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TechKnowSurge Catalog Videos DEMO: Troubleshooting Router-on-a-Stick Issue 3
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DEMO: Troubleshooting Router-on-a-Stick Issue 3

A systematic troubleshooting walkthrough for router-on-a-stick connectivity failures, covering how to isolate the fault using ping tests, routing table inspection, and interface status checks.

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About this video

Troubleshooting inter-VLAN connectivity in a router-on-a-stick topology requires a methodical approach that moves from the client outward toward the router, narrowing down the fault domain at each step. The process begins at the host by pinging the default gateway to confirm that basic Layer 3 reachability from the local subnet is intact. If the gateway responds but the destination subnet does not, the problem lies beyond the local interface, pointing toward the router or its configuration. Pinging the remote VLAN gateway address and the router's loopback interfaces provides additional diagnostic information. Loopback addresses are particularly useful because they remain stable regardless of physical interface state, so a successful ping to a loopback confirms the router itself is operational and that the source host is routing correctly. Failure to reach only the specific remote gateway subnet identifies that subinterface as the likely point of failure. Verifying this on the router with show IP interface brief exposes the issue directly: the subinterface associated with the unreachable VLAN is listed as administratively down, indicating a manual shutdown is in effect. Reviewing the running configuration for that interface confirms the shutdown command is present. Issuing no shutdown brings the subinterface up immediately, and connectivity to the previously unreachable host is restored without any additional changes. This scenario illustrates how a single misconfigured or disabled subinterface can silently block an entire VLAN while leaving all other paths functional.

What you'll learn

What's covered

Router on a Stick Troubleshooting

Aligned to

Cisco CCNA
2.1 Configure and verify VLANs (normal range) spanning multiple switches
1.4 Identify interface and cable issues
3.1 Interpret the components of routing table

Key terms

Virtual LAN
VLAN
A logical grouping of network devices that behave as if they are on the same network regardless of physical location.
Router
A network device that forwards data packets between networks based on IP addresses.
Default Gateway
The router that a device uses to send traffic to destinations outside its local network.
IP Address
A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Router-on-a-Stick
An inter-VLAN routing design in which a single physical router interface is connected to a switch trunk and divided into subinterfaces, one per VLAN, each configured with 802.1Q encapsulation and the default gateway address for that VLAN's subnet.
Subinterface
A logical subdivision of a physical router interface that allows it to carry traffic for multiple VLANs, each assigned a unique VLAN ID and IP address.

Topics

Networking Inter Vlan Routing Router On A Stick Subinterfaces Vlan Network Troubleshooting

Transcript

The symptom

Here is another issue when it comes to router on a stick. Here I have my demo laptop. I am pinging demo Pi 1 down here and I am successful. I am trying to ping demo Pi 2, but I am unsuccessful, so I have some sort of issue here that I need to troubleshoot. I am going to bring up a command prompt and start troubleshooting why I cannot reach out.

Working outward from the default gateway

I have already mentioned that one of the things I like to do is always ping the default gateway, to see if I can even ping the default gateway. Yes, in fact I can ping the default gateway, so then I can try something else.

In this case I am going to try the next hop, so this is 10.1.20.1. I cannot reach that. That seems to be stopping right there; I am not able to get past that point.

Then I can figure out whether there is another resource that I can ping. I believe that router actually has some loopback addresses, so let us take a look at the loopback address and see if we can ping that. The reason this makes a difference is because if I cannot ping anything to the outside world, one of the things I will think about is, well, it is the computer's problem, there is something wrong with the way it is routing or the way its default gateway is set up.

So I am 10.241.0.1, and see if I can ping, well, let us do 2.2 and ping that router. Yes, in fact I can ping the loopback addresses. This is one of the nice things about the loopback addresses on this equipment: I am able to get out and ping those devices and test to see if I have got some basic connectivity, and loopback addresses tend to be very stable compared to other interfaces.

On the router

So I know the router is up, and I know that my device can get to the outside world just fine. I am going to jump on router 1 and see what is going on with router 1.

I can close that out. On router one I can do a show IP route to see if there is even a 10.1.20.0 network in there, so I can think that could be an interface issue. Show IP interface brief, so we are going to take a look at that, and we can see that my interface is up, same thing with these sub-interfaces, except for fast ethernet 1.20, which says it is administratively down. That is a cue to me that there is something wrong with that interface right there, probably a shutdown.

So I am going to get into that interface, interface fa 1 R. I should have done a do go run interface fa 0 1 20, just so we can see the interface. We in fact do see the shutdown command, so I just bring this up with the no shutdown, and then pretty instantly we now have connectivity. I am able to ping out to demo Pi 2, so we are good to go.

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