Hello,
It is in release note XR .4.2:
http://www.cisco.com/en/US/docs/routers/asr9000/software/asr9k_r4.2/general/release/notes/reln_a9k_42.html
And some writing on Cisco doc:
http://www.cisco.com/en/US/docs/routers/crs/software/crs_r4.2/routing/configuration/guide/b_routing_cg42crs_chapter_01.html#concept_E6B1D3D6B8CA4742AED837B3A94E826F
Below is the example what i did in my lab running 4.2.3:
RP/0/RSP1/CPU0:ASR-9006-01#show run router bgp
Fri Jan 31 21:05:02.170 UTC
router bgp 1 instance IPV4
bgp router-id 72.15.48.5
bgp graceful-restart
ibgp policy out enforce-modifications
address-family ipv4 unicast
redistribute connected
allocate-label all
!
address-family vpnv4 unicast
retain route-target all
!
neighbor 172.16.8.3
remote-as 1
update-source Loopback0
address-family ipv4 labeled-unicast
route-policy PASS in
route-policy PASS out
!
!
!
router bgp 1 instance IPV6
bgp router-id 99.2.3.4
address-family ipv6 unicast
redistribute connected
!
!
thanks,
rivalino