VLAN, TRUNKING, VTP: INTERVIEW QUESTIONS

1. Can Native VLAN be tagged?
Yes, Native VLAN can be tagged

 

2. Is it possible :
Network: SA——SB——–SC
SA(Native VLAN is 10)——-(Native VLAN is 10)SB(Native VLAN is 20)——(Native VLAN is 20)SC
Will it work ?
Ans: Yes
At opposite end comparing trunk ports, Parameters should match, but may vary in whole network

 

3. Is it possible : SA——SB
SA(Native VLAN is 10:Tagged)————(Native VLAN is 10:Untagged)SB
Ans: No
Traffic will flow from SA to SB, but not from SB to SA

 

4. How many Native VLAN cane be made on a switch ?
Ans: At maximum, Number of Native VLAN’s=Number of Trunk ports

 

5. Can i delete Default VLAN. If deleted, what are the effects?
Ans: No
Default VLAN cannot be deleted. As , Default VLAN carries controlling traffic like CDP, VTP.
If CDP(Cisco Discovery Protocol) traffic gets stopped, devices will not be able to detect other physical devices

 

6. What is the concept of Management VLAN. What is the recommendation regarding its use?

 Ans.

Any Layer 3 VLAN can be the Management VLAN. Simply Configure Layer 3 VLAN, configure IP, Configure description as Management VLAN. It is recommended not to use VLAN 1 as management VLAN.

As, VLAN 1 is the default VLAN, which carries Controlling traffic like CDP, VTP, DTP.

 

7. What are the conditions for enabling trunking?

 Ans.

Example:  SWITCH-A(F0/0) ——–   (F0/1)SWITCH-B

>> Port F0/0  and  Port  F0/1,  both should be trunk ports

>> VLAN’s must be allowed on both the Trunk ports

>> For Ports F0/0  and  Port F0/1, Native VLAN should be same on both sides

>> Encapsulation type should be same on both sides, either  ISL  or  802.1Q

 

 

8. How to make Revision number=0 for a switch?

Ans. Two Options

> Make VTP domain name as any bogus(or Fake name), this will reset the revision number

> Change switch mode to Transparent mode, this will reset the revision number

 

9. In a VTP domain, how many Switches can be there in Server Mode?

Ans.

As such there is no limitation, no one is primary or secondary, Number of switches in server mode simply means different point in network to manage VLAN’s

 

10. In a VTP domain, What is the default mode of a switch?

Ans.

Server Mode

 

11. What is the difference between version 2 & version 3 in VTP?

REFERENCE LINK

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/vtp.html#wp1020429

 

12. Can we send ARP request from a L2 device ?

Ans.

We need Layer 3 capability for any VLAN, to send ARP request.

Real World Scenario

On Layer 2 switch, we can configure only one IP for management or simply to get remote access of switch.

we make Layer 2 VLAN’s on Layer 2 Switch. Corresponding to these Layer 2 VLAN’s, we configure Layer 3 VLAN’s on connected Layer 3 Switch. Then, Layer 3 switch sends ARP request.

Example

Switch A (Layer 2)  =======  ( Layer 3) Switch B

On switch we make any VLAN, say VLAN 10. So, we have to Layer 3 VLAN ( or SVI) on Switch B.  Now,  Switch A got the capability of sending ARP.

 

13.  Can we make two SVI’s for a single VLAN?

Ans.

Yes : In case we have two different routers. we do this kind of configuration in HSRP.

Example:

R1 Configuration:R1#sh running-config int vlan 3

interface Vlan3

ip address   10.144.3.253 255.255.255.0

standby 1 ip   10.144.3.1

standby 1 priority 110

 

R2 configuration:R2#sh running-config int vlan 3

interface Vlan3

ip address   10.144.3.254 255.255.255.0

standby 1 ip   10.144.3.1

 

No: In case of a single router

 

14. If  3  new  VLAN’s has to be added in the network, i have two options to configure

Option 1:

Do it one by one

switch(config)#vlan 10

switch(config)#vlan 11

switch(config)#vlan 12 

Option 2:

switch(config)#vlan 1-3

 

What is the effect, Revision number increased will be same or different using above configuration methods ?

Ans.

Change in Revision number is different

In first option, Revision no. gets increased by 3

In second option, Revision no. gets increased by 1

 

Reference Video: starts at 28:00

 

15.  Two Switches are in VTP Server mode with different VLAN’s. Revision Number of both the switches is 0. Which switch will overwrite his own VLAN Database onto other, if both switches are in the same VTP Domain?

Ans.

The one where change is performed first; Both switches don’t do anything until you add or remove any vlans, so that revision number goes up; then that switch will push vlans onto the other one

 

16.  Scenario: We have Switch A in Server mode with Revision number 10 & another Switch B in Client mode with Revision number 20. Who will win & overwrite the another’s Switch VLAN database ?

Ans.

In any case, Revision number always takes the precedence. That’s why it is recommended to add new switch in the network with revision number as ZERO. So, Switch B will overwrite the database of Switch A.

 

17.  If a Switch is in Client mode, Can we enter into the configuration mode ?

Ans.

No

 

18.  By the statement, ” VLAN is down “. What to conclude from this statement ?

Ans. 
If a Vlan is down, it simply means that Physical ports associated with the VLAN are down.

 

Example Scenario

Logs

Sep 2 07:29:25 EDT: %LINK-3-UPDOWN: Interface Vlan100, changed state to down
Sep 2 07:29:25 EDT: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan100, changed state to down

core#sh int des | i Gi7/2
Gi7/2   down    down    Connection to switch

 

Comment

Gi 7/2 is the port assigned to VLAN 100, Gi7/2 is in down state.

 

19. If a VLAN interface or SVI went down &  we want to make the VLAN interface or SVI to be up forcefully.

How to do that & in which scenario, this will be required ?

Ans.

Command to make interface VLAN to be up forcefully,

” no autostate ”  under SVI interface.

 

Scenario

We use it on cisco 800 series CPE ( Customer Premises Equipment ) to verify internet reachability from lan interface, when customer lan is not connected.

And this is just for testing only as it is not recommended to keep autostate off on svi port.