tried to trouble shoot this with juniper support, but nothing wrong seemed to be found.
tried with Microsoft IAS instead of ClearPass and then it works ...
checked the packetcaptures and it seems they are identical except that IAS sends the data with Radius tag 0x00 and ClearPass does it with tag 0x01.
[IAS]
AVP: l=6 t=Tunnel-Type(64) Tag=0x00: VLAN(13)
AVP: l=6 t=Tunnel-Medium-Type(65) Tag=0x00: IEEE-802(6)
AVP: l=4 t=Tunnel-Private-Group-Id(81): 21
[ClearPass]
AVP: l=6 t=Tunnel-Type(64) Tag=0x01: VLAN(13)
AVP: l=6 t=Tunnel-Medium-Type(65) Tag=0x01: IEEE-802(6)
AVP: l=5 t=Tunnel-Private-Group-Id(81) Tag=0x01: 20
anyone know if i can get the ClearPass to use tag 0x00?