Link aggregation (LACP / 802.3ad)
Run two (or more) cables between the same pair of devices and bond them into one virtual link. You get the combined speed AND a link that survives a single cable dying — the survivors carry on while you replace the dead one. The cost is a port on each side per cable.
When you run a duplicate cable between the same pair, the engine asks in #network whether to bond them. Approve to mint the bond; refuse to keep them as independent standby paths. A bonded group reports its combined health, not each cable's — which is exactly what makes it worth the extra ports.
Detailed explanation
N member links per LAG, hash-based per-flow load balancing across active members. min_active (default 1) is the threshold for considering the bundle up: members can drop one at a time and the LAG stays up (degraded) until you fall below it (failed). ActiveBackup is the failover-only cousin — one carrier, the rest idle. A LAG counts as one logical link, so it sidesteps spanning-tree.