Troubleshooting
Course Description
Knowing how a protocol works is one thing; finding out why it doesn’t work is another. This course teaches you how to troubleshoot networks with a structured approach and the right tools, and then applies it to real problems. We fix broken interfaces, VLANs, trunks, spanning tree, and EtherChannels, routing protocols like RIP, EIGRP, OSPF, and BGP, redistribution problems that cause sub-optimal routing and routing loops, IP services like NAT, DHCP, NTP, HSRP, and VRRP, IPv6, and packet drops on the Cisco ASA. We finish with a complete broken network and four trouble tickets to fix.
Course Highlights
In this course you will learn:
- How to troubleshoot with a structured approach, and when to use top-down, bottom-up, divide and conquer, or follow the traffic path.
- How to use ping, traceroute, conditional debugging, and show interface to find a problem.
- How to find and fix MTU problems with PPPoE, GRE, and IPsec.
- How to troubleshoot interfaces, VLANs, trunks, spanning tree, EtherChannel, and inter-VLAN routing.
- How to troubleshoot neighbor adjacencies and route advertisements for EIGRP, OSPF, and BGP.
- How to fix redistribution problems caused by metrics and administrative distance.
- How to troubleshoot NAT and PAT, DHCP, NTP, HSRP, and VRRP.
- How to troubleshoot IPv6 autoconfiguration, RIPng, OSPFv3, 6to4 tunnels, and redistribution.
- How to find out why a Cisco ASA drops packets.
Presented to you by instructor Rene Molenaar, CCIE #41726
Course at a glance
- 31 lessons in 10 units, plus a troubleshooting walkthrough.
- Format: self-paced lessons with broken Cisco IOS configurations, show and debug output, and video.
- Last updated: October 2026.
Requirements
- A good understanding of switching and routing; you can’t fix a protocol if you don’t know how it works.
- Basic knowledge of IP services like NAT, DHCP, and HSRP, and of IPv4 and IPv6 addressing.
Course Schedule
What is network troubleshooting?
Network troubleshooting is the process of finding and fixing the cause of a network problem. It starts when a user or your network monitoring reports a problem. Then you diagnose it: you collect information, analyze it, eliminate possible causes, come up with a hypothesis, and verify it. Once you have found the root cause, you implement a solution.
What makes a good troubleshooter is a method. If you follow your gut feeling, it’s easy to forget what you already tried. A structured approach like bottom-up, top-down, divide and conquer, or follow the traffic path helps you narrow down the problem step by step, from the physical layer up to the application. As you gain experience, you’ll skip steps and “shoot from the hip”, but the key is still to truly understand the protocols you troubleshoot.
Who needs troubleshooting skills? Every network engineer. Configuring a network is often the easy part; most of your time goes into fixing things that don’t work as expected. In real networks, you often have to fix problems under time pressure.
Troubleshooting Learning Roadmap
This roadmap follows the course units. Each topic has a short explanation and links to the lessons where we cover it in detail.
Troubleshooting Methods and Tools
We start with the structured troubleshooting approach and the different methods: top-down, bottom-up, divide and conquer, follow the traffic path, spot the difference, and replace components. Ping and traceroute help you narrow down where a problem is, and conditional debugging only shows the debug output you care about. MTU problems are tricky because pings work while applications fail; extra headers from PPPoE, GRE, or IPsec reduce the space for your data.
Layer 2 Troubleshooting
When you use the bottom-up approach, you start with your interfaces. The show interface command tells you whether an interface is up, and shows errors and duplex or speed problems. Then we fix VLAN assignment and trunk issues, spanning tree problems like an incorrect root port or blocked BPDUs, EtherChannels that don’t form because the configuration of the physical interfaces doesn’t match, and inter-VLAN routing.
