Nov 4

Networking is sometimes hard not because of the concepts that you need to apply, but because of the difference in implementation of some protocols on the equipment. For example, the default settings for DTP differ from one switch model to the next.

DTP (Dynamic Trunking Protocol) is used to negotiate a trunk link between two switches. From the DTP point of view, a port can be ‘desirable‘ (it will actively try to negotiate a  trunk), ‘auto‘ (it will form a trunk if the other side wants to be a trunk) and ‘non-negotiate‘ (port will not negotiate the link). The reason for this protocol is to have a working access or trunk link immediately after you connect the switch to the network.   Most of the combinations are:

  • auto – auto => access
  • auto – desirable => trunk
  • desirable-desirable => trunk
  • auto – trunk => trunk
  • auto – access => access
  • desirable – trunk => trunk
  • desirable – access => access

What you should pay attention to is the default setting of a port on different switch models. On a 2950 (Layer 2 switch) and a 3550 (Layer 3 switch), a port is, before any configurations, in desirable. If you connect two of these switches, you will have a trunk link formed. On the other hand, on a 2960 or a 3560, a port is in auto, so between these models, you will have an access port (by default, in VLAN 1). Even more problems could arrive when you have in a network switches of different models. If you connect a 2960 and a 2950, because the first is in auto and the second  is in desirable, a trunk link will be negotiated, so you should be careful when dealing with these kinds of situations.