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How-to: Configure trunking

A trunk port is a layer 2 port that may carry traffic from multiple VLANs. Trunk ports are mostly used to connect two switches, switch and a router for inter-vlan routing or a server.

Cisco switches support two types of trunking encapsulation protocols as listed below

1- ISL : frames are fully encapsulated with an ISL header before being sent o­n the trunk link.
2- IEEE 802.1Q (standard): frames are identified by inserting a 4 byte tag in the Ethernet frame. 802.1Q is essential in multi-vendor environments.

Trunk links can be established by manual configuration, or by auto-negotiation between the two interfaces (using DTP).

Note: for auto-negotiation to work properly switches must be in the same VTP domain.

Disabling interface trunking:

To fully disable trunking o­n a switchport just configure this port as a permenant access port as shown below

Rack1SW4(config-if)#switchport mode access

this command forces the port be a non-trunk port.

Permanent Trunk configuration:

The following steps are to be followed in order to configure a permanent trunk

1- Configure trunk encapsulation (ISL or dot1q).

2- Configure the interface in permanent trunking mode.

Rack1SW4(config-if)#switchport trunk encapsul
Rack1SW4(config-if)#switchport trunk encapsulation ?
dot1q Interface uses o­nly 802.1q trunking encapsulation when trunking
isl Interface uses o­nly ISL trunking encapsulation when trunking
negotiate Device will negotiate trunking encapsulation with peer o­n

Rack1SW4(config-if)#switchport trunk encapsulation dot1q
Rack1SW4(config-if)#switchport mode trunk

3- Preventing the interface from generating DTP frames is desirable in case of connecting to devices that don’t support DTP. The following command will disable DTP negotiation

Rack1SW4(config-if)#switchport nonegotiate

Configuration verification:

Rack1SW4(config-if)#do sh inter trunk

Port Mode Encapsulation Status Native vlan
Fa3/19 o­n 802.1q trunking 1

The output is telling us that interface f3/19 is in trunking mode and that encapsulation is dot1q for this link.
Note: make sure that native VLAN is the same o­n both ends when using dot1q encapsulation in order to avoid any problems that may occur by this misconfiguration.

Establishing trunks dynamically:

Switch can negotiate the interface capabilities and trunks can be brought up dynamically using DTP. DTP runs in o­ne of two modes

1- desirable: the interface actively attempts to convert the interface into a trunk if the other end is in trunking, auto or desirable mode.

2- auto: the interface state is determined by the other side’s mode. Interface will convert the link to trunk is the other side is in trunk or desirable mode.

Rack1SW4(config-if)#switchport mod dynamic ?
auto Set trunking mode dynamic negotiation parameter to AUTO
desirable Set trunking mode dynamic negotiation parameter to DESIRABLE

Rack1SW4(config-if)#switchport mod dynamic desirable
Rack1SW4(config-if)#switchport trunk encapsulation nego

checking and verification:

Rack1SW4(config-if)#do sh interface trunk

Port Mode Encapsulation Status Native vlan
Fa3/19 desirable n-isl trunking 1

The above output is telling us that this interface is now working as a trunk, it is in desirable mode and ISL encapsulation is was negotiated successfully “n-isl = negotiated ISL”.

Trunk operation verification:

you can use the command show interface trunk to display all trunking interfaces and their capabilities o­n the switch as shown in the above examples

Also the command show interface switchport can be used as shown in the below output

Rack1SW4(config-if)#do sh inter f3/19 switch
Name: Fa3/19
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: trunk
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: isl
Negotiation of Trunking: o­n
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL

Happy trunking 🙂