The EVPN split-horizon procedure ensures that BUM traffic originated by the multi-homed PE and sent from the non-DF to the DF is not replicated back to the CE in the form of echoed packets. To avoid echoed packets, the non-DF (PE1) sends all the BUM packets to the DF (PE2) with an indication of the source ES. That indication is the ESI Label (ESI2 in the following figure), previously signaled by PE2 in the AD per-ESI route for the ES. When it receives an EVPN packet (after the EVPN label lookup), PE2 finds the ESI label that identifies its local ES ESI2. The BUM packet is replicated to other local CEs but not to the ESI2 SAP.
The following figure shows the EVPN split-horizon concept for all-active multi-homing.
Figure: Split-horizon
