This article describes a scenario in which L3 incomplete are observed on devices due to an incorrect label value of an MPLS packet.

You may observe frequent increments in L3-incomplete over an interface due to malformed MPLS packets. L3 incompletes are defined as the number of packets discarded because they fail Layer 3 header checks. This counter increases when incoming packets fail Layer 3 (usually IPv4) checks in the header.

In this particular case, we were receiving IPv6 packets with an explicit null value of 0.

L3 incomplete can be easily verified with the command: show interfaces XE-x/y/z extensive | match "L3 incomplete"

Dump the incrementing exception, which can be captured from the FPC shell using the command: show jnh X exceptions terse X >>>> PFE instance.

From the decoded packet, it's evident that the packet type is MPLS, arriving with an explicit label of 0, yet it carries an IPv6 packet. In IPv6, the explicit null value is 2. Since we're receiving this from a Cisco device, we suggested the customer investigate with Cisco why they're sending IPv6 packets with an explicit null value of 0.

 Logs:

[May 9 21:28:02.795] [5848721785] jnh_exception_packet_trace:1456 ###############

[May 9 21:28:02.795] [5848721786] jnh_exception_packet_trace:1461 [iif:394,code/info:139 D(bad IPv4 hdr)/0x0,score:(0x0),ptype:2/0,orig_ptype:2,offset:18,orig_offset:18,len:366,l2iif:0,oif:0,BD 0,l2-off=0,token=0 ]

[May 9 21:28:02.795] [5848721787] jnh_exception_packet_trace:1485 0x00: 20 00 8b 00 00 00 01 8a 00 12 01 6e 80 00 00 20 

[May 9 21:28:02.795] [5848721788] jnh_exception_packet_trace:1485 0x10: 12 00 00 40 01 00 00 00 00 00 00 00 00 00 00 40 

[May 9 21:28:02.795] [5848721789] jnh_exception_packet_trace:1485 0x20: 8f 9d 8e 81 18 00 bc 60 67 fc 8a 88 47 00 00 0b 

[May 9 21:28:02.795] [5848721790] jnh_exception_packet_trace:1485 0x30: 29 6b 80 ff 0f 01 34 11 39 26 03 10 a0 0c 07 08 

[May 9 21:28:02.795] [5848721791] jnh_exception_packet_trace:1485 0x40: 1d 00 00 00 00 19 77 2a d3 26 03 10 a0 0c 07 08 

[May 9 21:28:02.795] [5848721792] jnh_exception_packet_trace:1485 0x50: 1d 00 00 00 00 19 77 af c7 00 00 00 00 01 34 80 

[May 9 21:28:02.795] [5848721793] jnh_exception_packet_trace:1485 0x60: f7 2a 2a 46 65 72 72 65 74 2a 2a 4e 65 74 53 63 

[May 9 21:28:02.795] [5848721794] jnh_exception_packet_trace:1485 0x70: 61 6e 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 35 

[May 9 21:28:02.822] [5848721795] jnh_exception_packet_trace:1456 ###############

 

Actual packet – 

 

40 8f 9d 8e 81 18 00 bc 60 67 fc 8a 88 47 00 00 0b 

29 6b 80 ff 0f 01 34 11 39 26 03 10 a0 0c 07 08 

1d 00 00 00 00 19 77 2a d3 26 03 10 a0 0c 07 08 

1d 00 00 00 00 19 77 af c7 00 00 00 00 01 34 80 

f7 2a 2a 46 65 72 72 65 74 2a 2a 4e 65 74 53 63 

61 6e 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 35

 

Decoded packet- 

Best