How to ensure trunk link in the switch are using dot1q encapsulation, when dtp (dynamic trunking protocol) is enable along with vtp.

I know when manually setting trunk links, you can force the trunk link to use dot1q encapsulation, but see that when using vtp and dtp, the trunk link says n-802 and does not change to dot1q.

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 9:30

The newer Cisco switches only support dot1Q encapsulation. You do not need to do anything because it is already configured that way. Older Cisco switches could support switchport trunk encapsulation isl or switchport trunk encapsulation dot1Q, but newer switches do not need the switchport trunk encapsulation command because they only support the 802.1Q trunking.

You should really be moving away from VTP and DTP. The current best practices are to set VTP to transparent mode and manually configure VLANs allowed across the trunk. VTP can inadvertently crash your whole network. Another best practice is to only allow a VLAN to be on a single access switch. This greatly reduces the chances of a broadcast storm. An access switch can have multiple VLANs, but those VLANs on that access switch should not extend to any other access switches. Modern networking is layer-3, and it is very rare to need devices on the same layer-2 network.

DTP is a vulnerable protocol that can be misused by end-users. If you have a trunk, you should nail it up and use the switchport nonegotiate command on the interface to disable DTP. You do not want the trunk to switch to access mode if someone plugs in a non-trunking device, nor do you want an end-user to be able to connect a device running DTP and get access to all your VLANs.

  • thanks for the reply, however I'm not looking for an enterprise best practice solution, I'm currently studying network and need to understand vtp and dtp. Using a layer 3 switch in auto mode, I see encapsulation as n-802.1q, shouldn't it be 802.1q ? – Novice Programmer Sep 16 '18 at 22:22
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    "I see encapsulation as n-802.1q, shouldn't it be 802.1q ?" It is 802.1Q, but it is a negotiated trunk, hence the n-802.1q. The 3950 switch only supports 802.1Q, not ISL, so it will only use 802.1Q. – Ron Maupin Sep 16 '18 at 22:28

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