This is what I ended up with when I knew a lot less about the 125 and RouterOS in general:
/interface bridge add name=bridge
/interface bridge port add bridge=bridge interface=ether1
/interface bridge port add bridge=bridge interface=ether2
/interface bridge port add bridge=bridge interface=ether3
/interface bridge port add bridge=bridge interface=ether4
/interface bridge port add bridge=bridge interface=ether5
/interface bridge port add bridge=bridge interface=ether6
/interface bridge port add bridge=bridge interface=ether7
/interface bridge port add bridge=bridge interface=ether8
/interface bridge port add bridge=bridge interface=ether9
/interface bridge port add bridge=bridge interface=ether10
/interface bridge port add bridge=bridge interface=ether11
/interface bridge port add bridge=bridge interface=ether12
/interface bridge port add bridge=bridge interface=ether13
/interface bridge port add bridge=bridge interface=ether14
/interface bridge port add bridge=bridge interface=ether15
/interface bridge port add bridge=bridge interface=ether16
/interface bridge port add bridge=bridge interface=ether17
/interface bridge port add bridge=bridge interface=ether18
/interface bridge port add bridge=bridge interface=ether19
/interface bridge port add bridge=bridge interface=ether20
/interface bridge port add bridge=bridge interface=ether21
/interface bridge port add bridge=bridge interface=ether22
/interface bridge port add bridge=bridge interface=ether23
/interface bridge port add bridge=bridge interface=ether24
/interface ethernet switch egress-vlan-tag add tagged-ports=ether23,ether24,switch1-cpu vlan-id=31
/interface ethernet switch egress-vlan-tag add tagged-ports=ether23,ether24,switch1-cpu vlan-id=32
/interface ethernet switch ingress-vlan-translation add customer-vid=0 new-customer-vid=31 \
ports=ether1,ether2,ether3,ether4,ether5,ether6,ether7,ether8,ether9,ether10,ether11,ether12,ether13,ether14,ether15,ether16,ether17,ether18
/interface ethernet switch ingress-vlan-translation add customer-vid=0 new-customer-vid=32 \
ports=ether19,ether20,ether21,ether22
/interface ethernet switch vlan add ports=ether1,ether2,ether3,ether4,ether5,ether6,ether7,ether8,ether9,ether10,ether11,ether12,ether13,ether14,ether15,ether16,ether17,ether18,ether23,ether24,switch1-cpu vlan-id=31
/interface ethernet switch vlan add ports=ether19,ether20,ether21,ether22,ether23,ether24,switch1-cpu vlan-id=32
/interface ethernet switch set drop-if-invalid-or-src-port-not-member-of-vlan-on-ports=ether23,ether24 forward-unknown-vlan=no unknown-vlan-lookup-mode=ivl
You should of course leave yourself a management port free somewhere as well until you’ve established exactly how you are going to manage the device. Otherwise you risk cutting yourself off while setting things up.