ARP entries building up is an old forum thread, know issue, and workaround: ignore and don’t worry, or clear out arp table.
http://forum.mikrotik.com/t/arp-entries-building-up/166300/1
Here is my question: can I determine the STATUS of an ARP entry via SNMP?
reachable, stale, failed, incomplete, or delay?
Sure, I could monitor arp tables via SSH, but we have a script that uses SNMP (faster than SSH) to get ARP table entries.
We use the OID at .1.3.6.1.2.1.4.22.1.4 to ignore all the INVALID entries, but both stale and reachable come back as MediaType 3 (dynamic).
We use .1.3.6.1.2.1.4.22.1.2 to build our view of the ARP table.