The default configuration in Ubuntu confirms that the 1Gps SFP and the WAN port share resources. They are bonded together and failover. I tested this with both, copper 1Gbps, and Fiber 1Gbps and it works as suggested.
eth0 is the fiber SFP, eth2 is the copper 1000baseT WAN
/etc/network/interfaces:
# bonding eth0 & eth2 with active-backup mode
auto eth0
iface eth0 inet manual
bond-master bond0
auto eth2
iface eth2 inet manual
bond-master bond0
bond-primary eth2
auto bond0
iface bond0 inet dhcp
bond-mode active-backup
bond-miimon 100
bond-slaves none
