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Understanding VLANs

Hello, I'm fairly new to networking and am currently studying for my CCENT. I've been given the opportunity to assist cleaning up my employers network infrastructure. When looking at these two separate VLAN's there are ports being used on both VLAN 111 and 112. In fact it seems that every port is identical across each VLAN for all 3 switches in the stack. I just need help understanding how this is exactly possible, is this a trunking situation?

cisco1.PNGcisco2.PNG

1 ACCEPTED SOLUTION

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Cisco Employee

Re: Understanding VLANs

Hello Shane,

 

Correct. The port is assigned to both vlan 111 and vlan 112. If the packet comes in with no tag, it will be put into vlan 112. However, phones are capable of putting a vlan tag on a packet so it will assume the phone will send all voice packets with a vlan tag of 111. Whenever we receive these packets, we will put them into vlan 111.

 

Hope that helps!

-Bradley Selzer
CCIE# 60833
8 REPLIES 8
VIP Expert

Re: Understanding VLANs

Hi,

If you are using VoIP, it could be that one vlan is configured for data and the other is for voice.

You can look at the config for one or multiple interfaces and see if 111 and 112 is configured.

example:

sh run int gi1/0/1

sh run int gi1/0/20

and so on...

 

HTH

Re: Understanding VLANs

Alright.cisco3.PNG

 

 

So gi1/0/1 is configured for both 111 and 112? A single port is configurable with multiple VLAN's?

Highlighted
Cisco Employee

Re: Understanding VLANs

Hello Shane,

 

Correct. The port is assigned to both vlan 111 and vlan 112. If the packet comes in with no tag, it will be put into vlan 112. However, phones are capable of putting a vlan tag on a packet so it will assume the phone will send all voice packets with a vlan tag of 111. Whenever we receive these packets, we will put them into vlan 111.

 

Hope that helps!

-Bradley Selzer
CCIE# 60833

Re: Understanding VLANs

Ahh okay, are there any security issues with this practice? Is it worth it to configure ports specifically for a VoIP VLAN? Or is that simply a waste of time?

Advocate

Re: Understanding VLANs

Hi @ShaneElarton0695 ,

 

The voice vlan must be configured only on the ports on which an IP telephone will be connected.
If only a PC is connected to a switch port, it is best to leave it alone with the data vlan.

 

Regards

Advocate

Re: Understanding VLANs

Hi @ShaneElarton0695 ,

 

This is possible when you configure IP telephony in the switch port.
This image illustrates a bit how vlan tags works in these cases:

CCNAVOICE3.5.jpg

 

Regards

VIP Expert

Re: Understanding VLANs

Yes, that is what I thought. Config looks correct.

Good Luck!

HTH

Rising star

Re: Understanding VLANs

Hi my friend,

Very good contributions, you can review the following post so you can see how a data and voice VLAN is configured:

https://community.cisco.com/t5/networking-documents/sample-configuration-for-voice-and-data-deployment-on-a-switch/ta-p/3123513

Regards
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