A Designated Forwarder (DF) is a leaf on the fabric that is elected to send incoming Broadcast, Unicast, and Multicast (BUM) traffic to the host device.
On a multi-homed host, with connections to two or more leafs on the fabric, only one leaf can forward BUM traffic to the host device. This leaf is the DF for the host for that ES. By default, this is the leaf with the lowest numeric IP address.
For the election of the DF, ESI must be configured on the Layer 2 interfaces/LAG on the leaf. When these Layer 2 interfaces/LAGs come up, BGP sends an Ethernet Segment Route and AD per ES route. It then starts a timer. On the expiry of this timer, each leaf in the ES builds an ordered list of IP addresses of the leafs which are a part of the same ES. The leaf with the lowest numerical IP address is then elected as the DF for the incoming BUM traffic.
Use the show bgp evpn ethernet-segment command to view details of the elected DFs for the selected ES.