Need Advice On Free/Opensource NIDS

So I need to deploy a network IDS in my intranet. The goals are:

1. detect and alert for network attacks such as port scanning, exploit, payloads etc.
2. Should be able to parse packets from a mirrored port of a switch/router. Deploying in host systems is not an option here.
3. Ease of use and support is also a factor.
4. Should be opensource or atleast free.
5. Should be able to handle large amount of data.

Please share your suggestions.