802.1Q Native VLAN on Cisco IOS Switch

Hello Aniket

There are two ways to implement the following scenario:
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One is to configure the Gi0/1 interface as a trunk. Let’s say the voice VLAN on our network is 137 and the data VLAN is 135. We would configure the Gi0/1 interface as a trunk, with a native VLAN of 135 and an allowed VLAN of 137. This means that frames on VLAN 135 destined for H1 would exit the Gi0/1 interface untagged. Any such frames reaching the IP phone would continue on to H1. Any frames on VLAN 137 destined for the phone would exit the Gi0/1 with a tag of 137. The phone receives this frame, removes the tag and takes the frame for itself to be processed.

A sample config would be:

interface gigabitetherent 0/0
  switchport mode trunk
  switchport trunk allowed VLAN 137
  switchport turnk native vlan 135

Now this is the “old” way of implementing voice in this manner. It is actually still used on Cisco switches when configuring third party vendor phones. The “new” way of implementing this is using the voice VLAN.

In this case, the interface is configured as an access interface with an assigned VLAN oif 135, which is the native VLAN. An additional command would be added indicating which is the voice VLAN, specifically, VLAN 137. A sample config is the following:

 interface gigabitetherent 0/0
      switchport mode access
      switchport access vlan 135
      switchport voice vlan 137

The configuration is a little more elegant and more intuitive, but the result is the same. Now there is the following important difference between these two configs: the latter involves CDP communicating with the IP phone and letting it know which is the voice VLAN and which is the data VLAN so it can react correctly to the appropriate tagged/untagged frames.

To find out more about the voice VLAN and how it is configured, take a look at this lesson:

I hope this has been helpful!

Laz

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