I have a RB2011UAS-RM (v6.13 - 3.14) and have set the Ethernet ports to the following configuration:
ether1 - Master Port
ether2-5 - Slaved to ether1
ether6 - Master Port
ether7-10 - Slaved to ether6
sfp1 - slaved to ether1

bridge - Loopback
bridge port - ether1
bridge port - ether6# may/27/2014 11:28:18 by RouterOS 6.13


This configuration seems to work just fine as a switch, which is what I was designing it for. However, I can't seem to get a mangle rule to detect any selected IP or Port. The only thing I can get it to do is detect "all" activity coming through the port, in this case..ether1. I want to mark traffic coming from a selected IP address and port but can't seem to manage it. Is this a bug, or have I got this configured wrong? Review the Packet Flow diagram, it should be possible.
# software id = YY4W-FNJ5
#
set [ find default-name=ether2 ] master-port=ether1
set [ find default-name=ether3 ] disabled=yes
set [ find default-name=ether4 ] master-port=ether1
set [ find default-name=ether5 ] master-port=ether1
set [ find default-name=ether7 ] master-port=ether6
set [ find default-name=ether8 ] master-port=ether6
set [ find default-name=ether9 ] master-port=ether6
set [ find default-name=ether10 ] master-port=ether6
set [ find default-name=sfp1 ] auto-negotiation=no master-port=ether1

/interface bridge
add l2mtu=1598 name=Loopback
/interface ethernet

/interface bridge port
add bridge=Loopback interface=ether6
add bridge=Loopback interface=ether1
/interface bridge settings
set use-ip-firewall=yes

"/firewall export compact"

"/firewall export compact"

I have a VERY simple firewall..Just troubleshooting Mangle

may/27/2014 13:46:24 by RouterOS 6.13
# software id = YY4W-FNJ5
#
/ip firewall mangle
add action=mark-connection chain=input new-connection-mark=SBS_3 protocol=tcp \
src-address=172.16.10.56 src-port=10001

I ask the firewall because i see the cause of your problem now:

if you want see bridge traffic, you must use forward chain, not input or output chain.

This two chain are reserved only for traffic directed or generated to router.