This is interesting. I also have problem with device discovery that take a long time. But it behaves a bit different. After two hours of discovery I have this numbers:

Total: 1883

Reachable: 1619

Unreachable: 264

Thats it. The job never ends. When I check the day after the numbers are still the same but discovery is still running. Do you have any suggestions in how to debug and find the cause?

Have a look if debugging for discovery is disabled: CS > Device and Credentials > Device Discovery > Discovery Logging Configuration

check the number of unreachable devices along with the timeout settings for snmp in discovery. These two settings do have a greate influence on the timeline for discovery.jclarke pointed it out here:

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=Network%20Management&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40^1%40%40.2cc24a15/6#selected_message

Check if you really need Layer3 discovery because it usually takes more time then Layer2 disco (CDP). If you want to enable Layer3 disco in big environment check if you can use the hop count settings and be careful with the option 'use DCR as seed'.

If you want to enable different discovery modules, a good practice is - test each module and the required settings seperately.

Got to CS> Device and Crtedentials > Device Discovery > Discovery Settings

First do a classical CDP disco and perhaps test with different settings (seeds, SNMP, Filters if needed..). If you have made your final settings, schedule a discovery job and name it 'CDP_discovery' - it will use your final settings.

Make the same with a Layer3 discovery module (test seed devices and hop count). If you have finshed your tests and decided to use it, schedule a new job e.g. 'ARP_discovery'. This job will use your last settings.

You can have *different* discovery settings for each disco job!! With this option of multiple discovery jobs you can have better control of the different discovey module.

In 'Discovery schedule' you have 2 buttons to edit a discovey job:

'Edit' with which you can change the schedule and

'Edit Settings' which lets you define the discovery settings *FOR THIS JOB* in detail!

So you can still change the discovery settings for each job at any time (but you cannot change the description of the job)

If you have finished configuration and scheduled different discovery jobs, you can see the results for the last cycle of each job in CS> Serve> Admin> Job Browser