Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual page 257

When IP packets arrive at the ingress LER, they are looked up in the relevant IP
forwarding table and then are forwarded into an LSP. Every IP route eventually points
to an IP interface. The IP interface contains IP attributes that affect how the IP packet
is forwarded. IPv4 routes point only to IPv4 interfaces and IPv6 routes point only to
IPv6 interfaces.
Because IP routes cannot point directly to MPLS major interfaces, MPLS automatically
creates up to four dynamic IP shared interfaces that are stacked on each MPLS major
interface. When you issue the mpls command in Interface Configuration mode, the
interfaces are created dynamically and provide the interfaces an IP route needs to
point to. You can specify a profile (created with the profile command) to configure
attributes for these interfaces with the mpls create-dynamic-interfaces command.
You can use the same command to enable or disable the creation of specific interface
types or all types.
Each dynamic interface is one of the following types:
By default, MPLS creates one dynamic IPv4 interface per MPLS major interface
for non-VPN traffic. This interface is used by default for VPN traffic as well.
By default, but only if IPv6 is enabled in the virtual router, MPLS creates one
dynamic IPv6 interface per MPLS major interface for non-VPN traffic. This
interface is used by default for VPN traffic as well.
If you configure it to do so, MPLS creates one dynamic IPv4 interface per MPLS
major interface for VPN traffic. If this interface is not created, then the VPN traffic
uses the default IPv4 interface for non-VPN traffic.
Typically, you request the creation of separate IPv4 interfaces for VPN traffic
only when you want the IPv4 interface for VPN traffic to have different attributes,
such as a different IP policy, from the IPv4 interface for non-VPN traffic. When
it is acceptable for the VPN traffic and the non-VPN traffic to receive the same
IP treatment, then you do not need to create separate IPv4 interfaces for the
VPN traffic.
If you configure it to do so, but only if IPv6 is enabled in the virtual router, MPLS
creates one dynamic IPv6 interface per MPLS major interface for VPN traffic. If
this interface is not created, then the VPN traffic uses the default IPv6 interface
for non-VPN traffic.
Typically, you request the creation of separate IPv6 interfaces for VPN traffic
only when you want the IPv6 interface for VPN traffic to have different attributes,
such as a different IP policy, from the IPv6 interface for non-VPN traffic. When
it is acceptable for the VPN traffic and the non-VPN traffic to receive the same
IP treatment, then you do not need to create separate IPv6 interfaces for the
VPN traffic.
IPv6 must be enabled in the parent virtual router so that IPv6 dynamic interfaces
can be created over MPLS interfaces. Otherwise, IPv6 VPNs do not work correctly,
All VPN traffic sent onto or received from the same layer 2 interface uses the same
IPv4 VPN or IPv6 VPN interface. Consequently, any policy attached to the interface
applies to all that VPN traffic.
Chapter 2: MPLS Overview
Mapping IP Data Packets onto MPLS LSPs
221