A link which connects the switches is known as trunk link, and ports used for trunk link are known as trunk ports.

As every interface has specific speed, sometime due to mismatch of speed, link gets down. You can use any port as trunk port, but you have to select the best one or faster than access ports, because heavy traffic of VLANs has to pass through trunk.

Types of trunk:

  1. Static (manual)
  2. Dynamic (automatic)
  • Dynamic trunk: By default, DTP (Dynamic Trunk Protocol) negotiation is enabled on each port of switch. So, if you connect the switches using any interface and by running no shutdown command on those interfaces, dynamic trunk will be created.

Two modes of DTP:

  1. Dynamic desirable ( in this mode switch initiates to create trunk link)
  2. Dynamic auto ( in this mode switch does not initiate to create trunk link, but accepts the request for trunk link to make trunk).

Case 1:                     Dynamic desirable     sending request     Dynamic auto

Dynamic desirable          accept                Dynamic auto

Dynamic desirable           trunk                Dynamic auto

Case 2:                       Dynamic desirable           trunk                Dynamic desirable

Case 3:                       Dynamic auto            no trunk link          Dynamic auto


  • Static trunk: You have to prefer static trunk because hackers can hack devices through DTP negotiation packets. You must have to switch off DTP negotiation by yourself, otherwise hacker can hack even on static trunk.
  • Encapsulation: Data always travel with the help of protocol. On layer 2 protocol is known as encapsulation. Following encapsulations are responsible to carry data among switches:
  1. ISL (Cisco’s proprietary)
  2. Dot1q (Standard)

When data travel through trunk link, it is either in encapsulated form or a tag is being attached with this data. (encapsulated form means, whole data is in packing ). When you use ISL for data travel through trunk link, data will travel in encapsulated form, and encapsulated data takes 20 bytes. When you use dot1q for data travel through trunk link, a tag will attach with this data, and tagged data takes 4 bytes.

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