Dear, asked about some problems that occur on an update of routeros on x86, the first ethernet interface is active when arp = reply-only, at times traffic stops working completely, and sometimes does on some ip, when I put arp = enabled, is solved. The network card is: 82574L Gigabit Network Connection (rev: 0), two ports.
Also in ip-firewall-connections, the connections do not appear as often appeared in version 5.
Waiting for a solution, I wish you a good new year.
arp=reply-only will only answer to entries in ARP list, and no new dynamic entries are going to be added. If entry times out - to that mac address responses will stop.
Also, changing setting will interrupt packet flow for few seconds.
Is it possible that arp mode reply-only is not working in v6rc6? I have exported the configuration of a RouterOS with v5.20 which is working perfectly with some VLAN interfaces configured as reply-only mode (and ARP entries added statically for these VLANs), however when I export this configuration to a new Cloud Core Router (with v6 rc6), everything works fine except for the VLAN interface set as arp reply-only (althought static ARP entries are added). If I set the VLAN as arp enabled, it works but this is a security fault in my system; I would like it to work with ARP reply-only, as it was working on v5.20
Any news about this? Thank you!
Good news! At first sight, it seems that the problem is solved in v6.0rc7 
I have all described problems with ARP on RC9:
http://forum.mikrotik.com/t/arp-static-records-and-arp-reply-only-on-interface-problem/63472/1