Switching Domains

Collision Domains

In the previous topic, you gained a better understanding of what a switch is and how it operates. This topic discusses how switches work with each other and with other devices to eliminate collisions and reduce network congestion. The terms collisions and congestion are used here in the same way that you use them in street traffic.

In legacy hub-based Ethernet segments, network devices competed for the shared medium. The network segments that share the same bandwidth between devices are known as collision domains. When two or more devices within the same collision domain try to communicate at the same time, a collision will occur.

If an Ethernet switch port is operating in half-duplex, each segment is in its own collision domain. There are no collision domains when switch ports are operating in full-duplex. However, there could be a collision domain if a switch port is operating in half-duplex.

By default, Ethernet switch ports will autonegotiate full-duplex when the adjacent device can also operate in full-duplex. If the switch port is connected to a device operating in half-duplex, such as a legacy hub, then the switch port will operate in half-duplex. In the case of half-duplex, the switch port will be part of a collision domain.

As shown in the figure, full-duplex is chosen if both devices have the capability along with their highest common bandwidth.

Broadcast Domains

A collection of interconnected switches forms a single broadcast domain. Only a network layer device, such as a router, can divide a Layer 2 broadcast domain. Routers are used to segment broadcast domains, but will also segment a collision domain.

When a device sends a Layer 2 broadcast, the destination MAC address in the frame is set to all binary ones.

The Layer 2 broadcast domain is referred to as the MAC broadcast domain. The MAC broadcast domain consists of all devices on the LAN that receive broadcast frames from a host.

Click Play in the figure to see this in the first half of the animation.

When a switch receives a broadcast frame, it forwards the frame out each of its ports, except the ingress port where the broadcast frame was received. Each device connected to the switch receives a copy of the broadcast frame and processes it.

Broadcasts are sometimes necessary for initially locating other devices and network services, but they also reduce network efficiency. Network bandwidth is used to propagate the broadcast traffic. Too many broadcasts and a heavy traffic load on a network can result in congestion, which slows down network performance.

When two switches are connected together, the broadcast domain is increased, as seen in the second half of the animation. In this case, a broadcast frame is forwarded to all connected ports on switch S1. Switch S1 is connected to switch S2. The frame is then also propagated to all devices connected to switch S2.

Alleviate Network Congestion

LAN switches have special characteristics that help them alleviate network congestion. By default, interconnected switch ports attempt to establish a link in full-duplex, therefore eliminating collision domains. Each full-duplex port of the switch provides the full bandwidth to the device or devices that are connected to that port. Full-duplex connections have dramatically increased LAN network performance, and are required for 1 Gbps Ethernet speeds and higher.

Switches interconnect LAN segments, use a MAC address table to determine egress ports, and can lessen or eliminate collisions entirely. Characteristics of switches that alleviate network congestion include the following:

• Fast port speeds – Ethernet switch port speeds vary by model and purpose. For instance, most access layer switches support 100 Mbps and 1 Gbps port speeds. Distribution layer switches support 100 Mbps, 1 Gbps, and 10 Gbps port speeds and core layer and data center switches may support 100 Gbps, 40 Gbps, and 10 Gbps port speeds. Switches with faster port speeds cost more but can reduce congestion.
• Fast internal switching – Switches use a fast internal bus or shared memory to provide high performance.
• Large frame buffers – Switches use large memory buffers to temporarily store more received frames before having to start dropping them. This enables ingress traffic from a faster port (e.g., 1 Gbps) to be forwarded to a slower (e.g., 100 Mbps) egress port without losing frames.
• High port density – A high port density switch lowers overall costs because it reduces the number of switches required. For instance, if 96 access ports were required, it would be less expensive to buy two 48-port switches instead of four 24-port switches. High port density switches also help keep traffic local, which helps alleviate congestion.

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