A trunk is a point-to-point link between one or more Ethernet switch ports and another network device, such as a router or a switch. Trunks carry the traffic of multiple VLANs over a single link thus allowing you to extend VLANs across an entire network.
Two trunking encapsulations are available, depending on the hardware:
● Inter-Switch Link Protocol (ISL)— ISL is a Cisco-proprietary trunking encapsulation
● IEEE 802.1Q— 802.1Q is an industry-standard trunking encapsulation
The Dynamic Trunking Protocol (DTP) manages trunk negotiation. DTP supports auto-negotiation of both ISL and 802.1Q trunks. In 802.1Q trunking , all VLAN packets are tagged on the trunk link, except the native VLAN. The native VLAN packets are sent untagged on the trunk link. This way, you can determine to which VLAN a frame belongs when you receive a frame with no tag. Native VLAN should be the same on both switches configured for trunking.
By default, VLAN 1 is the native VLAN on all switches.
● In CatOS, the native VLAN can be changed by issuing the set vlan vlan-id mod/ port command, where mod/ port is the trunk port.
● In Cisco IOS Software, the native VLAN can be changed by issuing the switchport trunk native vlan vlan-id interface command which is configured on the trunk port.
On Catalyst switches running CatOS, use these commands to verify trunking:
● show port capabilities module/ port
● show port module/ port
● show trunk module/ port
● show vtp domain
On Catalyst 6000 switches running Cisco IOS Software , use the following commands to verify trunking:
● show interfaces interface-type module/ port trunk
● show vlan In order to troubleshoot trunking, make sure that:
● cable is connected and the correct type of cable is used
● trunk is enabled on both interfaces
● encapsulation at both ends using same protocol (802.1Q or ISL)
● to verify that there are no restrictions on the either side of the trunk that are preventing a VLAN traffic
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1592-1603). . Kindle Edition.