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A network with 2 switches, each switch has 2 vlan, one called vlan 10 and another vlan 20. A port-channel with 2 uplinks (trunk). Why in a layer 2 scenario I don't need to configure the trunks with the command 'switchport trunk encapsulation dot1q'? and Why when that trunk goes to a router Do I have to encapsulate with dot1q?

I know this is done on the router to have routing between vlans, but would it have any relation with my router sending the encapsulated packet to another network?

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Before there was 802.1Q trunking, Cisco came up with ISL trunking. The IEEE bowed to the other industry vendors and came up with 802.1Q. Cisco added that to its repertoire, and the switches required you to choose one or the other. Cisco deprecated ISL, and the modern Cisco access switches only do 802.1Q.

The Cisco distribution switches and routers still needed to support both because a company may still have old switches and are using ISL, so those devices still need to be told which trunk encapsulation to use. In those devices, you must choose encapsulation of ISL or 802.1Q.

Obviously, you are using more recent access switches that do not have the option of using ISL, so Cisco has removed the trunk encapsulation option from those switches.


Technically, only ISL is an encapsulation because it adds a header to the frame, while 802.1Q only inserts a tag into the existing frame header.

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  • Now I see that my question doesn't make much sense... Thanks @Ron – Bruno Dias Dec 6 '20 at 6:51
  • Really, the access to old and new versions of IOS left me confused, but thanks to your help I see that the fact of not using a command to encapsulate does not mean that such an act is not taking place, thanks again. – Bruno Dias Dec 6 '20 at 6:55
  • One fact that made me think this is that my switch that needs to encapsulate is layer 3, and the switch that doesn't need is layer 2. It seems that my mind is now open! – Bruno Dias Dec 6 '20 at 7:06

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