VLAN trunks are links that carry multiple VLANs across them. When a packet traverses a trunk, it is "tagged" with a VLAN number.
Cisco's default protocol for trunking is ISL - or Inter Switch Link. ISL adds 26 bytes to the header with the VLAN info and 4 bytes to the end of the packet as a CRC. Because the packet has these additional 30 bytes, it may exceed the default 1518 byte MTU on Cisco switches and drop the packet. Because of this, an ISL trunk needs to be a point-to-point connection with no devices in between.
802.1q trunking is the IEEE's trunking protocol. Instead of adding to the header, it inserts the 4-byte VLAN information into frame after the source address. Because the original size of the packet changes, a new CRC must be calculated and the frame's FCS modified.
A native VLAN is a feature of 802.1q trunks and not ISL. Traffic reaching the switch on the VLAN defined as the native VLAN is not tagged as it goes across the trunk. For this reason, it is important to make sure the native VLAN on both sides of a trunk is the same, or data will leak from one VLAN to another.
Cisco has a proprietary protocol called DTP, or Dynamic Trunking Protocol. Depending on the port's condition, a trunk can be dynamically created when the switches are plugged in. The trunk port types are:
- Access - no trunking will take place
- Trunk - the port trunks only
- Dynamic desireable - the port will turn itself into a trunk if the other end is dynamic desirable, dynamic auto or trunk
- Dynamic Auto - the port will become a trunk if the other end is dynamic desirable or trunk
- Nonegotiate - the port will trunk permanently, but will not negotiate DTP to accommodate non-Cisco switches
R1#show interface trunk
command. This will show the status of the port as well as its trunking protocol and default vlan. If a port is not seen in the
R1#show vlan
command output, it is configured as a trunk port and will only appear in the show interface trunk output.
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