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 based on the CCNP TSHOOT topology.
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 complete troubleshooting lab based on the CCNP TSHOOT topology.
- Exams: covers the troubleshooting topics of CCNA 200-301, CCNP ENCOR 350-401, CCNP ENARSI 300-410, and CCIE Enterprise Infrastructure.
- 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. Troubleshooting is also a big part of CCNP ENARSI and the CCIE Enterprise Infrastructure lab, where you have to fix broken networks 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.
Lessons: How to troubleshoot networks, Ping troubleshooting, Traceroute, Conditional debug, MTU troubleshooting
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.
Lessons: Show interface explained, Troubleshooting interfaces, Troubleshooting VLANs and trunks, Troubleshooting spanning tree, Troubleshooting EtherChannel, Troubleshooting 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.
Lessons: Troubleshooting RIP, EIGRP neighbor adjacency, EIGRP 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 route advertisement problems like wrong network commands.
Lessons: OSPF neighbor adjacency, OSPF route advertisement, BGP neighbor adjacency, BGP route advertisement
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.
Lessons: Metric redistribution problems, AD redistribution problems
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 find out why a Cisco ASA drops packets: syslog, ASP drops, packet captures, and packet-tracer.
Lessons: Troubleshooting DHCP, Troubleshooting NAT and PAT, Troubleshooting NTP, Troubleshooting HSRP, Troubleshooting VRRP, Cisco ASA packet drop troubleshooting
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.
Lessons: IPv6 stateless autoconfiguration, RIPng, OSPFv3 neighbor adjacencies, Automatic 6to4 tunnel, IPv6 redistribution
Practice: Troubleshooting Lab
The best way to learn troubleshooting is to fix broken networks. In this lab, we walk through a complete network based on the official Cisco CCNP TSHOOT topology, 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
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 tells you whether a destination is reachable. A successful ping proves that layers 1 to 3 work, but when it fails, it doesn’t tell you where. Traceroute shows every router in the path, so you can see where packets stop. Learn how traceroute works.
Several items in the OSPF hello packet have to match: the area, authentication, hello and dead intervals, subnet mask, and network type. Also check that nothing blocks multicast address 224.0.0.5. The state where the neighbor gets stuck, like INIT or EXSTART, tells you where to look. Troubleshoot OSPF neighbor adjacencies.
BGP uses TCP port 179, so the routers need IP reachability and nothing can filter that port. When you peer between loopback interfaces, you need the update-source command, and for eBGP also ebgp-multihop, because eBGP packets have a TTL of 1 by default. Troubleshoot BGP neighbor adjacencies.
Redistribution problems come from two causes: metrics and administrative distance. When routes are redistributed back into the routing protocol they came from, a router can prefer the redistributed route, which leads to sub-optimal routing or routing loops. Route filtering, route tags, and changing the administrative distance fix this. Learn how to fix AD redistribution problems.
Extra headers from PPPoE, GRE, or IPsec reduce the space left for data, so large packets are fragmented or dropped while small packets like pings still work. Send pings with the DF-bit set and different sizes to find the largest packet that gets through. The ip tcp adjust-mss command fixes the TCP MSS for TCP traffic. Learn how to troubleshoot MTU.
Two things: truly understand the protocols you run, and gain experience by fixing broken networks in labs. Start with the structured approach and a checklist per OSI layer, and with practice, you’ll spot problems faster. Try the troubleshooting lab.
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