[MX] L3 incomplete drops are seen for bad IPv6 pkt len packets

Take the following steps to identify packets in the MX:

Check which interface shows the L3 incomplete errors

show interfaces et-1/1/9 extensive          
Physical interface: et-1/1/9, Enabled, Physical link is Up
  Interface index: 174, SNMP ifIndex: 590, Generation: 177
  Link-level type: Ethernet, MTU: 1518, MRU: 1526, Speed: 100Gbps, BPDU Error: None, Loop Detect PDU Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Enabled
  Pad to minimum frame size: Disabled
  Device flags   : Present Running
  Interface flags: SNMP-Traps Internal: 0x4000
  CoS queues     : 8 supported, 8 maximum usable queues
  Schedulers     : 0
  Hold-times     : Up 0 ms, Down 500 ms
  Damping        : half-life: 0 sec, max-suppress: 0 sec, reuse: 0, suppress: 0, state: unsuppressed
  Current address: 94:f7:ad:46:d3:a3, Hardware address: 94:f7:ad:46:d3:a3
  Last flapped   : 2020-04-22 16:08:15 PDT (00:29:13 ago)
  Statistics last cleared: 2020-04-22 16:03:54 PDT (00:33:34 ago)
  Traffic statistics:
   Input  bytes  :              8515948                    0 bps
   Output bytes  :              3805508                  200 bps
   Input  packets:                94004                    0 pps
   Output packets:                24722                    0 pps
   IPv6 transit statistics:
   Input  bytes  :              4486040
   Output bytes  :                    0
   Input  packets:                55984
   Output packets:                    0
  Dropped traffic statistics due to STP State:
   Input  bytes  :                    0
   Output bytes  :                    0
   Input  packets:                    0
   Output packets:                    0
  Input errors:
    Errors: 45613, Drops: 0, Framing errors: 0, Runts: 0, Policed discards: 0, L3 incompletes: 45613, L2 channel errors: 0, L2 mismatch timeouts: 0, FIFO errors: 0, Resource errors: 0
  Output errors:
    Carrier transitions: 2, Errors: 0, Drops: 0, Collisions: 0, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, Resource errors:

Check if bad IPv6 packet length errors show under PFE by issuing the commands below in MPC cards. It does not work on DPC cards.

show jnh 2 exceptions terse   

Packet Exceptions
----------------------
bad IPv6 pkt len                   DISC(111)        47813    5068178.
IP options                         PUNT(  2)         4557     483042

Enable debugging under PFE to check the source of the packet:

Enable the traces

a.   start shell pfe network fpc1
b.   debug jnh exceptions-trace 
c.  .debug jnh exceptions 111 discard  

[Apr 22 23:39:40.011] [2992] jnh_exception_packet_trace:1799 [iif:0,code/info:239 D(bad IPv6 pkt len)/0x0,score:(0x0),ptype:3/0,orig_ptype:3,offset:18,orig_offset:18,len:124,l2iif:0,oif:0,BD 0,l2-off=0,token=0 ]
[Apr 22 23:39:40.011] [2993] jnh_exception_packet_trace:1823 0x00: 30 00 ef 00 00 00 00 00 00 12 00 7c 80 00 00 30 
[Apr 22 23:39:40.011] [2994] jnh_exception_packet_trace:1823 0x10: 12 00 00 40 01 00 00 00 00 00 00 00 00 00 00 94 
[Apr 22 23:39:40.011] [2995] jnh_exception_packet_trace:1823 0x20: f7 ad 46 d3 a3 00 10 94 00 00 02 81 00 00 02 86 
[Apr 22 23:39:40.011] [2996] jnh_exception_packet_trace:1823 0x30: dd 60 00 00 00 a5 18 2f ff 20 01 00 00 00 00 00 
[Apr 22 23:39:40.011] [2997] jnh_exception_packet_trace:1823 0x40: 00 00 00 00 00 00 00 00 02 20 10 00 00 00 00 00 
[Apr 22 23:39:40.011] [2998] jnh_exception_packet_trace:1823 0x50: 00 00 00 00 00 00 00 00 01 3b 00 01 04 00 00 00 
[Apr 22 23:39:40.011] [2999] jnh_exception_packet_trace:1823 0x60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
[Apr 22 23:39:40.011] [3000] jnh_exception_
packet_trace:1823 0x70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

Disable the traces

start shell pfe network fpc<>
undebug jnh exceptions

How to identify the source of the packets:

First, check IPv6 ether type [86dd] in the trace. Notice 86dd in the packet

Apr 22 23:39:40.011] [2995] jnh_exception_packet_trace:1823 0x20:  86  
[Apr 22 23:39:40.011] [2996] jnh_exception_packet_trace:1823 0x30: dd 60 00 00 00 a5 18 2f ff 20 01 00 00 00 00 00

Before that, either VLAN-TAG or destination MAC, as per interface configuration. This interface has vlan-id 2 configured in my example router. Hence, we see 8100 for vlan identifier prior to that destination and source MAC addresses seen.
```
VLAN-ID :  81 00 00 02
Source MAC:00 10 94 00 00 02 
Destination MAC: 94 f7 ad 46 d3 a3 
```

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