Vlan Interview Questions and Answers

Vlan Interview Questions and Answers

Q1. Which switching technology reduces the size of a broadcast domain?

By using vlan technology we can reduces the size of Broadcast domain. Vlan (virtual local area network) is a logical grouping or segmenting the devices under a single broadcast domain. As a result provides security and flexibility.

Q2. Which protocols are used to configure trunking on a switch?

Vlan trunking protocol (vtp) is a cisco proprietary protocol that propagates the definition of virtual local area networks (vlan) on the whole local area network. To do this, vtp carries vlan information to all the switches in a vtp domain. Vtp Advertisements can be sent over 802.1q, and isl trunks.

Q3. Which protocol reduces administrative overhead in a switched network by allowing the configuration of a new vlan to be distributed to all the switches in a domain?


Q4. What is SVI?

A switched virtual interface (SVI) is a vlan of switch ports represented by one interface to a routing or bridging system. There is no physical interface for the vlan and the SVI provides the layer 3 processing for packets from all switch ports associated with the vlan.

Q5. What is meant by “router on stick”?

Router-on-a-stick is a term frequently used to describe a setup up that consists of a router and switch connected using one Ethernet link configured as an 802.1q trunk link. In this setup, the switch is configured with multiple vlans and the router performs all routing between the different networks/vlan.

Q6. Which are the two Trunking protocols?

There are two main types of Encapsulation Trunking protocols.

1. ISL (inter switch link) which is cisco proprietary protocol.

2. 802.1q which is an IEEE standard.

Q7. Which protocol encapsulates Ethernet frames?

ISL encapsulates Ethernet frames while 802.1q tags Ethernet frame.

Q8. Which is the vlan not tagged by 802.1q?

Native vlan

Q9. How to delete vlan information from switch?

Delete flash:vlan.dat

Q10. What is difference between Vlan access and trunk mode?

Access mode is used to connect end devices (host) to switches while trunk mode is used to connect between switches.

Q11. What is difference between dynamic auto and dynamic desirable?

Dynamic Desirable: attempts to negotiate a trunk with the other end.

Dynamic Auto: forms a trunk only if requested by the other end.

Q12. What is the use of none negotiate command in switch?

None negotiate command disables automatic formation of trunk links. It will be good to configure trunk manually and give none-negotiate command for security reason.

Q13. Explain different switch port modes?

Trunk: forms an unconditional trunk

Dynamic Desirable: Attempts to negotiate a trunk with the far end

Dynamic Auto: Forms a trunk only if requested by the far end

Access: It will never form a trunk

Q14. What is DTP?
Dynamic Trunking protocol is used to automatically establish trunks between capable ports (insecure method).

Q15. Can we see trunk interfaces in show vlan command?


Q16. Which is the command used to see trunk interfaces?

Show interface trunk

Show interface fa1/0/13 trunk

Show interface fa1/0/13 switchport

Show interface status | include trunk

Q17. What are frame tagging and different types of frame tagging?
Vlan frame tagging is a technology which is used to identify the vlan that the packet belongs to. The vlan frame tag is placed on the Ethernet frame when the Ethernet frame reaches a switch from an access port, which is a member of vlan.

Q18. What is the maximum number of vlans permitted in 802.1q and ISL?
Maximum vlan permitted in 802.1q is 4094

Maximum vlan permitted in ISL is 1000

Q19. What is the header size of 802.1q?

4 bytes

Q20. What is the difference between an access port and a trunk port?

A trunk does add dot1q or ISL tags directly to frames and can exist on all or multiple vlans. While an access port only passes traffic from a set vlan but does not modify the frame with a vlan tag.

Q21. What is a native vlan and what type of traffic will go through native vlan?

The native vlan is the only vlan which is not tagged in a trunk, in other words, native vlan frames are transmitted unchanged. Per default the native vlan is vlan 1 but you can change that:

#show interface fa0/8 trunk.

#Port mode encapsulation status native vlan

Q22. What is inter-vlan routing?

Virtual LAN (vlans) divides one physical network into multiple broadcast domains. But, vlan-enabled switches cannot, by themselves, forward traffic across vlans boundaries. So you need to have routing between these vlans which is called inter vlan routing.



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