Thanks for your help Ameya,
Unfortunately for ASR router, the OIDs provided in your link are those I am currently using but the result is :
snmpwalk -v 2c -c SNMP_community IP_adress 188.8.131.52.184.108.40.206.166
SNMPv2-SMI::enterprises.9.9.166 = No more variables left in this MIB View (It is past the end of the MIB tree)
IOS is : IOS-XE Software (PPC_LINUX_IOSD-IPBASEK9-M), Version 15.1(1)S1.
So, is it particular for this kind of IOS to see COS by SNMP on ASR routers ? Is there another diffrent OID with CBQos for this IOS ?
Another idea ?
Thanks in advance.