A trunk is an OSI Layer 2 point to point link between one or more Ethernet switch interfaces and another network device such as a router, an Ethernet switch or a server.
VLANs are stored in the VLAN database of each switch and it is local to the switch. Switches needed a mechanism to share VLAN database information with other switches and carry traffic for VLANs that span across other switches in the network. Unlike access ports Trunks can carry traffic for multiple VLANs.
This technology allows network engineers to extend VLANs across an entire network by allowing trunk switches to carry frames from multiple VLANs over a single physical connection. By default, trunk links carry all VLAN traffic. However, we can limit these links to carry only traffic for certain VLANs.
Trunk links must be configured to allow trunking on each end of the link.
Layer 2 devices such as switches use Ethernet frame header information to forward frames. Standard Ethernet frames do not carry any information that allows a switch to identify what VLAN the frame belongs. When a frame is placed on a trunk, information regarding which VLAN the frame belongs to must to be added to the frame in order for the switch at the other end of the trunk link to determine to which VLAN these frames belong. This process is called tagging.
Trunk links provide a mechanism for switches to identify traffic belonging to different VLANs by adding a tag to the original Ethernet frame. There are 2 types of trunking protocols we can use to tag a frame with VLAN information.
- Inter-Switch-Link (ISL): This is the Cisco proprietary method.
- IEEE 802.1Q: This is the most widely used IEEE open standard.
Figure Three VLANs extended over three Cisco Catalyst 3750 switches