Essential CLI commands for diagnosing network interface problems on Cisco switches, including ping, show interfaces, show ip interface brief, and show buffers.
Interface Troubleshooting Commands
Let's take a look at some commands that you would use to troubleshoot if you're having any issues with your device, or specifically with the interfaces on that device.
Let's say you've gone through and you've configured a device and there's something wrong with your interface. Perhaps it's not coming up, or perhaps it is functioning but you're seeing some weird network issues and you need to drill down to see what's happening on your network. Here are some commands that can help with that.
The first one that I'm going to mention is just the ping command. Ping is great because it can tell you if you just have basic connectivity. So I can ping something like 10.1.0.3 and ping that address and see if I have connectivity from, in this case, the switch to that device, which is my demo Pi that's on here. So yes, in fact I have connectivity there. Things are communicating and so I have at least some sort of connectivity. But what else could be wrong? Maybe it's having intermittent issues or something. So let's further take a look at this and see some other commands.
One that can be really helpful is the show run command, and this would show the running config. Or we can get more specific and see a specific interface, so I could do interface fa 0/1 and take a look at that interface and see how it is configured. So that can be helpful for taking a look to just see what the configuration is as you're going through this troubleshooting process.
Another command here is show interfaces. So I can do show interfaces, and I see that VLAN 1 is up. I see some stats associated with it. I'm going to scroll through to take a look at this interface 0/1, which is an actual port that's physically plugged into a device, and I see packets that have gone across here, bytes that have gone across here, if there's any buffers, and we can take a look at the buffer here in a little bit, if there's any runts or giants or throttles or input errors. Each one of these will be significant. Is the whole packet not coming in, or is the packet too large? So it can really zero down into what's happening on your device.
If I continue with this show interfaces, you'll notice that it shows me a lot of different interfaces, and that can be problematic if I'm looking for a specific interface, especially if it's later on. So if I want to just look at fa 0/14, I can use that same command but I can type in the interface that I want to look at, and it would show me just that interface.
The other command that I really like using is the show ip interface. So I can use that command, and same thing, it's going to scroll through all of the interfaces. Or I can use that command and say fa 0/14 to look at just the interface that I want to look at. This is a switch port, and since I typed in ip in there, it's looking from a layer 3 perspective and layer 3 information on here, and 0/14 is not giving me much, although we did see that VLAN 1 is a layer 3, so it gives us much more information with it.
The next thing, one of my favorite commands, is show ip interface brief. This shows me a table of all my interfaces. So I use this a ton, and it gives me information such as assigned IP addresses. It gives me information such as if these are up, if they're administratively shut down, which means I've actually got the shutdown command on the interface, it'll show me that. In these cases where it doesn't say administratively down, it's just because these ports, nothing is plugged into them. So a real quick way to see all of this information at a quick glance.
Of course these commands will also work with IPv6, so I can do a show ipv6 interface brief and hit enter, and then it shows me from a layer 3 perspective what is going on with these different switch ports here too. So layer 3 and an association with IPv6.
And then one other one that I'm going to do on here is a show buffers. A switch needs to buffer the information, buffer the frames as it comes into the switch, to see what the MAC address is before it forwards it on to a port. So it's got to buffer that information before it can forward the information. So if I look at this, this can show me if I'm having any buffering issues and perhaps this switch is saturated or something is going on with the switch. So I can look at here and find all sorts of information on the buffers, and if there are details that will help me troubleshoot, this could be a good location for that.
So quite a few commands there that can help us. There are some that I use really often, like the show ip interface brief. I love that command. Obviously you're doing show runs. So these are the commands that you're going to constantly be using as you're troubleshooting and you're connecting in different devices and you're experiencing maybe something is not functioning the way you want. You'll definitely be using these.
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