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LAN & WAN Basics

Playing with Ports

NOTE!Note: All features and functions described can be supported by both "smart" and "managed" switches unless otherwise noted.

With the definitions out of the way, let's dig into the benefits that a smart switch can provide. The most basic smart switch functions involve basic port control and monitoring.

In the GS108T, some of these features are located on the Switching > Ports page (Figure 6).

Port Configuration screen
Click to enlarge image

Figure 6: Port Configuration screen

The Port Configuration page both summarizes link status and enables setting of port speed and mode and disabling flow control. There is also a Default Priority control that allows you to set a priority from 0 (lowest) to 7 (highest) for untagged packets coming into the port.

Things you can do with these settings are:

  • Shut off troublesome users/devices - The Port Speed control can be set to 100M, 10M, Auto and Disable. The last setting effectively unplugs the device from the network. Although you could always yank a cable out, a smart switch lets you do it remotely. This can be handy if the switch is located someplace where you, the network administrator, usually are not, like a basement or closet.
  • (Crudely) control bandwidth - Setting port speed also provides brute-force bandwidth control. By forcing a port to 10 or 100 Mbps, you can put a real crimp in local bandwidth hogging. I say local because the resolution of this setting isn't fine enough to provide effective bandwidth control of Internet traffic. If you have only 5 Mbps of Internet download speed, setting Suzie's port speed to 10 Mbps isn't going to limit her ability to hog the family Internet connection.

Tracking Traffic

But how do you know who is hogging bandwidth? All you need to do is check the port statistics. In the GS108T, these are on the Monitoring > Ports page (Figure 7). I would have preferred that the positions of the Summary and selected port statistics were reversed, since you generally have to look at the overall picture before you drill down to a specific port.

Port Configuration screen
Click to enlarge image

Figure 7: Port Statistics

One glance at the Summary Bytes fields shows that Port 8 has the highest traffic, which makes sense because it is used to uplink the switch to my main LAN switch. The upper part of Figure 7 shows the detailed statistics for Port 8, which show nothing particularly unusual.

In addition to seeing where your bandwidth is going, statistics can also help you troubleshoot slow or intermittent connection problems. The Summary Drop Packets field could be helpful for finding overloaded uplink ports, but it only reflects packets dropped due to Flow Control back pressure or switch memory overload. More helpful fields might be the Errors and Collision fields for Transmit and Error, Fragments, Jabbers, UnderSizePkts and OverSizePkts fields.

Mirroring to Sniff

The last basic port feature provided by smart switches is port mirroring, which is used when you need to packet sniff with Wireshark. Doug Reid's series will take you through the basis of using this popular, free packet capture and analysis program. But unless the traffic you want to analyze is flowing to or from the system that Wireshark is running on, you'll need to mirror the port(s).

In the days before inexpensive switches, network hubs were used to create LANs. Hubs are Layer 1 devices, which simply copy all data coming into each port out to all other ports in the hub. This was bad for busy LANs because individual ports could easily become overloaded by traffic on other ports. But it was great for packet sniffing, since you could simply plug your sniffer into any port and see traffic on all other connected devices.

Switches send received data only to the port that connects to the device with the data packet's destination MAC address. So plugging a packet sniffer into a switch port will only detect traffic intended for the device that the sniffer is running on.

But port mirroring lets you copy data on other switch ports to a destination port. Figure 8 shows the GS108T's port mirroring (Monitoring > Mirroring) configured to copy transmit and receive data on port 8 to port 1. (You can also configure transmit only or receive only mirroring.) Since port 8 is being used as the uplink port, this would allow me to sniff all traffic in and out of the switch.

Port mirroring

Figure 8: Port mirroring

These are some of the most basic features that smart switches provide. Next time, we'll explore features that can enhance your small LAN's security.

More Basics

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