Trunking

TRUNK

A trunk port is a switch port that can simultaneously carry traffic for multiple VLANs

A trunk port can have multiple VLANs

There are two main types of frame tagging:

Modern switches use one primary mechanism of trunking between devices: IEEE 802.1Q, which allows traffic from multiple VLANs to be transported across a trunk. It performs this task using two different methods: a native VLAN, and a tagging mechanism.

Traffic in the native VLAN is sent between devices without adding anything to it . By default, the native VLAN on switches is set to VLAN 1, as are all of the switchports ; this default setting allows traffic to be passed between all ports of multiple switches without any additional configuration.


 Traffic that is not in the native VLAN will be tagged, adding 32 bits to the Ethernet header. This tag identifies the VLAN for any traffic sent between Ethernet devices

 We  need to make sure that both switches agrees on which one was supposed to be the native VLAN.Otherwise we can have a VLAN Hoping :so different switches with different VLAN  with untaged frames are assuming that belongs to Natives VLANs of the other switch


We have different trunking mode:

Dynamic Trunking Protocol (DTP) is a Cisco proprietary trunking protocol, which is used to automatically negotiate trunks between Cisco switches. Dynamic Trunking Protocol (DTP) can be used negotiate and form trunk connection between Cisco switches dynamically.
 Imagines that we have 2 switches:sw1 and sw2:

Remember that we will have access port (non trunk)

This is the result of different ways to form a trunk:

This is our personal configuration at home

Please take into consideration two things:

1.-Before we do anything in this switch make VERY sure that you delete ALL previous configuration on your switches:


#write erase
#delete flash:vlan.dat
#reload


2.-Adjust using your own interface


We will do the configuration in our system with Switch-A



 SWITCH A

• conf t
• int fa0/3
• switchport trunk encapsulation ?
•  switchport trunk encapsulationdot1q
•   switchport trunk native vlan50
• switchport mode dynamic desirable
• end 

• conf t
• int fa0/15
• switchport trunk encapsulation ?
•  switchport trunk encapsulationdot1q
•   switchport trunk native vlan50
• switchport mode trunk
• end

 SWITCH B

• conf t
• int fa0/10
• switchport trunk native vlan 50
• end

SWITCH C

• conf t
• int gi0/11
• switchport trunk native vlan 50
• end

SWITCH A

• show int trunk

We see the interface fa03 and fa0/15..They are dynamic desirable, the trunking is VLAN 1 ..the default one..The trunking encapsulation is negotiated

We will do several things:

1. Make encapsulation  dot.1q in the ports
2. Make the native VLAN 50
3. Make F03 and F0/15 dinamic desirable to form the formation of the trunk

First we will check out if the is any trunk  : #show interface trunk

Let's configure Switch  C

Let's do the same in Switch B

Now let's get back to Switch A

We can see that we got on SWITCH A in desirable mode, the other one in trunk mode.., using encapsulation dot1q and VLAN 50 as a native VLAN.


Now checking again in our old 2900 switch (Switch C), which have a different command we can see as well the successful  configuration

This exercise was successful and done!