After upgrading to RouterOS v6.6 on two 1100 AHx2, I found that I was not able to ping certain interfaces. The investigation revealed that the ARP cache on the switches were learning 00-00-00-00-00-00 MAC addresses dynamically for other RouterOS v6.6 interfaces, and so were clients. As seen by /ip arp print.

A network sniffer showed that the RouterOS v6.6 router was generating an ARP REPLY with 00-00-00-00-00-00 as the MAC address for its interface IP when it received an ARP request, pretty bad, and very quickly pulled the network apart...

Force downgrading the RouterOS to v6.2 resolved the issue. No more 00-00-00-00-00-00 MAC addresses in the arp table, or in ARP Reply messages from the RouterOS devices.

I also note that VRRP interfaces do not show up after a software upgrade. In fact, '/interface vrrp export' would indicate no vrrp interfaces defined. To fix I simply had to reboot the switch again, then the VRRP interfaces showed up again. Another way of 'returning' these interfaces was to re-add one of them, then RouterOS would say that it was already present, and magically 'restore/return' the other VRRP interfaces. I would note that I have seem this VRRP 'bug' over a number of RouterOS versions now...

However, the ARP reply problem is a pretty major issue...

I would suggest contacting support@mikrotik.com directly with this issues.

If you provide them with a supout and wireshark logs, they should fix it in next RC.