BUG: RouterOS v4.6 on RB750 drops frames with certain MAC

RouterOS General
1

Recently has installed the RouterOS v4.6 on my RB750. After reboot I noticed, that one of my computers is unable to connect to RB750 (neither with winbox, nor with telnet, pinging doesn’t work too - the MAC-address of RB750 doesn’t resolve with ARP). After doing some experiments I stated, that any computer having 00, 01, 02, 03 or 04 in the 2nd octet of its MAC-address (00:13:76:28:00:06, for example) is affected. Changing this address to 00:13:76:28:05:06 makes it work. Writing the 00:13:76:28:00:06 address to another machine leads to similar connection loss. Changing any other nibble wasn’t helpful. No MAC-filtering was made - the device is just reset to default configuration.

This was observed on RB750 with RouterOS 4.6 only - downgrading to 4.5 solves the problem. Another boards (RB600, RB411, RB433) wasn’t known to produce such a bug.

To wait or not to continue ROS v4.x?..
2

Yes, we have some problem here. Looks like this problem only happen on RB750s, I haven’t seen any other board have this issue.

3

Wrote to MikroTik’s support. They couldn’t reproduce it, and asked for additional information

4

Have the same problem with v4.6 on RB750. RB just keeps sending who-was arp.

tcpdump snapshot:

listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
22:14:37.360570 00:0c:42:56:26:62 > ff:ff:ff:ff:ff:ff, ethertype 802.1Q (0x8100), length 60: ethertype ARP, arp who-has y.y.y.y tell x.x.x.x
22:14:37.361304 00:00:5e:00:01:02 > 00:0c:42:56:26:62, ethertype 802.1Q (0x8100), length 64: ethertype ARP, arp reply y.y.y.y is-at 00:00:5e:00:01:02
22:14:38.360575 00:0c:42:56:26:62 > ff:ff:ff:ff:ff:ff, ethertype 802.1Q (0x8100), length 60: ethertype ARP, arp who-has y.y.y.y tell x.x.x.x
22:14:38.368668 00:00:5e:00:01:02 > 00:0c:42:56:26:62, ethertype 802.1Q (0x8100), length 64: ethertype ARP, arp reply y.y.y.y is-at 00:00:5e:00:01:02
22:14:39.360612 00:0c:42:56:26:62 > ff:ff:ff:ff:ff:ff, ethertype 802.1Q (0x8100), length 60: ethertype ARP, arp who-has y.y.y.y tell x.x.x.x
22:14:39.361771 00:00:5e:00:01:02 > 00:0c:42:56:26:62, ethertype 802.1Q (0x8100), length 64: ethertype ARP, arp reply y.y.y.y is-at 00:00:5e:00:01:02
22:14:40.390802 00:0c:42:56:26:62 > ff:ff:ff:ff:ff:ff, ethertype 802.1Q (0x8100), length 60: ethertype ARP, arp who-has y.y.y.y tell x.x.x.x
22:14:40.391999 00:00:5e:00:01:02 > 00:0c:42:56:26:62, ethertype 802.1Q (0x8100), length 64: ethertype ARP, arp reply y.y.y.y is-at 00:00:5e:00:01:02

You can workaround this problem by adding static arp

[admin@N3] > ip arp print where interface=vlan66
Flags: X - disabled, I - invalid, H - DHCP, D - dynamic 
 #   ADDRESS         MAC-ADDRESS       INTERFACE                                                                           
 6   y.y.y.y   00:00:5E:00:01:02 vlan66
5

It didn’t work for me - the affected computers still couldn’t neither ping, nor winbox/telnet that RB750

6

MikroTik support group finally reproduced this bug and promised to patch it in the next version of RouterOS