Interface Issues – CompTIA Network+ N10-009 – 5.2

A network interface can tell a lot about the health of the network. In this video, you’ll learn about interface monitoring, CRC errors, runts, giants, drops, error disabled states, and port status messages.


Network administrators use a great deal of their time to monitor different interfaces on important devices. This allows us to see any problems that might be developing, either with a bad cable or a bad interface, and we may be able to resolve that problem before it becomes an outage. Sometimes these interface statistics can warn us of congestion or overutilization on a network, and we may be able to make some decisions about how to change the design of the network to better deal with that oversaturation.

Many operating systems can also give you a great deal of feedback on how your interface may be performing in your device. You can look at this under the network configurations, depending on what operating system you’re using. Network administrators obviously don’t have time to log into every single system to watch these interfaces, so they automate the process using the simple network management protocol or SNMP. Many devices will support a standard set of SNMP statistics known as a MIB-II. This is a management information base.

MIB-II provides us with a set of standard statistics that are common across many different devices. But you may find that your firewall or your switch might have some statistics that are unique to that particular device, and there may be a proprietary MIB that you can integrate into that process to get even more visibility into that interface. One piece of information that we can gather using these techniques is to determine whether the link is up or the link is down.

This is our link status. This may be a problem with the cable, an interface, or it may be an issue with the device rebooting. Utilization is also a great metric to monitor, especially if you’re concerned about total throughput through the network. You want to be sure that you have enough bandwidth for all of the services running over that network connection, and if you’re not quite sure the total amount of throughput you can get over a link, you may want to run some bandwidth tests and see what the throughput might be.

And if you want some type of notification that problems might be brewing with that particular connection, then you’ll want to look at all of the errors that are on that particular interface. This includes CRC errors, runts, giants, drops, and other interface errors as well. This often points to a problem with the cable or a problem with the interface, so we may need to do additional troubleshooting to determine what’s causing these specific errors. Before we dive into those errors, let’s look at the structure of an ethernet frame.

The first part of an ethernet frame is a part we normally don’t see. It’s the preamble and the SFD, or the start frame delimiter. These two fields identify the beginning of an ethernet frame and lets a system know that everything after the start frame delimiter is a normal ethernet frame. If you are capturing these frames on a packet analyzer, you probably would not see the preamble and the start frame delimiter. Instead, you would see the beginning of the frame with the destination Mac address and the source Mac address.

This identifies the device that is sending this particular frame and the destination of where this frame should be going. This destination Mac is one that is used by switches, for example, so that it knows where to forward a particular frame. The next field of the frame is the ether type. This describes what type of data we would expect to see in the rest of this particular frame. Then you would have the payload of the frame itself, and at the very end, we have the frame check sequence.

This is a checksum that can be calculated very quickly that tells us if all of the information that we’ve received for this particular frame came through without any type of corruption. And when this frame check sequence does not match the rest of the data in the frame, we have a cyclic redundancy check checksum error or CRC error. The CRC error is often our first Warning. That there’s some type of problem with the signal on this particular connection, and we might want to look at our cable or our interfaces to remove those CRC errors.

That CRC is calculated when you receive the frame. That CRC is recalculated internally on your ethernet adapter, and it’s compared to the checksum that is included in the frame check sequence. If those two match, then we’ve received the frame without any type of corruption. But if there is a mismatch between that data in the frame and the information that’s in the frame check sequence, we know that there is an error and the CRC counter is incremented by one.

If your error counter for runts is increasing, it means that we’ve received a frame that is less than 64 bytes. 64 bytes is the minimum size of a frame that you should see. So if you happen to see a frame that is less than 64 bytes, that would be qualified as an error. We don’t often see runts any longer because most of our switch networks are full duplex. But if you happen to be working with a network that is running at half duplex, you may very well find runts occurring whenever there’s a collision. The default maximum frame size on ethernet is 1,518 bytes.

And if you happen to have a frame come across the network that is larger than that value, we describe that as a giant. Of course, you can also have jumbo frames that are much larger than 1,518 bytes, but those jumbo frames are defined within the switches and the devices that you’re using. So those also have a maximum size. And if those frames are larger than the maximum size that’s configured, those are also defined as giant. And of course, if there’s some type of contention or buffering problem with the devices on your network, these frames may be lost because there’s no room to hold them in any type of buffer.

In that case, you may see the drop counter is incremented on your system, and that may indicate that you have some type of communication problem on your network. Most devices and operating systems give you a way to view these error counters. This is a set of counters from a Cisco device. You can see I’m showing the interface statistics for one of the fast ethernet interfaces, and you can see that I’ve highlighted the section for runts, giants, CRCs, and you can see there are some other errors associated with this list as well.

This is where we can start to get a warning that something might be going wrong. If we happen to see the CRC error is slowly incrementing, that may indicate that we’re having a problem with a cable or an interface, and we may need to have some downtime to replace either one of those. You may find that some interface issues create a much larger problem if they’re left to their own devices. It might be a better idea to turn off certain interfaces if we happen to run into a particular kind of problem.

If you can see the problem happening, you can log in to a device and administratively disable a particular interface. That obviously isn’t fixing the problem. We’re more addressing the symptom. But at least we’ve now gotten the interface into a mode where it won’t cause additional problems on the network. Instead of you as the network administrator having to interactively log in to a device and administratively disabling an interface, it would be much easier if the device would automatically recognize this problem and automatically disable that interface.

We refer to this state as error disabled, where the device has disabled this interface without any type of human intervention. You might see this happen if an interface on a switch is flapping, which means that the interface is up, then the interface goes down, then the interface comes up again, then the interface goes down. So a switch might error disable that interface so that it’s no longer flapping up and down and causing problems with spanning tree and other switch functions. Or we may be limiting how many devices can connect to any particular switch interface.

So if we have configured this port security and someone unplugs a device and plugs their own laptop into it, your switch will go into an error disabled state for that interface. Or there might be a configuration problem or an increase in number of errors, and all of those can cause the interface to move into an error disabled state. When a switch disables one of its interfaces because of these errors, it’s not going to turn that interface back on. You have to log into the switch and administratively re-enable that particular interface.

Only then will that interface be able to operate again. But if that interface has the same problems occur again, it could possibly move back into an error disabled state. Instead of error disabled, you might see an interface is administratively down. That means that an administrator logged into that system and specifically turned off that interface. This was an intentional act by the administrator, and it wasn’t something that was done automatically. To be able to use that interface again, you would have to log in to that device and administratively enable that interface.

And on some switches we have a mode that’s very similar to error disabled, except it’s an error that occurs the moment you turn that interface on. This is a suspended port status and this means that we’ve connected this interface to a configuration that is incompatible with the settings of that interface. For example, you may be configuring link aggregation between two switches and you’ve enabled the Link Aggregation Control Protocol or LACP. If you configure LACP on one side, but you don’t configure LACP on the other, you may find that the interface immediately moves into a suspended state.