BUM traffic forwarding is based on egress PE filtering. Figure 1 shows an example of EVPN E-Tree forwarding behavior for BUM traffic.
In Figure 1, BUM frames are handled as follows when they ingress PE or PE2.
If the BUM frame enters a root-ac, the frame follows regular EVPN data plane procedures.
If the BUM frame enters a leaf-ac, the frame handling is as follows:
The frame is marked as leaf and forwarded or replicated to the egress IOM.
At the egress IOM, the frame is flooded in the default multicast list subject to the following.
Leaf entries are skipped when BUM traffic is forwarded. This prevents leaf-to-leaf BUM traffic forwarding.
Traffic to remote BGP EVPN PEs is encapsulated with the EVPN label stack. If a leaf ESI label present for the far-end PE (L1 in Figure 1), the leaf ESI label is added at the bottom of the stack; the remaining stack follows (including EVI label). If there is no leaf ESI label for the far-end egress PE, no additional label is added to the stack. This means that the egress PE does not have any E-Tree enabled service, but it can still work with the VPLS E-Tree service available in PE2.
The BUM-encapsulated packet is received on the network ingress interface at the egress PE or PE1. The packet is processed as follows.
A normal ILM lookup is performed for each label (including the EVI label) in the stack.
Further label lookups are performed when the EVI label ILM lookup is complete. If the lookup yields a leaf label, all the leaf-acs are skipped when flooding to the default-multicast list at the egress PE.