Configure Trunking between switches

Vlan trunking requires two types of configurations to be applied in order to operate

  1. the type of trunking encapsulation { IEEE 802.1Q, ISL, or negotiate }
  2. An administrative mode specifying if the port is { trunking, not trunking or negotiating }

Step 1 Connecting Cables

Connect the switches together with crossover cables, crossover is needed because the switches are like devices and the pinout of a straight through cable would not allow traffic to properly forward.

Step 2 Configure Encapsulation ( for old switches )

sw(config)#switchport trunk encapsulation {dot1q, ISL, negotiate }

Notice new switches don't have this option see below, taken from  C2950 Version 12.1(22)EA4

Older switches however do have this option because they still have ISL built into the configuration, most newer switches do not.

Taken from C2900XL Version 12.0(5)WC17

Step 3 Configure the administrative mode 

There are three options when creating a trunk link: trunk, dynamic desirable, or dynamic auto

Trunk - always uses trunking for that port
dynamic desirable - initiates negotiation and responds to negotiation messages to dynamically chose whether or not to start using trunking
dynamic auto - passively waits to receive trunk negotiations 

The only case in which the trunking mode would be an issue is if both switches are in dynamic auto mode, if they are the switches will not trunk because neither one is starting negotiation. 

Note: Trunk ports do not show up in a vlan brief commands

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