Split Horizon

In a multihomed network with a set of Provider Edge (PE) devices on an all-active Ethernet Segment (ES), when a Customer Edge (CE) device sends Broadcast, Unknown Unicast, and Multicast (BUM) packets to a non-Designated Forwarder (non-DF) PE, that non-DF PE forwards the packet to all other PEs in the Ethernet Virtual Private Network (EVI), including the Designated Forwarder (DF) PE for that ES.

To prevent duplication of packets, the DF PE must drop the BUM packet rather than forwarding it back to the CE. This process is known as split-horizon filtering.

For an Ethernet Virtual Private Network Instance (EVI), if a BUM packet is received from a CE, the PE acting as the DF must send the packet to every ES for which it is the DF, except the one from which it originated. Additionally, the PE must send the packet to all other PEs in the EVI. Conversely, if the BUM packet is received from another PE (for example, PE2), the PE (for example, PE1) must only send the packet on the ESs where it is the DF and should avoid sending it to other PEs, as the other PE (PE2) has already handled that. Split Horizon prevents forwarding loops for BUM packets.

Parent topic: BGP EVPN Multihoming