The following figure shows scenarios that may cause potential transient issues in the network.
In the preceding figure, the scenario on the left shows an example of transient packet duplication caused by delay in PE3 to learn MAC1.
In an all-active multi-homing scenario, if a specified MAC address (for example, MAC1), is not yet learned in a remote PE (for example, PE3), but it is known in the two PEs of the ES (for example, PE1 and PE2), the latter PEs might send duplicated packets to the CE.
Configuring ingress-replication-bum-label in PE1 and PE2 resolves the issue. PE1 and PE2 know that the received packet is an unknown unicast packet; consequently, the NDF (PE1) does not send the packets to the CE, which prevents transient and duplication.
In the preceding figure, the scenario on the right shows an example of transient black hole caused by delay in PE1 to learn MAC1.
In an all-active multi-homing scenario, MAC1 is known in PE3 and aliasing is applied to MAC1. However, MAC1 is not yet known in PE1, which is the NDF for the ES. If PE3 hashing picks up PE1 as the destination of the aliased MAC1, the packets are blackholed. To resolve this issue, unknown unicast traffic that arrives with a unicast label should not be blocked on the NDF. If PE1 and PE2 are configured using ingress-replication-bum-label, PE3 sends unknown unicast packets with a BUM label and known unicast with a unicast label. In the latter case, PE1 considers it safe to forward the frame to the CE, even if it is unknown unicast.
This is a transient issue that is resolved as soon as MAC1 is learned in PE1 and the frames are forwarded as known unicast.