[j-nsp] JunOS 18, ELS vs non-ELS QinQ native vlan handling.

[Alexandre Snarskii]

Hi!

Looks like JunOS 18.something introduced an incompatibility of native 
vlan handling in QinQ scenario between ELS (qfx, ex2300) and non-ELS 
switches: when ELS switch forwards untagged frame to QinQ, it now adds 
two vlan tags (one specified as native for interface and S-vlan) instead 
of just S-vlan as it is done by both non-ELS and 'older versions'.

As a result, if the other end of tunnel is non-ELS (or third-party)
switch, it strips only S-vlan and originally untagged frame is passed
with vlan tag :( 

Are there any way to disable this additional tag insertion ?

PS: when frames sent in reverse direction, non-ELS switch adds only 
S-vlan and this frame correctly decapsulated and sent untagged.

ELS-side configuration (ex2300, 18.3R1-S1.4. also tested with 
qfx5100/5110):

[edit interfaces ge-0/0/0]
flexible-vlan-tagging;
native-vlan-id 1;
mtu 9216;
encapsulation extended-vlan-bridge;
unit 0 {
    vlan-id-list 1-4094;
    input-vlan-map push;
    output-vlan-map pop;
}

(when native-vlan-id is not configured, untagged frames are not
accepted at all).