Merci, vous m'avez mis sur la bonne piste !!
Depuis un routeur RB750GR3 en config usine, voici la config :
/interface vlan add interface=ether1 loop-protect-disable-time=00:05:00 loop-protect-send-interval=00:00:05 name=VLAN832 vlan-id=832
/interface bridge add name=Bridge-Wan
/interface bridge port add bridge=Bridge-Wan interface=VLAN832
/interface bridge filter add action=set-priority chain=output dst-port=67 ip-protocol=udp log=yes log-prefix="Set CoS6 on DHCP request" mac-protocol=ip new-priority=6 out-interface=VLAN832 passthrough=yes
/ip dhcp-client option add code=60 name=vendor-class-identifier value=0x736167656d
/ip dhcp-client option add code=77 name=userclass value=0x2b46535644534c5f6c697665626f782e496e7465726e65742e736f66746174686f6d652e4c697665626f7834
/ip dhcp-client option add code=90 name=authsend value=0x00000000000000000000001a0900000558010341010d6674692FXXXXXXXXXXXXXX
/ip dhcp-client add interface=Bridge-Wan dhcp-options=vendor-class-identifier,userclass,authsend add-default-route=yes default-route-distance=1 use-peer-dns=yes use-peer-ntp=yes disabled=no
/ip firewall nat add action=masquerade chain=srcnat comment="NAT LAN to WAN" out-interface=Bridge-Wan
Avec XXXXXXXXXXXXXX la partie login APRES fti/ en HEXA.
Merci !