Why is a designated forwarder required in a multihomed CE-to-PE VXLAN environment using EVPN signalling?

In an EVPN multihoming scenario, a CE device is connected to multiple PEs within the same Ethernet Segment. When the CE sends Broadcast, Unknown Unicast, or Multicast (BUM) traffic, all connected PEs receive it. Without a control mechanism, each PE would flood the packet into the EVPN core. This would cause remote PEs to receive multiple copies of the same packet, which they would then forward to their connected CEs. The Designated Forwarder (DF) election process ensures that only one PE (the DF) is responsible for forwarding BUM traffic from the CE, thus preventing the delivery of duplicate packets to remote hosts.

A. Loop prevention between PEs in the EVPN core is primarily handled by split-horizon rules, not the DF election for CE-originated traffic.

B. MAC addresses are learned and distributed via the BGP control plane (EVPN Type-2 routes), a process separate from the DF's data plane forwarding role for BUM traffic.

C. While duplicate packets increase traffic, the specific problem solved by the DF is duplication itself, not necessarily a self-propagating traffic storm. "Duplicate packets" is the most precise answer.

1. IETF. (2015). RFC 7432: BGP MPLS-Based Ethernet VPN. Section 8.5, "Forwarding BUM Traffic," states, "In order to prevent duplicate packets from being delivered to a CE, only one of the PEs attached to a given Ethernet segment is allowed to forward BUM traffic... This PE is called the Designated Forwarder (DF)." (p. 20). Retrieved from https://doi.org/10.17487/RFC7432

2. Juniper Networks TechLibrary. (2023). EVPN Multihoming Overview. "For a given Ethernet segment, only one of the PE routers, called the designated forwarder (DF), forwards BUM traffic to the CE. This restriction prevents duplication of BUM packets." Retrieved from https://www.juniper.net/documentation/us/en/software/junos/evpn-vxlan/topics/concept/evpn-multihoming-overview.html