This article explains how to troubleshoot if we are getting Interface Input Errors registered as Runts Errors.

• Interface shows Input errors incrementing and the Runts packet counter incrementing.

Physical interface: ge-0/0/27, Enabled, Physical link is Up
     Interface index: 673, SNMP ifIndex: 545, Generation: 164
     Link-level type: Ethernet, MTU: 1514, LAN-PHY mode, Link-mode: Half-duplex, Speed: 10mbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled, Remote fault: Online, Media type: Copper, IEEE 802.3az Energy Efficient Ethernet: Disabled, Auto-MDIX: Enabled
     Device flags   : Present Running
     Interface flags: SNMP-Traps Internal: 0x0
     Link flags     : None
     CoS queues     : 12 supported, 12 maximum usable queues
     Hold-times     : Up 500 ms, Down 0 ms
     Current address: 24:fc:4e:c1:e7:de, Hardware address: 24:fc:4e:c1:e7:de
     Last flapped   : 2024-08-28 08:00:05 UTC (12w1d 20:40 ago)
     Statistics last cleared: Never
     Traffic statistics:
      Input  bytes  :             11494388                    0 bps
      Output bytes  :            204047466                   88 bps
      Input  packets:               115330                    0 pps
      Output packets:              1629063                    0 pps
      IPv6 transit statistics:
      Input  bytes  :                    0
      Output bytes  :                    0
      Input  packets:                    0
      Output packets:                    0
     Input errors:
       Errors: 3853, Drops: 0, Framing errors: 0, Runts: 3853, Policed discards: 0, L3 incompletes: 0, L2 channel errors: 0, L2 mismatch timeouts: 0, FIFO errors: 0, Resource errors: 0

• Runt packets are the frames which are smaller than the Runt threshold in size.
• For Juniper devices the Runt threshold will be packets which are (< 64 bytes) in size. This frame include the header, payload and crc.
• Typically these frames are invalid and are dropped by the device.

To find out more about the Runt packets we may do a port mirror and then identify the Runt frames and based on the investigation the necessary action can be taken to stop runt input counter from incrementing.

We may have to check the remote device and understand why we are receiving the runt frames on the interface.