EVPN multihoming
The way to attach a server or switch to two leaf switches at once with no proprietary link between them. The replacement for MC-LAG, with all the coordination moved into BGP.
It starts with an identifier, the ESI, which both leaves configure identically for the shared link. Four mechanisms then do the work. The type-4 route lets the switches sharing a segment find each other and elect a designated forwarder for broadcast and multicast traffic, so the customer does not receive duplicates. Split horizon, implemented with an ESI label, prevents traffic that came in from the customer being sent back out towards the same segment. Aliasing, via a per-EVI route, tells remote nodes that a MAC address announced by only one of the leaves is reachable through both — which is what makes the load genuinely active-active. And mass withdraw lets a single route be pulled on link failure instead of thousands of MAC withdrawals, which is the difference between a second of reconvergence and a minute.