In This Lesson We Will
- Discuss Switching History
- Learn about Broadcast and Collision Domains
- Review How Switches Forward Traffic and the Methods Used
A Little Backstory
Today, switches are almost exclusively used to connect end devices to the network. This is because switches contain an algorithm that allows them to make forwarding decisions, i.e., whether to forward a packet or not to an end device. In the past, legacy devices such as hubs and bridges were used, and these did not have the ability to make forwarding decisions. Hubs and bridges simply forwarded packets to every user, out every interface save for the one it received it on, and it was up to the user device to discard these packets if it wasn’t meant for them. As you might imagine, this isn’t great for security, and would bog down the network with unnecessary traffic.
This also meant that collisions were more frequent. If a frame collided, it would stop all traffic on the network for a set period of time. This was discussed further in a prior section (link here: Introduction to Computer Networking – Every Day Tech (every-day-tech.com)), but a quick review would help:
Older networks were basically daisy chained computers along a large (and I mean large [
Credit to David Bombal:
]) physical bus. All network traffic between the computers was shared along this bus, and if a collision occurred, the end devices would react appropriately, ceasing all traffic until the collision was cleared up.
This issue was somewhat resolved with the addition of bridges to a network, as a bridge would segment the LAN into two collision domains. However, the collisions still remained a problem until the switch was developed.
Collision and Broadcast Domains
A collision domain is a set of LAN interfaces whose frames could collide with each other. Switches reduce collisions by creating a dedicated connection to the host. This of course improves bandwidth because switches provide full duplex, dedicated bandwidth to each network segment. However, it’s worth noting that out of the box, a switch provides no relief from broadcast traffic, this is because a switch requires a router or a VLAN to segment broadcast domains. Without a broadcast domain, a switch will flood the packet out every active port, save for the one it received the packet on.
There are three methods in which switches forward packets:
- Store and Forward Switching – As the name suggests, the switch will store the entire frame, the CRC (cyclic redundancy check) is calculated, and if the CRC passes, the frame is forwarded to it’s destination.
- CRC will analyze and evaluate the data integrity in the frame trailer. This ensures the switch forwards to the correct destination, and forwards an in-tact packet.
- Cut-through switching – The switch will ‘cut-through’ half of the frame to read the destination MAC address, and determine which port to forward the data to. It will only forward a frame when the switch determines a match between the destination MAC and it’s MAC address table. This happens all while the rest of the frame is still being received. Meaning, no error checking.
- Fragment Free Mode – Switch ensures that no fragmentation of the packet has occurred. Switch waits for the collision window (the first 64 bytes) to pass before forwarding the frame, so that each data field is checked. This switching methods is as quick as cut-through, and provides better error checking.
Other Bits about Switching:
Symmetric switching – This provides a switched connection between ports with the same bandwidth, i.e., all 100mbps ports.
Asymmetric switching – Provides switched connections between ports of unlike bandwidth. The opposite of symmetric.
Switches will store frames for a brief time in a memory buffer. The two memory buffering methods are:
Port based memory – Frames stored in queues that are linked to specific incoming ports
Shared memory – Frames ar e deposited into a common memory buffer that all ports on the switch share.
L2 and L3 Switching
A layer two switch will perform switching and filtering of only layer 2 protocols, MAC addressing. A layer 2 switch is completely transparent to network protocols and user applications. An L3 switch provides all the functionality of a L2 switch, but can also route IP packets. L3 switches can work in place of a router.
So Why Use a Switch?
As discussed in the legacy section, hubs and bridges provide a number of downsides with their positives, mainly, increasing the number of collision domains. Switches will help eliminate collision domains, assuming you’ve set them up correctly (remember, each broadcast domain should be separated into a VLAN.) A switch can do this by interpreting the bits received in the frame so that it can send the frame out the one required port instead of flooding it out all other ports. Additionally, if the switch does need to flood the frame, it will buffer the frame, storing it memory, and sending it out to each device one by one to avoid collisions.
If a switch has only one device directly connected, it can operate in full duplex, meaning the NIC on the host and switch can send and receive information at the same time. So instead of one 100mbps connection, you can use two 100mbps connections together at 200mbps, effectively doubling the bandwidth. This also means that no collision will occur if only a single device attached, as the switch is designed to receive and forward frames concurrently. Also, if more than one device is attached to the switch, each switch port has its own dedicated 100mbps, so the bandwidth is not shared per device, but dedicated per port.
Read back through the lesson and answer these following questions:
- What are the benefits of using a switch over a hub or bridge?
- What role did hubs serve before the introduction of switches?
- Describe the differences between symmetric and asymmetric switching
- Describe the 3 switching methods
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