Trunks

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