Trunking and DTP

Dynamic trunking protocol (DTP) automatically connects and configure trunks among switches.

We will use DTP here and will discuss pros and cons of this protocol.


There are two standards for trunk ISL and 802.1q. ISL is Cisco proprietary and not much used; 802.1q is open standard and used almost everywhere.

When you have trunking mode to auto then you will see the following configuration.


Notice that the encapsulation is n-802.1q which tells that the encapsulation was negotiated from the neighbor interface.

There are two types of auto mode for DTP.

Active this trunk mode keeps telling its connected port hey you are trunk if you are let’s do trunk everybody is doing this lets have fun.

Passive this mode is a silent mode where interface does not tells its neighbor interface that it is trunk and does not sends request to be trunked. But when someone asks it hey are you trunk then it tells that yes it is trunk and can do trunking operations.


Here we can see that there are two trunk modes. Auto is passive and desirable is active mode.

The interfaces will get trunk in following cases:

Switch-1 Mode

Switch-2 Mode




Trunk establish



Trunk establish



Trunk establish



No Trunk






Look this is the result of Dynamic trunk protocol.

Notice that here the negotiated encapsulation is isl and if we want to hardcode the 802.1q we can do this on either switch and other side will negotiate. But it is highly recommended that you shutdown the interface before changing the protocol parameters.

It is highly recommended that you hardcode the ports to access mode that are used for computers and if you are using port switch to switch then you can use the dynamic mode.

By default trunk port carries the data of all VLANs but if we want to restrict the trunk to carry only data of specific VLANs we can do this by executing following command.



Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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