ISL, dot1q and DTP

When we talk about encapsulation protocols in a switching network, there are actually 3 different protocols known which is ISL, dot1q and DTP. The most protocols that we heard of is probably ISL and dot1q however, in this blog the DTP will also be discussed.

Here are the differences between ISL and dot1q:

ISL	dot1q
Cisco Proprietary	Intercompatible
Contains header (26 bytes) + trailer (4 byte CRC)	Contains header (4 bytes) only
Native VLAN unknown, all VLANs encapsulated	Native VLAN (Default = VLAN 1) known, all VLANs except native VLAN are encapsulated

Although there are major differences between ISL and dot1q, there is also a similarity in these protocols which is that both are point to point protocols (suitable for trunks).

To configure any of the protocol go to the interface on the switch and type the command “switchport trunk encapsulation dot1q/isl/negotiate”. If the negotiate option is selected, then the port will negotiate which protocol is suitable for both of the switches. If both of the protocols are suitable, then the ISL will be prioritized. If there are no encapsulation options (which can occur on older cisco switches) then the switch supports only the dot1q protocol.

The last protocol is also a cisco proprietary protocol which is the DTP (Dynamic Trunking Protocol). This protocol actively negotiates a trunk line with its pair for every 30 seconds (which overhead is also a sideback of this protocol). This protocol should be applied only when the switch’s port is set on negotiate, if it is not the protocol does not have any use or it should also not be applied to ports which peers cannot negotiate (such as firewalls). To disable this protocol use the command “switchport nonegotiate” on the desired interface on the switch. However, make sure the interface itself is configured as either in access mode or trunk mode. If the interface is configured in the dynamic mode, than the switchport nonegotiate command cannot be applied.

In conclusion, all these protocols have their advantage and disadvantages. It is up to you using which protocols.