When Q-in-Q tunneling is configured on EX Series switches with support for the Enhanced Layer 2 Software (ELS) configuration style, "vlan-id-list" statement is used under [edit interfaces interface-name unit logical-unit-number] configuration stanza to map packets from customer VLANs (C-VLANs) to a logical interface on user-network interfaces [UNI].

• Changing vlan-id-list C-VLAN configuration - for example changing from all-in-one bundling (vlan-id-list 1-4094) to a more specific (vlan-id-list 2245-2248) - may lead to packet loss on C-VLANs during the time when change is being applied.
• In steady operations, i.e. with no configuration vlan-id-list changes, mapped C-VLAN traffic should be running without traffic drops.

Q-in-Q tunneling configuration on UNI is changed for example from (vlan-id-list 1-4094) to (vlan-id-list 2245-2248) 

user@switch> configure

user@switch# show | compare 

[edit interfaces interface-name unit logical-unit-number]

-  vlan-id-list 1-4094;

+  vlan-id-list 2245-2248;

user@switch# commit 

configuration check succeeds

commit complete

and packet loss on C-VLANs may be observed between "commit" command is initiated and "commit completed" in some cases for x100ms. At that time, IFL interface flap would be seen. With no configuration vlan-id-list changes, traffic would be running without traffic drops.

As per the current design, when the vlan-id-list configuration changes, switch ASIC Q-in-Q reconfiguration is taking place. It's worth considering to perform such changes in planned Maintenance window.

2024-12-03 : Added a number of internal PR 
2024-04-03 : Article Created

Configuring Q-in-Q Tunneling and VLAN Q-in-Q Tunneling and VLAN Translation