DEMO: Identifying Interfaces

Physical interfaces

One thing that is really important is for you to be able to identify what the ports are physically compared to what you see in the operating system, so you know what you are looking at when you go through the different ports. Your device has a lot of physical interfaces, but it also has virtual interfaces, and there is a question of how you address each port that you have on your switch.

One of the things we have talked about in the past is the difference between copper, fiber, and wireless, which goes through the air. Copper and fiber are two of those physical interfaces that connect into the switch. We are primarily going to be talking about copper in this case, and there are still a lot of different types of connectors when it comes to copper. There are RJ45 connectors, and there are a lot of different types of serial connectors out there; there is a wide range of connectors. But the thing we are going to be working with most in this class is Ethernet connections, so I am going to go through how those Ethernet connections look and how to associate each port with what it looks like in the operating system.

Virtual interfaces

I do want to call out that there are a lot of virtual interfaces as well, and we are going to have to figure out how we address each of those virtual interfaces. Just a few examples:

• The VTY connection, which is when we are making a connection into a device so we can control it and program it.
• The switch virtual interface. This is associated with VLANs, where we have a virtual interface associated with a VLAN.
• Loopback addresses. Loopback addresses are generally used for testing, but there are a lot of other things we can do with loopback addresses as well.
• Sub-interfaces. This is when we have a physical connection into a router or a switch, and that physical connection has lots of VLANs being passed across it, and it is a routed interface.
• The null interface. The null interface is where we send traffic if we want to kill that traffic.
• Port channels. Port channels are when we group up multiple physical interfaces into a grouping, so that we have a connection that can use multiple interfaces to have more bandwidth from one switch to another.
• Tunnels. Tunnels are when we have two devices that are physically connected to each other and we tunnel specific traffic across there, and a lot of times that is encrypted traffic we are tunneling across.

There is more to do with all of those, but I wanted to call out that there is a different way that you address each one of these interfaces, just like there is a different way to address a physical interface.

The Catalyst 2960

Right now I have my computer plugged into a Catalyst 2960, so we are going to look at what a 2960 looks like. If you look at this, we have 48 ports, and each one of those ports is labeled. We have the first port right here, the second port right here, the third port, the fourth port, and each one of those ports is labeled on this machine all the way up to port 48.

Then we have these connectors right here. These are actually just a single connector, so there are two of these. It might look like there are four, but one of them is if you want to connect via Ethernet and one of them is if you want to slide in a GBIC to give it some sort of extra connection or extra functionality. Maybe you are connecting fiber between these switches, maybe you have a long run between these switches and you want some sort of media that is going to go a longer distance. I would slide a GBIC into here, then I would connect a fiber in here and run a fiber off between the different switches. These are gigabit connections here, while these other ones are fast Ethernet, or 100 megabits per second.

Seeing the interfaces in IOS

For this particular command we have to be in privileged exec mode, so I am going to type in enable here to get into privileged exec mode, and then I am going to do a show ip interface brief. I hit it, and it is showing us a lot of different interfaces that are on this device.

If I scroll up, I can see right here at the top that the first interface that shows up is actually VLAN 1. That is one of those virtual interfaces, a switched virtual interface, an SVI, showing up in this list. The next one we see is fast Ethernet 0/1, and then fast Ethernet 0/2, so we have some sort of labeling here: 0/1, 0/2, 0/3. That second digit, the one, two, three, four, is associated with that interface I showed you a second ago. That is how I know which interface is associated with which one I am seeing on this IOS.

I am going to scroll down, and I see giga Ethernet 0/1 and 0/2. Those are the two extra that I showed you on the end, that could be either a GBIC that you slide in there for something like fiber, or it could be just an RJ45 copper end that you plug in there. These are the gig Ethernet, so that is operating at a gigabit per second, versus these others operating at a hundred megabits per second.

What the first number means

One note about this first zero right here. A lot of times we have several modules that can slide into these devices, and when you have add-on devices that you slide in, then the next one would be one. If I have a regular switch that maybe has 48 ports, but I slide a little interface in and it gives it an extra four ports, then the first 48 ports would be 0/1 through 0/48, where that little extra would be 1/1, 1/2. So it designates a separate little card that is slid in for that additional functionality.

We do not see that on this switch, because these are actually two different speeds, so one is called gig Ethernet and one is called fast Ethernet. If these were the same speeds, let us say it is all gig Ethernet, then these would be gig Ethernet 0/1 through 0/48, and these extras with the GBIC that slide in would be giga Ethernet 1/1 and 1/2. But because they are separate speeds, it actually just calls it gig Ethernet 0/1.

The 3750 and StackWise

Here is another example. We have a 3750, and the 3750 has all these ports. It still has 48 ports along here, but we see a separate little section right here. Let us say these are all operating at gigabit Ethernet speeds, so we would address all of these as 0/1, 0/2, 0/3. It would be this first one, and usually the upper-most left one is the first port, and the second port is right below that, and the third port is just to the right of the first port, and so on and so forth. Generally that is the way these are laid out. If I were to slide an extra card in here that gives an extra four or eight ports, that is where we would see a 1/1 or a 1/2, and so on, labeling those.

I use this as an example because we can clearly see where there is a spot here to slide in some additional functionality or some additional ports. But I also bring this up because these are switches that have StackWise. They have a connection in the back which will connect the backplane, and then these will operate all as one switch. You program one switch and it is actually programming all the switches. So we add another element to this: there is a whole other slash in here. This would be 0/0/1, 0/0/2, 0/0/3, and this is identifying the switch in there. As we connect these together, it would be 1/0/1, and the next one would be 2/0/1, 3/0/1. So this is identifying the equipment that it is connected into, this is identifying which of the components within the switch it is connected to, and then this is identifying the specific port.

Let us look at what the 3750s and those interfaces look like on the IOS. I am going to type in my command, show ip interface brief, and hit enter. Now it is going to scroll through here. I see the same VLAN 1, and I have a fast Ethernet connection to this. Here is where I get into what is a little different here: there are three numbers here. Remember, that first one designates the switches, so in this case, with StackWise, it will start from one and count up. Although if you have a router and there are several little modules that can slide into it, I have seen this start at zero as well. In this case, with StackWise, this identifies the switch and it starts with the number one. Then this second number identifies the module that it is a part of, and these are all part of those initial 48 ports, so that is zero. Then we have a one right here designating the port that it is on, and this is going to count up from one all the way to 48.

Let us continue to scroll down through here and see what else they have on this particular switch. Here is an example of the switch where it has a module with some sort of additional functionality installed in there, and it has got four ports to that, and then it has this connection right here. Then we get into the second switch, so we see a two at the beginning of this, and then the first module here, and then we have the port number here. I am going to scroll through the rest of this, and it goes all the way up to 48, and then we see that same numbering here, so these are connected together using the StackWise connection.

Just so you have a visual of what is going on, here is the StackWise. There are these connectors here on the bottom that connect directly into the backplane, and you connect from one to the other. In fact, you see here there are two connections for each one of these, so you would probably have another one that would connect these two in case one of these fails, and it would then still be connected. Or what I have done is you have a stack of maybe seven of these together and they are all daisy chained together. You would have one connected into this one, and then this one connected into the next switch, and this switch connected into the next switch, all the way down, and the last switch would come in and plug into this one. So you have full redundancy amongst all of those switches. You could have a single switch fail and all the rest of the switches would still be up and running.

It is important to know the association between the two, because we are going to be plugging switches and other devices into these devices, and we will want to know what port we are plugging that into so we can identify it on the IOS.