In the TX direction, packets received by VPP an the mirror Tap/Tun
are cross-connected to the VPP interfaces. For IP packets, IP output
features are applied.
+ If MPLS is enabled on a VPP interface, state is synced to Linux and
+ in TX direction a special feature is enabled to pass MPLS packets through
+ untouched.
The "linux_nl" plugin listens to netlink messages and synchronizes the IP
configuration of the paired interfaces.