I was reading L2 VPN architecture book chapter 7 LAN Protocol over MPLS Case studies and I am not able to understand the following paragraph:
In port-tunneling mode, the packet does not have ingress port information. For inclusion of ingress information, the port-tunneled interface is put into the QinQ mode. A hidden VLAN is then created and added onto the packet. A hidden VLAN is a VLAN that is numbered outside the allowed range for VLAN IDs. This is how the NP learns the ingress information. The hidden VLAN concept applies to a switch platform (that is, 6500 and 7600 platforms). In contrast, VLAN-tunneled mode does not require a hidden VLAN. The NP can discern the ingress information from the packet's dot1Q header.
A similar concept applies to routers (VLAN stacking method). It involves the use of subinterfaces.
Another difference between the port-tunneling mode and VLAN-tunneling mode is in the handling of VLAN IDs. In port-tunneling mode, the VLAN ID is transparently passed from the ingress PE to the egress PE over MPLS in a single VLAN. In VLAN-tunneling mode, however, the VLAN ID at each end of the EoMPLS tunnel can be different. To overcome this, the egress side of the tunnel that is mapped to a VLAN rewrites the VLAN ID in outgoing dot1Q packets to the ID of the local VLAN.
It will be great if some one can explain me what it says.