Trunking is one of the most important practices when planning and building your computer network. The most popular protocol to use is IEEE 802.1q or in most cases, abbreviated to just dot1q. ISL is technically the Cisco proprietary protocol for trunk link traffic, however 802.1q has become the standard and can be used on Cisco, Juniper, and many other brands of equipment. In this article, we will focus on how to configure 802.1q trunking between two Cisco switches.
The advantage of using a trunk link is that it keeps all the traffic coming from different virtual LANS (VLANs) straight and consistent with where they are supposed to go. In simple terms, a trunk link is meant to keep you from having to place a link between the two switches for each VLAN that must transmit information to the next switch.
Say we hypothetically have two Cisco 2960 series access switches. They are connected by a single crossover cable. Both switches, S1 and S2, are connected at port Fa0/1 (Fast Ethernet 0/1). Before you create the trunk link on the switches, make sure both have a native VLAN installed on them. To create a native VLAN or any VLAN for that matter, just follow the following commands.
S1>enable
S1# vlan database
S1# vlan 99 name Management/Native (you cannot have a VLAN name that is more than one word)
It should tell you that the VLAN has been created. Make sure both switches have the same VLAN number and name assigned to them for the native VLAN. A native VLAN mismatch will cause your trunk link to fail. I use VLAN 99 as the number for my native VLAN because that is usually the default that most networkers use, but the number you choose for the native VLAN does not matter as long as it is the same for all equipment.
To create the trunk link on S1, you will need to enter the switch and input the following commands.
S1> enable
S1# configure terminal
S1 (config)# interface fa0/1
S1 (config-if)# switchport mode trunk
S1 (config-if)# switchport trunk native vlan 99
S1 (config-if)# no shutdown (This command keeps your port up and running until you shut it down yourself)
It will tell you that you have a CDP mismatch right away because you have not implemented trunking on the fast ethernet port on S2 yet. Both sides must have the same thing done to them. Therefore, it is probably ideal if you use the S1 (config)# no ip domain-lookup command to stop the switch from giving you mayday messages every 10 seconds about the incomplete trunk. Once you have completed the step above, simply input the same commands on S2, as shown below.
S2> enable
S2# configure terminal
S2 (config)# interfact fa0/1
S1 (config-if)# switchport mode trunk
S1 (config-if)# switchport trunk native vlan 99
S2 (config-if)# no shutdown
After all these commands have been inputed, you should have a working 802.1q trunk link. These commands work for Cisco equipment and are made to make trunking a simple process. Traffic in the same VLAN on both switches should be able to contact each other. If you cannot, check the types of cables used, the commands you put in, and for any silly errors. I hope this composition is helpful for configuring trunk links on your networks.