Description

Why is the "L3 incompletes" counter incrementing on the interface?
 

Symptoms

When viewing show interfaces <intf_name> extensive , " L3 incompletes " output, the counters may increase over time.

> show interfaces ge-0/0/0 extensive    
Physical interface: ge-0/0/0, Enabled, Physical link is Up
  Interface index: 132, SNMP ifIndex: 622, Generation: 133
  Link-level type: Ethernet, MTU: 1514, Speed: 1000mbps, Loopback: Disabled, Source filtering: Disabled, Flow control: Enabled,
  Auto-negotiation: Enabled, Remote fault: Online
  Device flags   : Present Running
  Interface flags: SNMP-Traps Internal: 0x4000
  Link flags     : None
  CoS queues     : 4 supported, 4 maximum usable queues
  Hold-times     : Up 0 ms, Down 0 ms
  Current address: 00:90:69:4a:94:00, Hardware address: 00:90:69:4a:94:00
  Last flapped   : 2008-09-30 11:47:42 EST (4d 06:40 ago)
  Traffic statistics:
   Input  bytes  :         527915033990              6046912 bps
   Output bytes  :         486972641696              5435312 bps
   Input  packets:           1767601016                 1946 pps
   Output packets:           1714506848                 1840 pps
  Input errors:
    Errors: 241151, Drops: 0, Framing errors: 0, Runts: 0, Policed discards: 4020, L3 incompletes: 241151, L2 channel errors: 0, 
    L2 mismatch timeouts: 0, FIFO errors: 0, Resource errors: 0
  Output errors:
    Carrier transitions: 0, Errors: 0, Drops: 0, Collisions: 0, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, 
    Resource errors: 0

Solution

The "L3 incompletes" counter is incremented when the incoming IP packet (IP Ethertype 0x0800) fails the L3 sanity checks as follow:

  1. Checksum error : checksum value is not correct.
  2. Packet length error : Real packet length is less that total_packet_length of IP header
  3. Packet header error : Other packet header error, for example incorrect Version value, etc...
For example, a frame with less than 20 bytes of available IP header would be ditched and the counter incremented.
In most cases, this is an indication that the interface is receiving corrupted IP packets from other devices on the same Ethernet segment.

Other reason for the counter incrementing could be that the interface is connected to a Cisco device,   Cisco devices can send CTP (Configuration Test Protocol Ethertype 0x9000) packet which is not IEEE 802.3 standard.  JUNOS drops these packets with "L3 incompletes" counters.

In order to stop "L3 incompletes" counters increasing, you can either:
  1. Configure Cisco side to disable keepalives
  2. Configure Juniper device side to ignore this type of errors using option " ignore-l3-incompletes "
     

Modification History

2019-12-19: Added link to ignore-l3-incompletes option.

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