Summary for the impatient
The Link Aggregation Control Protocol (LACP) can be used to create link aggregation groups with active and standby links in them. Those standby links can be made active when one or more of the previously active links fail or removed from the group.
“The special magic” required for this to work needs only to be done by one side of LAG; the only requirement for the “other” side is that it supports standard LACP.
Telecom Occasionally 