I realise this is an old thread - but in a hope to assist others.

I found the issue resolved, by SSH'ing to the router, in this case a 2830 and issuing:

ip arp accept 1
ip arp accept 3
sys commit

By default acceptance of illegal source and destination mismatched ARP's is disabled.

Vigor2830> ip arp accept status
Accept illegal source mac arp: disable
Accept illegal dest mac arp: disable

Vigor2830> ip arp accept ?
Ethernet source/dest address doesn't match ARP sender/receiver mac address.
Accept illegal ARP REPLY packets or not.
%% ip arp accept <0/1/2/3/status>
%% 0: disable to accept illegal source mac address
%% 1: enable to accept illegal source mac address
%% 2: disable to accept illegal dest mac address
%% 3: enable to accept illegal dest mac address

This has resolved my issues. For reference:

http://www.draytek.co.uk/support/kb_vigor100_setup.html (Additional Setup for MPoA section)

http://support.sharedband.com/kb/kb3/Turn+off+Anti-Spoofing+Detection+on+DrayTEK+Routers