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Create a bridge interface and assign the bridge with the AT&T residential gateway's MAC address.
/interface/bridge add name="BR1" mtu=auto actual-mtu=1500 l2mtu=1592 arp=enabled arp-timeout=auto mac-address=xx:xx:xx:xx:xx:xx protocol-mode=none fast-forward=yes igmp-snooping=no auto-mac=yes ageing-time=5m vlan-filtering=yes ether-type=0x8100 pvid=1 frame-types=admit-all ingress-filtering=no dhcp-snooping=no