Configuring ISL Trunk Links

Earlier in Chapter 3, we looked at ways to connect switches with trunk links, such that VLANs could span multiple switches. The two main ways to accomplish this are via the Inter-Switch Link (ISL) and 802.1q frame tagging methods. The Cisco 1900 series does not support 802.1q, leaving ISL as the only option for creating trunk connections with other Cisco switches.

Recall that two main types of links exist between switches – access links, which provide a link for only a single VLAN, and trunk links, which are capable of carrying the traffic of many VLANs through the use of a tagging method like ISL. ISL can only be configured on switch ports with speeds of 100 Mbps and above. On a Cisco 1900, this means that only ports 26 and 27 can be used to define ISL trunk links.

The command to configure a trunk port is simply trunk, followed by the state of the port. There are five states into which you can place a trunk port, as shown below.

  • auto – Will configure the port as a trunk link if the connected system is set to on or auto.
  • desirable – Port will negotiate to become a trunk link if the other system is set to auto, desirable, or on.
  • nonegotiate – Port will become a permanent trunk link, will not negotiate the connection.
  • off – Disables the trunk port and negotiates with the connected system to turn trunking off.
  • on – Configures the port as a permanent ISL trunk link, and negotiates with the other system to convert their port to trunk mode.

In this example we’ll configure port e0/26 as a trunk port, turning ISL on.

Cisco1912#config t
Enter configuration commands, one per line. End with CNTL/Z
Cisco1912(config)#int f0/26
Cisco1912(config-if)#trunk ?
auto Set DISL state to AUTO
desirable Set DISL state to DESIRABLE
nonegotiate Set DISL state to NONEGOTIATE
off Set DISL state to OFF
on Set DISL state to ON
Cisco1912(config-if)#trunk on

Once trunking is configured for a given port, that port will trunk traffic for all configured VLANs. However, it is also possible to deny traffic from certain VLANs from being trunked by issuing the no trunk-vlan command. In the example below, traffic from VLAN 50 will not be trunked across the port 26 link.

Cisco1912(config-if)#no trunk-vlan 50

Once trunk links have been configured, their state can be viewed using the show trunk command, followed by a letter associated with that link. Since port 0/26 is the first FastEthernet port, it is known as trunk A, while port 0/27 would be known as trunk B.

Cisco1912#show trunk ?
A Trunk A
B Trunk B
Cisco1912#show trunk A
DISL state: On, Trunking: On, Encapsulation type: ISL

Going a step further, you can also view which VLANs are allowed to use the trunk link by appending the command to include allowed-vlans, as shown below.

Cisco1912#show trunk a allowed-vlans
1-49, 51-1005

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.