8.3. Split Horizon

8.3. Split Horizon

Consider a CE that is multihomed to two or more PEs on an Ethernet segment ES1 operating in All-Active redundancy mode. If the CE sends a broadcast, unknown unicast, or multicast (BUM) packet to one of the non-Designated Forwarder (non-DF) PEs, say PE1, then PE1 will forward that packet to all or a subset of the other PEs in that EVPN instance, including the DF PE for that Ethernet segment. In this case, the DF PE to which the CE is multihomed MUST drop the packet and not forward back to the CE. This filtering is referred to as "split-horizon filtering" in this document.

When a set of PEs are operating in Single-Active redundancy mode, the use of this split-horizon filtering mechanism is highly recommended because it prevents transient loops at the time of failure or recovery that would impact the Ethernet segment -- e.g., when two PEs think that both are DFs for that segment before the DF election procedure settles down.

In order to achieve this split-horizon function, every BUM packet originating from a non-DF PE is encapsulated with an MPLS label that identifies the Ethernet segment of origin (i.e., the segment from which the frame entered the EVPN network). This label is referred to as the ESI label and MUST be distributed by all PEs when operating in All-Active redundancy mode using a set of Ethernet A-D per ES routes, per Section 8.2.1 above. The ESI label SHOULD be distributed by all PEs when operating in Single-Active redundancy mode using a set of Ethernet A-D per ES routes. These routes are imported by the PEs connected to the Ethernet segment and also by the PEs that have at least one EVPN instance in common with the Ethernet segment in the route. As described in Section 8.1.1, the route MUST carry an ESI Label extended community with a valid ESI label. The disposition PE relies on the value of the ESI label to determine whether or not a BUM frame is allowed to egress a specific Ethernet segment.