Below is the topology of lab (similar is customers, interfaces are different),

CE1-ACX710 (xe-0/0/0) <------> (xe-0/0/1) PE1-ACX710 (xe-0/0/0)<----> (xe-0/0/0) Core-MX960 (xe-0/0/1) <-----> (xe-1/3/1) PE2-ACX2100 (xe-1/3/0) <-----> (xe-1/3/0) CE2-ACX2100

ACX710 and ACX2100 connect to 2 CE devices, interface facing CE on ACX710 and ACX2100 are configured native-vlan because CE devices only accept untag vlan packets.

ACX710 and ACX2100 have l2circuit connections to MX960.

In this setup, the packet from CE1 "ACX710" is sent to PE1 ACX710 as a un-tag packet, when the same packet received on CCC interface xe-0/0/1 on PE1 it should add VLAN 100 as configured and send with vlan-tag toward Core router MX960.

But when native-vlan is configured, due to the bug it is not adding the VLAN id while sending it to core device and packet is getting blackhole. 

Below is the configuration PE1 device, 

lab@acx710> show configuration interfaces xe-0/0/1 | display in

description XConnect-CE;

flexible-vlan-tagging;

native-vlan-id 100;

mtu 9000;

encapsulation flexible-ethernet-services;

unit 0 {

  encapsulation vlan-ccc;

  vlan-id 100;  >>>>>>>>>>>>>>>>>>>>

}

Issue is to bug in code, it is now fixed through the PR https://prsearch.juniper.net/PR1793829