Cisco IOS Cookbook, 2nd Edition

Connecting VLAN Trunks with ISL

Problem

You want to connect an InterSwitch Link (ISL) VLAN trunk to your router.

Solution

The following set of commands allows you to connect an ISL trunk to your router:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface FastEthernet0/0
Router1(config-if)#no ip address
Router1(config-if)#speed 100
Router1(config-if)#full-duplex
Router1(config-if)#exit
Router1(config)#interface FastEthernet0/0.1
Router1(config-subif)#encapsulation isl 1
Router1(config-subif)#ip address 172.25.1.5 255.255.255.0
Router1(config-subif)#exit
Router1(config)#interface FastEthernet0/0.2
Router1(config-subif)#encapsulation isl 2
Router1(config-subif)#ip address 172.16.2.1 255.255.255.0
Router1(config-subif)#exit
Router1(config)#interface FastEthernet0/0.3
Router1(config-subif)#encapsulation isl 574
Router1(config-subif)#ip address 10.22.1.2 255.255.255.0
Router1(config-subif)#exit
Router1(config)#end
Router1#

Discussion

A trunk is a point-to-point link containing one ore more Virtual LANs (VLANs). The main purpose of a trunk is to save physical interfaces. Without VLANs, if you wanted to connect two LAN segments into a router, you would need two Ethernet interfaces. Unfortunately, this does not scale well. It is relatively common for a switch to support many VLANs. So there is a clear advantage to using trunks to bundle the VLANs together into a single link.

While trunks carry traffic for many different VLANs, they ...