Here is a useful list of tips that network administrators can leverage in their everyday travails.
1. Assume nothing. Where does the real fault lie? Is it really the network or something else? Go to #10...
2. Is IP addressing correct? Proper subnetting and VLANs?
3. Conduct a visual inspection of the equipment.
4. Validate proper device power-up after a power failure.
5. RTM or Read The Manual (polite version); believe it not, the manufacturer release notes have valuable information.
6. Configuration of equipment. More often than not a change, that is either managed or not, has a cascade effect. Check settings for auto negotiation, for example.
7. Check MTU settings. This is the cause of most black holes.
8. Are the router tables correct? Is the gateway correct? show iproute is a useful tool.
9. If using a service provider/telco. Are your PE routers setup correctly and with the right MTU for WAN links?
10. Leverage network management and monitoring tools to rapidly and preemptively locate issues and to get 360o visibility into, actionable intelligence about and complete control of your IT infrastructure.