Build a CCNA lab in Packet Tracer, then break it on purpose

Reading about a trunk and finding out why one is not passing VLAN 20 are different skills. Only one of them appears in the simulation.

Why the lab is half the course

CCNA simulations ask you to type commands on virtual devices. Someone who has only read the material can describe a trunk, yet may never have looked at the output of show interfaces trunk. At least a third of every week should be in the lab, and more if you are on the light track.

Cisco recommends Packet Tracer or Cisco Modeling Labs. Packet Tracer is free through a Cisco Networking Academy account and covers most exam topics.

The starter network

Build this once, early, and keep extending it.

Give each VLAN its own subnet, and make the router the gateway. Once everything pings, save a copy of the file. That copy is your clean baseline.

Faults worth introducing

Break one thing at a time, then find it using only show commands and ping.

1. Put a host's switch port in the wrong VLAN.
2. Remove a VLAN from the trunk's allowed list.
3. Set a different native VLAN on each end of the trunk.
4. Give a host the wrong subnet mask.
5. Delete the default gateway on a host.
6. Use a different OSPF area or hello timer on one router.
7. Apply an access list in the wrong direction.
8. Mismatch the duplex setting on a link.

A late-collision count on a point-to-point link is the classic sign of the last one.

A troubleshooting order

Work from the bottom up.

1. Interface. Is it up and up? Check show ip interface brief.
2. Address. Does the host have the right address, mask and gateway?
3. VLAN and trunk. Is the port in the right VLAN, and is that VLAN allowed across the trunk?
4. Routing. Does the routing table have a route to the destination?
5. Services and filters. Is an access list or NAT rule interfering?

Say the layer out loud before you type anything. It stops you changing three things at once.

A weekly rhythm

• One lab for every topic you cover.
• One fault introduced and fixed per week.
• Five minutes of subnetting drills most days.
• A written line each time: the fault, the clue that revealed it and the command that confirmed it.

Keep the written lines. After ten weeks you will have a record of faults that reads like a short troubleshooting guide, and it makes a strong CV project later.

When to stop reading and start typing

If your plan slips, cut from the reading in the first coverage stage, not from the lab. The book's practice stage, which takes about 30 per cent of your hours, is built from troubleshooting labs, subnetting drills and question sets by domain.

Where to find the plan

Break something small this weekend, and write down how you found it.