A trunk encapsulation will allow multiple VLANs to transport between switches & can be configured in two ways, the Cisco proprietary Inter-Switch Link, ISL, or with the standard IEEE 802.1Q
Inter-Switch Link
ISL is Cisco restrictive in how it includes the VLAN tag to a outline. It’ll typify a entire outline, including a 26-byte header and a 4-byte trailer. The VLAN number is put within the header.
Configure Trunks
A trunk between a switch can be designed physically or arranged consequently — utilizing Energetic Trunking Convention. I’m not a fan of DTP since I like to know what’s going on in my arrange. Also, a client can plug in a switch into the organize and your arrange switches will consequently make a trunk with the obscure switch causing other issues which I will not depict here. So lets plunge straight into designing a trunk port. Identify the interface that’s associated to the other switch and enter interface arrangement mode.
SW1# conf t SW1(config)#interface f0/24
On SW1 I will specify the trunk encapsulation. On the lower end switches you don’t have to specify this. Some of the higher end switches will allow you to specify the encapsulation in case you have to support ISL.
SW1(config-if)#switchport trunk encapsulation dot1q
switchport trunk encapsulation dot1q command sets the encapsulation to the industry standard.
It’s statically set and I know that it will never automatically change to an access port.
Dynamic desirable means the switchport desires to be a trunk but it won’t be a trunk if the other end is not a trunk or willing to become a trunk.
Dynamic auto means the switchport is willing to become a trunk but it will not desire to unless the other end initiates the conversation of becoming a trunk.
Since we don’t want our trunk ports to use DTP we can disable it with switchport no negotiate
SW1(config-if)#switchport mode trunk SW1(config-if)#switchport nonegotiate
Configure the far end the same way
SW2#conf t SW2(config)#interface f0/24 SW2(config-if)#switchport trunk encapsulation dot1q SW2(config-if)#switchport mode trunk SW2(config-if)#switchport nonegotiate
Notice our interface is not listed anywhere. That's because only access ports are listed in show vlan
Let’s view the configuration of the interface with show interface f0/24 switchport:
SW1#show interface f0/24 switchport Name: Fa0/24 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: Off
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