Network

Setting up a DHCP relay is a pretty common task that performed by network administrators when setting up a new LAN. If you’re not familiar with a DHCP Relay, take a look at the example below. In order for a client to get an IP Address from a DHCP Server, it sends out a broadcast once it’s plugged into the network. The broadcast is asking for any servers that are DHCP servers to reply. Remember that a broadcast is a frame that is forwarded to all hosts on a Local Area Network. The DHCP Server will reply and the client will get it’s IP Address. ...

If you’ve been in a situation where you need to test connectivity, you’ve probably used the ping command. But what do you do when you’re trying to test connectivity from an ESXi host? Luckily there is a command called vmkping that will allow you to test from the host. The first thing that you need to do is to SSH into your ESXi host. Turn the SSH Service on from the Configuration –> Security Profile Tab. Then you can use your favorite ssh client and remote into your host. ...

This is a series of posts designed to help readers understand how the Internet works. This specific post looks directly at collision domains. Ethernet uses a process called “Carrier Sense Multiple Access with Collision Detection” or CSMA/CD for short. This is a very long way of explaining the process of how network adapters can share the same media to communicate. Think about it if you have 10 machines on a network that are all sharing the same wires or devices, how can any of the devices understand anything with all those frames? ...

If you find yourself in an unfamiliar network and want to understand how the networks are connected, it would certainly be nice to be able to tell what is connected to each other. Luckily there are a couple of protocols that are responsible for just that. Cisco Discovery Protocol (CDP) As you can probably guess from the name, the Cisco Discovery Protocol is a proprietary protocol from Cisco Systems. ...

I’ve recently had to label more network cables than I care to discuss, but found my mind wondering over the best method to label these cables. I’ve come up with three different ways to label networking cables and wanted to get some thoughts from other Engineers about how they go about this. Method 1: Same label on both sides This method creates 2 labels that are identical and puts one label on each side of the cable. This give the advantage that if you’re running multiple batches of cables all at once, you can determine exactly which cable you’re working with. ...

One of the benefits of using HP Virtual Connect in C-class blade Chassis is the ability to have MAC Addresses and WWNs set on a server bay as opposed to the physical server. I’m sure you’re aware that each device that has a network card has a Media Access Control (MAC) address which is a burned in identifier that makes that NIC unique. HP decided that it might be nice to control those MAC Addresses in their blade chassis. Before you setup any server profiles, you have the option to choose “Virtual Connect Assigned MAC Addresses”. These are addresses that are assigned to each server bay so that no matter what blade is put into the bay, the MAC addresses will stay the same. You might find this very useful in the case of a failed blade. If you receive a new blade from HP and throw it into the same bay, it will retain all of the same MAC Addresses and thus look the same to your switches. ...

I often hear Port Address Translation (PAT)referred to as Network Address Translation (NAT). Its a pretty common to hear this and is really not a big deal because the two are similar and I know what is meant. But to clear things up I decided to put together a quick post. Network Address Translation NAT is the process of “translating” an IP Address in a router or firewall. This is most commonly done to present a private IP Address into a Public IP Address that is accessible on the Internet. For instance, you may want to have your E-mail server have a public address so that it can route mail. ...

Jumbo frames can be useful to optimize IP networks, especially in storage networking. This post should help to explain why using jumbo frames can be useful. I’m not Jumbo, I’m just big boned! First, let’s define what we mean by the term jumbo frame. As you can imagine it’s bigger than a normal frame. A Jumbo frame simply means any frame with an MTU larger than 1500 bytes. What exactly does that mean? To really understand that we need to look at an Ethernet frame. The diagram below shows a hastily thrown together Ethernet frame and most of the frame we’re not concerned with for this topic. Parts of the frame are used for determining where the frame is headed, where it came from and to make sure it arrived intact. The section we’re looking at is the “Data” or “Payload” section of the frame. ...

Suppose you have multiple virtual machines that you would like to distribute load across that are housed inside of your virtual environment. How do we go about setting up Network Load Balancing so that it will still work with things like DRS and VMotion? Switch Refresher In most networks we have switches that listen for MAC addresses and store them in their MAC Address Table for future use. If a switch receives a request and it knows which port the destination MAC address is associated with, it will forward that request out the single port. If a switch doesn’t know which port a MAC Address is associated with, it will basically send that frame out all of it’s ports (known as flooding) so that the destination can hopefully still receive it. This is why we’ve moved away from hubs and moved towards switches. Hubs will flood everything because they don’t keep track of the MAC Addresses. You can see how this extra traffic on the network is unwanted. ...

A question often comes up about what to do when you have a segmented virtual network that needs to be able to traverse subnets. This might happen if you’re doing some testing and don’t want the machines to contact the production network, or perhaps doing a test SRM failover and having the virtual machines in their own test network. Virtual machines in subnet (A) might need to contact other virtual machines in subnet (B) but don’t have access to the physical router any longer, so they can’t communicate. To solve this issue, how about we try a virtual router? ...