# 8 Implementing Ethernet Virtual LANs
###### tags: `CCNA`

### Virtual LAN Concepts
However, an alternative narrower definition of a LAN can help in understanding the concept of a virtual LAN:

**A LAN includes all devices in the same broadcast domain.**

A broadcast domain includes the set of all LAN-connected devices, so that when any of the devices sends a broadcast frame, all the other devices get a copy of the frame.

**So, from one perspective, you can think of a LAN and a broadcast domain as being basically the same thing.**



Limiting the number of hosts that receive a single broadcast frame reduces the number of hosts that waste effort processing unneeded broadcasts.

The following list summarizes the most common reasons for choosing to create smaller broadcast domains (VLANs):
- **To reduce CPU overhead on each device, improving host performance**, **by reducing the number of devices that receive each broadcast frame**
- To reduce security risks by reducing the number of hosts that receive copies of frames that the switches flood (broadcasts, multicasts, and unknown unicasts)
- To improve security for hosts through the application of different security policies per VLAN
- **To create more flexible designs that group users by department**, or by groups that work together, **instead of by physical location**
- To solve problems more quickly, because the failure domain for many problems is the same set of devices as those in the same broadcast domain
- To reduce the workload for the Spanning Tree Protocol (STP) by limiting a VLAN to a single access switch

### Creating Multiswitch VLANs Using Trunking
The design shown in Figure 8-3 works, but it simply does not scale very well.
It requires one physical link between switches to support every VLAN.
**If a design needed 10 or 20 VLANs, you would need 10 or 20 links between switches, and you would use 10 or 20 switch ports (on each switch) for those links.**
使用VLAN Trunk，以這個舉例就可以不需要兩個link

拓樸大一些的應用使用VLAN Trunk時

### The 802.1Q and ISL VLAN Trunking Protocols
**This 12-bit field** supports a theoretical maximum of 212 (4096) VLANs, but in practice it **supports a maximum of 4094.** (Both 802.1Q and ISL use 12 bits to tag the **VLAN ID, with two reserved values [0 and 4095].)**

### Forwarding Data Between VLANs
**The Need for Routing Between VLANs**

**Routing Packets Between VLANs with a Router**

### VLAN and VLAN Trunking Configuration and Verification

Creating VLANs and Assigning Access VLANs to an Interface