hierarchy level in order to configure an internal loopback on the interface and conduct a loopback test. This is used to verify the functionality of the local Ethernet port on the router.

   When an internal loopback is configured, the physical link should come up with no alarm. 

   [edit interfaces ge-5/0/1]
   root@router# set gigether-options loopback
   [edit interfaces ge-5/0/1]
   root@router# commit
   commit complete
   
   [edit interfaces ge-5/0/1]
   lab@Rack1-MX480> show interfaces ge-5/0/1 media 
   Physical interface: ge-5/0/1, Enabled, Physical link is Up
     Interface index: 317, SNMP ifIndex: 1602
     Link-level type: Ethernet, MTU: 1514, Speed: 1000mbps, BPDU Error: None, MAC-REWRITE Error: None, Loopback: Enabled,
     Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled, Remote fault: Online, Speed-negotiation: Disabled,
     Auto-MDIX: Enabled
     Device flags   : Present Running
     Interface flags: SNMP-Traps Internal: 0x4000
     Link flags     : None
     CoS queues     : 8 supported, 8 maximum usable queues
     Current address: 2c:6b:f5:xx:xx:xx, Hardware address: 2c:6b:f5:xx:xx:xy
     Last flapped   : 2012-11-30 01:25:37 UTC (03:46:55 ago)
     Input rate     : 880 bps (1 pps)
     Output rate    : 312 bps (0 pps)
     Active alarms  : None
     Active defects : None
     MAC statistics:
       Input bytes: 901296, Input packets: 9799, Output bytes: 976587, Output packets: 10451
     Filter statistics:
       Filtered packets: 68, Padded packets: 0, Output packet errors: 0
     Autonegotiation information:
       Negotiation status: Complete
       Link partner:
           Link mode: Full-duplex, Flow control: Symmetric/Asymmetric, Remote fault: OK
       Local resolution:
           Flow control: Symmetric, Remote fault: Link OK
     Interface transmit statistics: Disabled
   

   Note:  Remember to delete the loopback statement after completing the test.

   Hardware loopback test:

   If the local Ethernet port is a fiber interface or an RJ-45 connector, you can do a physical loopback:

   For more information, refer to the technical documentation on Performing Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces

   • For an Ethernet PIC with a fiber optic interface, you can physically loop the TX and RX ports and check the status of the physical link.

   • For an RJ-45 Ethernet interface, you can make a loopback plug by crossing pin 1 (TX +) to pin 3 (RX +) together and pin 2 (TX -) to pin 6 (RX -) together.

5. Verify the integrity of the cabling.

   To do this, you must use an external cable-testing tool.

   Important:  Before doing this, verify that you have the correct cables for the interface port. Make sure you have a single-mode fiber cable for a single-mode interface and a multi-mode fiber cable for a multi-mode interface.

   For a fiber optic connection, you can measure the received light level at the receiver end to make sure that it is within the receiver specification of your Ethernet interface.

   You can also measure the transmit light level on the TX port to verify that it is within specification.

6. If this Ethernet interface is connected to the remote Ethernet device over multiple patch panels, check to see whether the connection can be looped back at the different patch panels so you can conduct a loopback test.

For more information, refer to the technical documentation on Performing Loopback Testing for Fast Ethernet and Gigabit Ethernet InterfacesRefer.

      7. Check if any L2 transport equipment/network is involved in between and ask the customer to check for any errors, alarms in their L2 transport.

8. If there are no issues with the physical interface, check the following common reasons:
   • Check if physical interface and logical interfaces are not administratively disabled, using the command ' show interfaces <interface> terse ':

user@router> show interfaces ge-5/0/1 terse 

     Interface               Admin Link Proto    Local                 Remote

     ge-5/0/1                up    up

     ge-5/0/1.0              up    up   inet     a.b.c.d/24     

8. • Verify that the speed, duplex, and auto-negotiation settings are configured correctly for the interface.
     Tip: Specifically check if the hardware you're using, like FPC/DPC/MPC, MIC/PIC, or SFP/XFP, supports auto-negotiation and speed settings.
   • For additional tips, refer to the technical documentation on Monitoring Fast Ethernet and Gigabit Ethernet Interfaces
9. Check the Hardware Compatibility Tool, identify if the transceiver used is compatible with your device. If the transceiver used isn't listed, please read the article: KB69868 [juniper.net].
10. Reseat the transceiver and clean it, or swap it for a known working transceiver. Do this at both ends.
11. If, after all the above troubleshooting, the port is still down, there may be a hardware problem; you may need to contact JTAC so they can determine if an RMA is needed for the part.

2020-06-12:​ ​Added EX and QFX product series. Article also reviewed for accuracy.

2025-11-13:​ ​Added MX2020 to the categories
2025-12-22: removed MAC/IP details 

2026-02-25: Corrected minor issues