What is the purpose of an EPVN Type 4 route?

According to Juniper Networks' EVPN documentation, an EVPN Type 4 route, also known as an Ethernet Segment route, is a critical component of the BGP EVPN control plane used specifically in multihoming scenarios. When a Customer Edge (CE) device is connected to two or more Provider Edge (PE) routers via a Link Aggregation Group (LAG), these PE routers must coordinate to manage traffic flow and prevent network loops.

The primary purpose of the Type 4 route is to enable Designated Forwarder (DF) election. In a multihomed Ethernet Segment (identified by a unique Ethernet Segment Identifier or ESI), only one PE router—the Designated Forwarder—is permitted to forward Broadcast, Unknown unicast, and Multicast (BUM) traffic from the EVPN core to the CE device. Without this election process, multiple PE routers would forward the same BUM packets to the CE, leading to traffic duplication and potential broadcast storms.

The workflow begins when a PE router is configured with an ESI on a physical or logical interface; it then generates and advertises a Type 4 route to all other PE routers in the network. This route carries the ESI and an "ES-import" extended community, which allows other PEs connected to the same segment to automatically discover their peers. Once these "peering" PEs are discovered, they collectively execute a DF election algorithm—such as the default modulo-based algorithm or the newer preference-based algorithm—to select the DF and Backup Forwarder (BDF) for each VLAN or bridge domain. By ensuring a deterministic and synchronized selection of the forwarder, Type 4 routes provide the foundational stability required for active-active and active-standby multihoming in modern campus and data center fabrics.