Hi , I tried this in our lab, it seems to work.
i configured this on an ASR9000 running 5.3.3
RP/0/RSP0/CPU0:ASR-9001-A(config)#int Gi0/0/1/17
RP/0/RSP0/CPU0:ASR-9001-A(config-if)#loopback internal
RP/0/RSP0/CPU0:ASR-9001-A(config-if)#commit
Thu Jun 24 21:34:38.749 GMT
RP/0/RSP0/CPU0:ASR-9001-A(config-if)#exit
RP/0/RSP0/CPU0:ASR-9001-A(config)#interface gigabitEthernet 0/0/1/17.100 l2tr$
RP/0/RSP0/CPU0:ASR-9001-A(config-subif)#encapsulation dot1q 100
RP/0/RSP0/CPU0:ASR-9001-A(config-subif)#rewrite ingress tag pop 1 symmetric
RP/0/RSP0/CPU0:ASR-9001-A(config-subif)#commit
Thu Jun 24 21:35:01.794 GMT
RP/0/RSP0/CPU0:ASR-9001-A(config-subif)#ethernet loopback
RP/0/RSP0/CPU0:ASR-9001-A(config-if-ethlb)#permit internal
RP/0/RSP0/CPU0:ASR-9001-A(config-if-ethlb)#commit
Thu Jun 24 21:35:08.402 GMT
RP/0/RSP0/CPU0:ASR-9001-A(config-if-ethlb)#do show version
Thu Jun 24 21:35:14.667 GMT
RP/0/RSP0/CPU0:ASR-9001-A(config-if-ethlb)#do show version
Thu Jun 24 21:35:22.012 GMT
Cisco IOS XR Software, Version 5.3.3[Default]
Copyright (c) 2021 by Cisco Systems, Inc.
ROM: System Bootstrap, Version 2.01(20130627:172232) [ASR9K ROMMON],
ASR-9001-A uptime is 2 weeks, 2 days, 11 hours, 40 minutes
System image file is "bootflash:disk0/asr9k-os-mbi-5.3.3.sp12-1.0.0/0x100000/mbiasr9k-rp.vm"
cisco ASR9K Series (P4040) processor with 8388608K bytes of memory.
P4040 processor at 1500MHz, Revision 2.0
i have found a router in our lab running 5.1.3, it fails
RP/0/RSP0/CPU0:ASR9001-K(config)#int Te0/0/2/2
RP/0/RSP0/CPU0:ASR9001-K(config-if)#loopback internal
RP/0/RSP0/CPU0:ASR9001-K(config-if)#commit
Thu Jun 24 05:23:19.744 UTC
RP/0/RSP0/CPU0:ASR9001-K(config-if)#exit
RP/0/RSP0/CPU0:ASR9001-K(config)#int Te0/0/2/2.100 l2transport
RP/0/RSP0/CPU0:ASR9001-K(config-subif)#ethernet loopback
RP/0/RSP0/CPU0:ASR9001-K(config-if-ethlb)#permit internal
RP/0/RSP0/CPU0:ASR9001-K(config-if-ethlb)#commit
Thu Jun 24 05:23:36.986 UTC
% Failed to commit one or more configuration items during a pseudo-atomic operat ion. All changes made have been reverted. Please issue 'show configuration faile d [inheritance]' from this session to view the errors
RP/0/RSP0/CPU0:ASR9001-K(config-if-ethlb)#show configuration failed
Thu Jun 24 05:23:41.392 UTC
!! SEMANTIC ERRORS: This configuration was rejected by
!! the system due to semantic errors. The individual
!! errors with each failed configuration command can be
!! found below.
interface TenGigE0/0/2/2.100 l2transport
ethernet loopback
permit internal
!!% 'Ethernet Loopback' detected the 'warning' condition 'Loopback is not suppor ted on this interface type in this direction.'
!
!
end
RP/0/RSP0/CPU0:ASR9001-K(config-if-ethlb)#do show ver
Thu Jun 24 05:23:52.538 UTC
Cisco IOS XR Software, Version 5.1.3[Default]
Copyright (c) 2014 by Cisco Systems, Inc.
ROM: System Bootstrap, Version 2.04(20140424:063844) [ASR9K ROMMON],
ASR9001-K uptime is 34 weeks, 1 day, 22 hours, 35 minutes
System image file is "bootflash:disk0/asr9k-os-mbi-5.1.3.CSCur18995-1.0.0/0x100000/mbiasr9k-rp.vm"
cisco ASR9K Series (P4040) processor with 8388608K bytes of memory.
P4040 processor at 1500MHz, Revision 3.0
ASR-9001 Chassis
2 Management Ethernet
4 TenGigE
4 DWDM controller(s)
4 WANPHY controller(s)
219k bytes of non-volatile configuration memory.
2868M bytes of hard disk.
3915760k bytes of disk0: (Sector size 512 bytes).
Configuration register on node 0/RSP0/CPU0 is 0x2102
Boot device on node 0/RSP0/CPU0 is disk0:
Package active on node 0/RSP0/CPU0:
iosxr-security-5.1.3.CSCur54568, V 1.0.0[SMU], Cisco Systems, at disk0:iosxr-security-5.1.3.CSCur54568-1.0.0
Built on Thu Nov 27 13:16:46 UTC 2014
By iox-lnx-008 in /san1/smu/EFR/smu_r51x_5_1_3/workspace for pie
asr9k-px-5.1.3.CSCur54568, V 1.0.0[SMU], Cisco Systems, at disk0:asr9k-px-5.1.3.CSCur54568-1.0.0
Built on Thu Nov 27 13:16:48 UTC 2014
By iox-lnx-008 in /san1/smu/EFR/smu_r51x_5_1_3/workspace for pie
--More--
Cisco IOS XR Software, Version 5.1.3[Default]
Copyright (c) 2014 by Cisco Systems, Inc.
it looks like there was a change in version, newer versions seem to work.
the older one has an issue, are you able to upgrade?
Thanks
