It is because the router is sending the SNMP V3 traps which supports encryption so the packets can be read by the SNMP manager which is configured in the router for which you require admin rights at the router and you shall be able to view and modify the router configuration . using the manual provided.
Other way to remove encryption is that at the router you configure it to through SNMP V1 or V2c traps at your IP. and then you can capture it from ethereal and view them.