As a best practice, when I'm troubleshooting network issues, I typically start at layer 1 and work my way up the stack. If you've been managing networks for very long you've probably learned that the first three steps to troubleshooting a network issue are "check the cable, check the cable, check the cable..". Once you've checked the cable though, it's probably time to move up to layer 2.
Using layer 2 protocols and methods to troubleshoot layer 3 problems has long been a favorite of mine. Understanding where layer 2 is working but layer 3 isn't is a key aspect of solving many types of network issues. When troubleshooting LAN connectivity issues using the Switch Port Mapper within the Engineer's Toolset can tell you what sort of issue you're having - whether it be a duplex mismatch or an ACL issue, the Switch Port Mapper makes quick work of helping you find the issue.
When it comes to troubleshooting WAN issues, I've primarily relied on the CLI within my routers. The fact that you can see the adjacent device with CDP (Cisco Discovery Protocol) or LLDP (Link Layer Discovery Protocol) but can't get to them with ICMP is a clear indicator that you have a layer 3 issue. While the CLI is still a great tool for this sort of troubleshooting, email@example.com and the rest of the guys that write new apps for the Engineer's Toolset have been hard at work again and have just released a new tool called the "CDP/LLDP Neighbor Map". It does exactly what you'd expect - it does a layer 2 discovery and automatically creates a map displaying the CDP/LLDP connectivity of devices on the network. It's a great new tool and has already helped me solve several connectivity issues both here in the lab and for some customers.
You can of course download the latest SolarWinds Engineer's Toolset from the SolarWinds.com website. Give it a try and let us know what you think or if you have any suggestions for new tools/improving the existing ones.
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