12. Processing of Unknown Unicast Packets

12. Processing of Unknown Unicast Packets

The procedures in this document do not require the PEs to flood unknown unicast traffic to other PEs. If PEs learn CE MAC addresses via a control-plane protocol, the PEs can then distribute MAC addresses via BGP, and all unicast MAC addresses will be learned prior to traffic to those destinations.

However, if a destination MAC address of a received packet is not known by the PE, the PE may have to flood the packet. When flooding, one must take into account "split-horizon forwarding" as follows: The principles behind the following procedures are borrowed from the split-horizon forwarding rules in VPLS solutions [RFC4761] [RFC4762]. When a PE capable of flooding (say PEx) receives an unknown destination MAC address, it floods the frame. If the frame arrived from an attached CE, PEx must send a copy of that frame on every Ethernet segment (belonging to that EVI) for which it is the DF, other than the Ethernet segment on which it received the frame. In addition, the PE must flood the frame to all other PEs participating in that EVPN instance. If, on the other hand, the frame arrived from another PE (say PEy), PEx must send a copy of the packet on each Ethernet segment (belonging to that EVI) for which it is the DF. PEx MUST NOT send the frame to other PEs, since PEy would have already done so. Split-horizon forwarding rules apply to unknown MAC addresses.

Whether or not to flood packets to unknown destination MAC addresses should be an administrative choice, depending on how learning happens between CEs and PEs.

The PEs in a particular EVPN instance may use ingress replication using RSVP-TE P2P LSPs or LDP MP2P LSPs for sending unknown unicast traffic to other PEs. Or, they may use RSVP-TE P2MP or LDP P2MP for sending such traffic to other PEs.