Cisco CRS Configuration Manual page 198

Packet Forwarding
• The first label (IGP route label) directs the packet to the correct PE router on the eBGP border edge
router. (For example, the IGP label of ASBR2 points to the ASBR2 border edge router.)
• The second label (VPN route label) directs the packet to the appropriate PE router or eBGP border edge
router.
Figure 31: Forwarding Packets Between MPLS VPN Inter-AS Systems with ASBRs Exchanging VPN-IPv4 Addresses
Cisco IOS XR Virtual Private Network Configuration Guide for the Cisco CRS Router, Release 6.1.x
184
Implementing MPLS Layer 3 VPNs