VLANs Basic

Hi Guys,

1. Today I will talk about VLAN (Virtual Local Area Network). VLANs (Virtual LANs) are logical grouping of devices in the same broadcast domain. VLANs are usually configured on switches by placing some interfaces into one broadcast domain and some interfaces into another. VLANs can be spread across multiple switches. VLANs offer the following advantage. 

•        Easy Administration 
•       Increase  the broadcast domain

• ·        Restricting broadcast packets (broadcast storms), saving bandwidth, and improving network processing capability.
• Broadcast packets are restricted within a VLAN. If the switch that is connected to multiple VLANs is not a Layer-3 switch, frames are not directly transmitted from one VLAN to another.
• ·        Enhancing VLAN security.
• VLANs cannot directly communicate with each other. That is, the users on one VLAN cannot directly visit the users on other VLANs. The users on different VLANs can visit each other through Layer-3 devices such as routers or Layer-3 switches. 

VLAN Terminology

Trunking

· Extending multiple VLANs over the same physical connection

Native VLAN

· By default, frames in this VLAN are untagged when sent across a trunk

Access VLAN

· The VLAN to which an access port is assigned Voice VLAN If configured, enables minimal trunking to support voice traffic in addition to data traffic on an access port.

Dynamic Trunking Protocol (DTP)

·Can be used to automatically establish trunks between capable ports; carries a security risk. Switched Virtual Interface (SVI) A virtual interface which provides a routed gateway into and out of a vlan.

Vlan Trunking

Trunks allow carrying traffic for more than one VLAN on the same link. There are two types of trunks supported on

Cisco switches:

1.     Inter Switch Link (ISL): encapsulated original frame into 30-bytes ISL frame (26-bytes for ISL and 4-bytes for CRC). Cisco  propriety method

2. IEEE 802.1Q: an open standard. Instead of encapsulatingit embeds tag 4-bytes in the Ethernet frame.