Stacking and Rewriting Gigabit Ethernet VLAN Tags Overview
Stacking and rewriting VLAN tags, commonly known as Q-in-Q tunneling, allows you to use an additional (outer) VLAN tag to differentiate between customer edge (CE) routers that share one VLAN ID. A frame can be received on an interface, or it can be internal to the system (as a result of the input-vlan-map statement).
On IQ2 interfaces, 10-Gigabit Ethernet LAN/WAN PIC, 40-Gigabit Ethernet MIC, 100-Gigabit Ethernet MIC, IQ2-E interfaces, and MX Series interfaces, when a VLAN tag is pushed, the inner VLAN IEEE 802.1p bits are copied to the IEEE bits of the VLAN or VLANs being pushed. If the original packet is untagged, the IEEE bits of the VLAN or VLANs being pushed are set to 0.
When swap-by-poppush is configured on the interface, when a VLAN tag is swapped, the inner VLAN IEEE 802.1p bits are copied to the IEEE bits of the VLAN being swapped. If swap-by-poppush is not configured on the interface, the VLAN IEEE 802.1p bits of the of the VLAN being swapped remains same.
You can stack and rewrite VLAN tags on the following interfaces:
Gigabit Ethernet IQ
10-Gigabit Ethernet LAN/WAN PIC
40-Gigabit Ethernet MIC
100-Gigabit Ethernet MIC
Gigabit Ethernet IQ2 and IQ2-E
10-Gigabit Ethernet IQ2 and IQ2-E interfaces, and MX Series router Gigabit Ethernet Interfaces
Tri-Rate Ethernet copper, and 10-Gigabit Ethernet interfaces with the VLAN encapsulation type configured to support Layer 2 tunneling protocols such as circuit cross-connect (CCC) or virtual private LAN service (VPLS) (as described in 802.1Q VLANs Overview)