VLANs Deep Dive

Brief History of VLANs

After successful experimentation with VoE (Voice Over Ethernet) W. David Sincoskie, with Bellcore tackled a problem that Ethernet itself was facing. Ethernet is a broadcast network, meaning that all signal from the host is forwarded to all connected devices. This resulted in high CPU on the endpoint devices, while also clogging the network with unnecessary traffic. Moreover, at the time, there was no proven way to connect multiple Ethernet networks. IP routing was one possible solution, but the drawback was that IP routing was slow and expensive.

Sinconskie reinvented a technique called transparent bridging. Unfortunately, this approach created new issues, namely turning central switches into bottlenecks that limited scalability.

Transparent Bridging

Transparent Bridging was a technique used in modern Ethernet switches. However, using switches to connect multiple Ethernet networks in a fault-tolerant fashion requires redundant (unnecessary) paths through that network, which in turn requires a Spanning Tree configuration. This ensures that there is only one active path from any source node to any destination on the network. This causes centrally located switches to become bottlenecks, limiting scalability as more networks are interconnected.

Creation Of VLANs

To help alleviate this concern, Sinconskie invented VLANs by adding a tag to each Ethernet Frame.

Destination MAC	Source MAC	Type (VLAN)	VLAN Tag	User Data
6	6	2	4	46 - 1500 Bytes

These tags could be thought of as colors, say red, green, or blue. In this scheme, each switch could be assigned to handle frames of a single color, and ignore the rest.

Configuration Of VLANs

Early network designers often segmented physical LAN's with the aim of reducing the size of the Ethernet collision domain, resulting in improved performance.

Collision Domain and Hubs

A collision domain is a network segment connected by a shared medium or through repeaters where simultaneous data transmissions collide with one another. A network collision occurs when more than one device attempts to send a packet on a network segment at the same time. Because in older days of networking we used Hubs, not modern switches.

IEEE 802.1Q

IEEE 802.1Q, is the networking standard that supports VLANs, on an IEEE 802.3 Ethernet network. The standard defines a system of VLAN Tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames. The standard al