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VLAN Interview Questions and Answers

January 18, 2016

What is a VLAN and how it will reduce the broadcast traffic?
A VLAN is a logical grouping of network users and resources connected to administratively defined ports on a switch. VLAN divides the Broadcast Domain So, the frames that will be broadcasted onto the network are only switched between the ports logically grouped within the same VLAN.

What is the difference between an access port and a trunk port?
Access port - Access Port belongs to and carries the traffic of only one VLAN. Anything arriving on an access port is simply assumed to belong to the VLAN assigned to the port. Any device attached to an access link is unaware of a VLAN membership as switches remove any VLAN information from the frame before it’s forwarded out to an access-link device. Access-link devices can’t communicate with devices outside their VLAN unless the packet is routed.
Trunk Ports - Trunk Port can carry the traffic of multiple VLANs from 1 to 4094 VLans at a time. Normally Trunk link is used to connect switches to other switches or to routers. Trunk ports supports tagged and untagged traffic simultaneously.

What is Frame Tagging and different types of Frame Tagging?
Frame tagging method uniquely assigns a VLan ID to each frame. It is used to identify the VLAN that the Frame belongs to.
There are mainly two types of Frame Tagging Method:-
1.Inter-Switch Link (ISL)
2.802.1Q
These are also known as Frame Encapsulation Protocols.

Explain difference between 802.1Q and ISL ?
802.1Q - It is an open standard created by the Institute of Electrical and Electronics Engineers (IEEE). To Identify to which VLAN a frame belongs to, a field is inserted into the frame's header. It is a Light Weighted Protocol & adds only 4 Byte within Frame's Header.
ISL (Inter-Switch Link) - This protocol is Cisco proprietary which means unlike 802.1Q, it can be used only between Cisco switches. ISL works by adding Header (26 Bytes) and Trailer(4 Bytes) with Original Ethernet Frame.

What is a Native VLAN and What type of traffic will go through Native VLAN?
The Trunk port is assigned a default VLAN ID for a VLAN that all untagged traffic will travel on. This VLAN is called the Native VLAN and is always VLAN 1 by default (but can be changed to any VLAN number). Similarly, any untagged or tagged traffic with unassigned VLAN ID is assumed to belong to the Native VLAN.

What is Inter-Vlan Routing?
VLANs divide broadcast domains in a LAN environment So, by default only Hosts that are members of the same VLAN can communicate. Whenever hosts in one VLAN need to communicate with hosts in another VLAN, the traffic must be routed between them. This is known as Inter-VLAN routing.
This can be done by two methods - Router-On-Stick & Switch Virtual Interfaces (SVI)

Give the commands to create VLAN?
Switch(config)# vlan 10
Switch(config-vlan)# name sales
Switch(config-vlan)# exit

How can we add an interface to a VLAN?
Switch(config)# interface fastethernet0/0
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10

How to configure trunk link?
Switch(config)# interface fa0/24
Switch(config-if)# switchport trunk encapsulation <dot1q/isl>
Switch(config-if)# switchport mode trunk

How can we change Native Vlan?
Switch(config)# interface fa0/0
Switch(config-if)# switchport trunk native vlan 100

Which command is used to see trunk interfaces?
Switch# show interface trunk

Which command is used to see all VLANs information?
Switch# show vlan

 

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