Hello,
Here are some answers to you questions:
1. What's the difference between EXEC mode and admin EXEC mode? Or simply, when will I need use admin EXEC mode?
The following URL gives some info about the exec modes.
http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.8/general/command/reference/cm38mode.html
In short, Admin mode gets you to the admin plane. This has access to SDRs and chassis info, like fabric cards, etc.
2. Is there a way to clear the configuration of an ASR9000 (something like "erase startup-config" in IOS)?
This has been asked may times in the forums. Here is an example. In short, you do a commit replace with a blank config.
https://supportforums.cisco.com/message/828839#828839
3. Is there any reference link or document which describes in detail about builit-in user groups? e.g. what commands the "operator" group can issue?
Here is a URL which breaks down commands and task-groups needed.
http://www.cisco.com/en/US/partner/docs/ios_xr_sw/iosxr_r3.9/task_ids/reference/guide/td39tid.html
4. My client is trying to migrate a Cisco 7609 to an ASR9K, what commands do the same things as the statement "mls qos aggregate-policer"?
There isn't the equivalent of an aggregate policer that applies to the chassis. You can do policing via MQC on a per interface basis.
http://www.cisco.com/en/US/docs/routers/asr9000/software/qos/configuration/guide/qcasr9kover.html
Hope that helps,
Bryan
