Network Terminology

 Network Terminology

There are several technologies used in a network that almost everybody working with networks has heard about. 

• Ethernet
• Topologies
• Traffic Flows
• Bandwidth
• Virtualization 

ETHERNET

Ethernet is the most used LAN technology today for interconnecting devices on a network. When sending data over Ethernet, a standard Type II Ethernet frame is 1518 bytes long and carries a MAC header (which includes addressing information about the sender and receiver), a payload (data), and a checksum (CRC).

Ethernet is used to connect different network segments together and is considered a shared medium. That means collisions can occur, so each segment needs to use CSMA/CD* (even with a switch). These segments are called a collision domain*. In a HUB all ports connect to the same network segment*, so each port in a HUB belongs to the same collision domain. Using a switch instead – all switch ports connect to a different network segment, so each switch port is a different collision domain.

When using a switch, it will make a frame forwarding decision based on the destination MAC address used in the MAC-header. Since a switchport is its own collision domain it means a frame will be forwarded between multiple network segments. The frame can be sent as a unicast frame, a multicast frame, or a broadcast frame. A unicast frame is sent to a single destination on the network segment, a multicast frame is sent to a selected group of addresses on the network segment, while a broadcast frame is sent to all hosts on the network segment.

The switch will make the forwarding decision using its CAM table. A unicast frame is forwarded out of a single switch port. A multicast frame is forwarded out of all switch ports that are members of the multicast group (selected group). The broadcast frame will be sent out of all switch ports. The scope of how far the broadcast frame will be forwarded is called the broadcast domain*. But a switch will never forward a frame back out on the same switch port that it was received on.

Ethernet supports multiple topologies and multiple network designs, and if you are not careful, you can end up with a very large multicast group or a very large broadcast domain. Both will impact your overall network performance, so it’s a good general rule to keep your broadcast domain as small as possible.

TOPOLOGIES

A network topology refers to how the network nodes (hosts) are interconnected, including the physical cabling and the logical signaling. A very important skill for any network engineer is the ability to discuss different network topologies. That also includes being able to understand the difference between a physical topology and a logical topology.

In many cases, the physical topology is a star topology, while the logical topology is a bus topology. When I discuss network topologies, I often need to explain the differences between the physical topology and the logical topology to explain a problem. A common mistake is to believe that the physical topology and the logical topology are the same.

“In many cases, the physical topology is a star topology, while the logical topology is a bus topology.” What does this mean?

Physical Topology

The physical topology is easy to understand. Just draw how the devices are actually cabled and wired and you get the physical topology. Examples used today are:

#1. Star Topology                                                      #2. Ring Topology                                          #3. Tree Topology

Logical Topology

It’s much more difficult to try to understand how the logical topology looks. The logical topology is equal to how the communication appears from the perspective of the connected users (hosts).

What impacts the logical topology?

In general, depending on how your network is cabled, it will impact how your logical topology will look.– But what’s often overlooked is that multiple other components also affect your logical topology. Depending on which protocols you are using and how they are configured, you will create different logical topologies.

The most common protocol used in Ethernet is STP* (Spanning Tree Protocol). It’s used to prevent bridge-loops* (often called switch loops). STP will make sure that all redundant connections that can cause a bridge loop will be blocked when you cable multiple switches together. Since STP will block links in that case, it affects the logical topology. The communication path that hosts can take in your network is changed because of the links that STP blocked to avoid bridge loops.

I only mention STP briefly to show that it affects the logical topology and there is much more to learn about how it works. Let's look at an example of how a logical topology looks before STP and after STP.

  Topologies before STP

                              Physical Topology                                      Multiple Logical Topologies

The logical topology between User 1 and User 2 have multiple paths available. User 1 could reach User 2 via multiple paths, for example:

ASW1->DSW2->ASW3

ASW1->DSW1->ASW3

ASW1->DSW1->CSW1->DSW2->ASW3

(There are more available paths, which I left out.)

A network cabled like that will not perform well without a protocol to prevent loops caused by the multiple paths available. STP is going to help us block some of the redundant links, and you can configure it and tune it to meet your network requirements. Depending on your STP configuration, you can end up with different logical topologies even if the physical topology remains the same. From the perspective of the connected hosts, STP allows you to build these logical topologies:

In this case, STP can create multiple logical topologies based on a physical topology. It can be tuned and configured according to best practices you want your core switch to be the root bridge* so that traffic will flow over CSW1. This section doesn't cover any of the advanced features of STP or how it works other than to demonstrate that it will affect the logical topology.

STP is not the only protocol that will build a logical topology; there are too many protocols to discuss in this post to cover them all. I’ll just name a few others for reference purposes:

• EtherChannels (used to hide physical topology for the protocols that build logical topologies)
• Routing protocols (provides a logical view of interconnected networks and how to reach them)
• GRE (used to build a “virtual” or logical point-to-point link)