The switch ports are connected to various devices – routers, switches, hosts, etc. The software has “trunk” command to allow a port to “trunk” with a VLAN (only works with the two fast ethernet ports f0/26 and f0/27). Cisco defines switch trunking as the connecting of two switches via a LAN. This can be direct cable (crossover), through a hub, or through multiple switches. If a router is involved, then you need to use the ISL “encap” command to configure encapsulation. By default, Trunks use the ISL (Inter-Switch Link) protocol, covered later. There are 5 trunk states (all are either Static ISL or DISL -Dynamic ISL), using the following commands :
1) trunk on – configures the port into permanent ISL status and creates a trunk if it finds a connected switch. If the port is f0/26 the trunk is assigned the letter “A”. . If the trunk is f0/27 the port is assigned the letter “B”.
2) trunk off – removes the trunk
3) trunk desirable – only becomes a trunk if the connected switch is in trunk on, desirable, or auto state
4) trunk auto - only becomes a trunk if the connected switch is in trunk on or desirable
5) trunk nonegotiate – same as trunk on – but it does not negotiate with the connected switch (stays at 10 Mbps). This is used for stability, since auto-negotiation can sometimes cause problems.
NOTE: a link between two switches is not necessarily a trunk. If you issue a “trunk off” command, then the trunk is gone, but the link remains. A link is physical, and a trunk is logical.
To view the trunk status: sh trunk a or sh trunk b