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Trunk & Native VLAN

sadik.bash
Level 1
Level 1

‎11-02-2012 08:43 AM - edited ‎03-07-2019 09:49 AM

Hi,

I inhireted a number of Cisco switches, routers, and firewalls in my new job. I was reveiwing the config file of the switches and ran across a few Interfaces that were created as Trunk ports(802.1q) but also assiged two VLANs as Native VLANs. I got confused since my understanding about Trunk ports is that they would allow the passing of ALL VLANs. My question is why was or what is the purpose of assigning the Trunk Port the Native VLANs?

Much appreciated.

~sK

!

interface GigabitEthernet2/0/41

switchport access vlan 10

switchport trunk encapsulation dot1q

switchport trunk native vlan 2

switchport mode trunk

----------------------------------------------------

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ALIAOF_
Level 6
Level 6

‎11-02-2012 09:03 AM

By default Native VLAN is VLAN 1.  And Native VLAN traffic does not get tagged (802.1q).  Find out what is the purpose of that VLAN2.  So any traffic that is on VLAN 2 does not get tagged and will pass even if there is an issue with the trunk itself.  Couple of useful links:

http://etherealmind.com/basics-cisco-ios-native-vlans/

http://www.sans.org/security-resources/idfaq/vlan.php

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