JunOS ‘L3 incompletes’, what and why?
There is quite often chatter about L3 incompletes, and it seems there are lot of opinions what they are. Maybe some of these opinions are based on some particular counter bug in some release. Juniper has introduced also toggle to allow stopping the counter from working. It seems very silly to use this toggle, as it is really one of the few ways you can gather information about broken packets via SNMP.
What they (at least) are not
- Unknown unicast
- CDP
- BPDU
- Packet from connected host which does not ARP
- Packet from unconfigured VLAN
What they (at least) are
- IP header checksum error
- IP header error (impossibly small IHL, IP version 3, etc)
- IP header size does not match packet size
Troubleshooting
So if you are seeing them, what can you do? As it is aggregate counter for many different issues, how do you actually know which one is it and is there way to figure out who is sending them? Luckily for Trio based platforms answers and highly encouraging, we have very good tools to troubleshoot the issue.
To figure out what they exactly are, first you need to figure out your internal IFD index (not snmp ifindex)
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