Nagios is reliable in monitoring network alerts. It also has a huge amount of information that it can use to view the monitoring

The little alert bar that tells you instantly whats going on. As well as the various map types allows you to explore your network looking for links to servers or switches that might have issues, it allows for rapid detection of exactly where your issues lie.

Once it's set up, the web interface is great. We use it extensively so that those in dev ops can monitor the server environment. Very flexible and configurable.