Troubleshooting RIP and EIGRP
RIP doesn’t form a neighbor adjacency, so most RIP problems are about missing routes: wrong network commands, passive interfaces, version mismatches, split horizon, or auto-summarization. EIGRP does form neighbor adjacencies, so you check those first: the AS number and K values have to match, the neighbors need to be in the same subnet, and nothing can block multicast address 224.0.0.10. Once the adjacency works, you can troubleshoot route advertisement.
Troubleshooting OSPF and BGP
For OSPF neighbors, the area, authentication, hello and dead intervals, subnet mask, and network type have to match, and the state where a neighbor gets stuck tells you a lot about the problem. BGP neighbors need IP reachability, TCP port 179, the correct update source, and for eBGP neighbors that aren’t directly connected, a higher TTL with ebgp-multihop. Once the neighbors are up, we fix OSPF and BGP route advertisement problems like wrong network commands.
Troubleshooting Route Redistribution
Redistribution is easy to configure, but if you are not careful, you end up with sub-optimal routing or routing loops. All redistribution problems boil down to two causes: metrics and administrative distance. You learn how to fix them with route filtering, route tagging, seed metrics, offset-lists, and by changing the administrative distance.
IP Services, Gateway Redundancy and Firewalls
Many problems that users report turn out to be an IP service: a host that doesn’t get an IP address from DHCP, a NAT or PAT translation that doesn’t happen, or a device with the wrong time because NTP doesn’t synchronize. We also troubleshoot HSRP and VRRP, where interface tracking does nothing without preemption, and a misconfiguration can leave you with two active routers. Finally, you learn the tools to troubleshoot Cisco ASA packet drops: syslog, ASP drops, packet captures, and packet-tracer.
IPv6 Troubleshooting
IPv6 brings its own problems. A router only sends router advertisements for stateless autoconfiguration when IPv6 unicast routing is enabled. Routing protocols like RIPng and OSPFv3 use link-local addresses for their neighbors, automatic 6to4 tunnels need the correct addressing, and IPv6 redistribution behaves a bit differently than IPv4 redistribution.
Practice: Troubleshooting Walkthrough
The best way to learn troubleshooting is to fix broken networks. In this troubleshooting walkthrough, we go through a complete network with four trouble tickets: from a client without connectivity to an IPv6 prefix that can’t be reached. You use the network diagrams and everything you learned in this course to fix them one by one.
Frequently Asked Questions
It’s normal to struggle at first: book knowledge doesn’t automatically turn into troubleshooting skills. Two things make the difference: truly understanding how the protocols work, and fixing a lot of broken networks. Break things in your lab on purpose, follow a structured method so you don’t go in circles, and write down what you checked.
First, someone or something reports a problem. Then you diagnose it: collect information, analyze it, eliminate possible causes, form a hypothesis, and verify it. When you have found the root cause, you implement a solution. Following these steps keeps you from going in circles. Learn how to troubleshoot networks.
Top-down starts at the application layer of the OSI model and works its way down. Bottom-up starts with cables and interfaces and works its way up; it’s thorough but takes time. Divide and conquer starts in the middle with a ping: if it works, layers 1 to 3 are fine and you go up, and if it fails, you go down. Compare the troubleshooting methods.
Ping and traceroute are where you start: ping tells you if a destination is reachable, and traceroute shows where packets stop. On the routers and switches, show commands tell you the state of interfaces, neighbors, and routes, and debug commands show what happens in real time. When that’s not enough, a packet capture shows exactly what’s on the wire. Learn how to troubleshoot with ping.
Most problems come down to a handful of causes: bad cables or interfaces, VLAN and trunk mistakes, IP addressing errors, DHCP or DNS failures, routing problems like a missing route or a neighbor that doesn’t come up, and filters like access lists or NAT that block traffic. That’s why it pays to check the basics first before you dive into the complex stuff. Learn how to troubleshoot DHCP.
First, find out what “slow” means: one user or everyone, one application or all of them, and since when. Then check interface utilization, errors, and CPU on the devices in the path, and look for duplex mismatches or drops. If pings work but large transfers or certain applications fail, suspect an MTU problem. Learn how to troubleshoot MTU.