Any tips on how I might find out more information as to which device is responsible for this?
I'm not knowledgeable (at all) about IPX/DECnet, but am I right to assume that these L3 protocols are encapsulated in ethernet just the same as IP and are arriving on my router's interface and being dropped there for "not being IP"?
If the above is correct, then I think my culprit can be in the entire L2 domain and might perhaps be recognizable by its MAC address. How might I go about and figuring out where the IPX/DECnet is coming from?
For the interested: By polling the OID 188.8.131.52.184.108.40.206.1.15 (ifInUnknownProtos) from the IF-MIB, I was able to check this on more devices since not all supported the CLI output of "unknown protocol drops", but all had the info via SNMP.
It turned out to be UDLD configured on the trunk on the switch side (2950/3550) which the routers (2811/1803/1841) didn't understand and they counted these frames as "unknown protocol drops".
I'd like to point out that UDLD, according to the documentation I have read, operates on Layer2, so it wasn't enough to just look at IPX/DECnet on Layer3.
Thanks for posting back and indicating the information that allowed you to find the solution to your question. It is very helpful to see this aspect of the unknown protocol drops. I believe that it deserves the rating that I gave it.
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