I have a master asset list and I need to give them tagging so that when I type something like src_asset_tag=firewall I should get all source as firewall in logs.
If i understand your question you want to map an asset tag (lets say ID0001) to a device. (I assume 'firewall' is your hostname)
Create a CSV file which maps your assets like so:
assettag,hostname
ID0001,firewall
ID0002,router01
ID0003,email-server
upload and add this file as a lookup file - assets.csv
Define a lookup table - AssetsLookup.
You can now use the Lookup to populate a form on a dashboard, and allow your users to select an asset ID.
The input will give you the hostname, so you can build a dashboard which searches
host=$asset.hostname$
If this is not enough detail - let me know and i'll try and come back with more detailed info when i am back at my desk