Trunking And Native VLAN


It’s time to talk about trunking and native vlan. Well perhaps it would be better if we post this article before VTP but not so much problem i think !Trunking works at Layer-2 of OSI referance model and carries multi vlan information between VLANs. Each frame sets the VLAN information. ( That is known as “tagging” too ). There are two types of trunking, first one is ISL trunking, other type is 802.1Q.



– ISL is a cisco proprietary protocol ( Just used in Cisco devices )

– All frame is encapsulated.

– You can see the ISL header scheme here :



– 802.1Q is not a cisco proprietary protocol, it is an industry standart that is used in all vendors devices.

– Frame is not encapsulated, it is tagged.

– You can see the 802.1Q header scheme here :

dot1q header


If you make a topology like this, trunk ports receive the untagged packets but normally trunk ports just carry tagged packets. Native VLAN is the VLAN that carried untagged packets. This concept is used on IP phones too. IP phone and PC is in different VLANs mostly. IP phone’s VLAN is called as “voice VLAN” and coded “switchport voice vlan 10”.


Switch’s ports have five different port states ;

– Access

– Trunk

– Dynamic Auto

– Dynamic Desirable

– Non negotiate

  • Access port is used for access devices such as PCs, printers etc.
  • Trunk port is used for links that goes to another switch.
  • Dynamic desirable makes the ethernet port willing to convert the link to a trunk link. The port becomes a trunk port if the neighboring port is set to trunk or dynamic desirable mode. This is the default mode for all ethernet ports.
  • Dynamic auto is a dynamic port. If you plug that port a switch, that becomes a trunk, if you plug that port a PC, that becomes an access port.This may occur security problems.
  • Non negotiate shuts the DTP up

* You can display the port state using ”Show interfaces f0/25 switchport” command.

* You can display trunk port details using ” show interfaces f0/25 trunk ” command.

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