How to configure Trunking between VLANs with ISL?

Trunking is a technique to carry different VLAN traffic using point to point link between two devices. ISL (InterSwitch Link) is a cisco proprietary protocol can work with Ethernet token ring and Fddi also. Trunking changes the formatting of the packets. The ports need to be in agreement as to which format is being use to transmit data on the trunk, if there is different trunking encapsulation on the two ends of the link they will not able to communicate. Similar situation will occur if one of your ports is configured in trunking mode and other one as in access mode.

First create multiple VLANs with name, assign multiple ports to them then create ISL trunk link between the two switches to allow communication between VLANs. 

To create a VLAN, first enter global configuration mode to run the following commands.

Configuration to create VLAN 2 and 3 on switch A 

SwitchA(config)#configure terminal               (enter in global configuration mode) 

SwitchA(config)#vlan 2                                        (defining the vlan 2) 

SwitchA(config)#vlan 2 name marketing       (assigning the name marketing to vlan 2) 

SwitchA(config)#vlan 3                                        (defining the vlan 3) 

SwitchA(config)#vlan 3 name management    (assigning the name management to vlan 3)

SwitchA(config)#exit                                            (exit from vlan 3) 

Now assigning the ports 2 and 3 to VLAN 2, it must be done from the interface mode. Enter the following commands to add port 2 and 3 to VLAN 2. 

SwitchA(config)#configure terminal                                 (enter in global configuration mode) 

SwitchA(config)#interface fastethernet 0/2                     (select the Ethernet 0 of port 2) 

SwitchA(config-if)#switchport access vlan 2                  (allot the membership of vlan 2)

SwitchA(config-if)#exit                                                        (exit from interface 2) 

SwitchA(config)#interface fastethernet 0/3                     (select the Ethernet 0 of port 3) 

SwitchA(config-if)#switchport access vlan 2                  (allot the membership of vlan 2)

SwitchA(config-if)#exit                                                        (exit from interface 3)

 Now assigning the ports 4 and 5 to VLAN 3, enter the following commands to add port 4 and 5 to VLAN 3. 

 SwitchA(config)#configure terminal                                 (enter in global configuration mode) 

SwitchA(config)#interface fastethernet 0/4                     (select the Ethernet 0 of port 4) 

SwitchA(config-if)#switchport access vlan 3                  (allot the membership of vlan 3)

SwitchA(config-if)#exit                                                        (exit from interface 4) 

SwitchA(config)#interface fastethernet 0/5                     (select the Ethernet 0 of port 5) 

SwitchA(config-if)#switchport access vlan 3                  (allot the membership of vlan 3)

SwitchA(config-if)#exit                                                        (exit from interface 5) 

  

Configuration to create VLAN 2 on switch B 

SwitchB(config)#configure terminal               (enter in global configuration mode) 

SwitchB(config)#vlan 2                                        (defining the vlan 2) 

SwitchB(config)#vlan 2 name marketing       (assigning the name marketing to vlan 2) 

SwitchB(config)#vlan 3                                        (defining the vlan 3) 

SwitchB(config)#vlan 3 name management    (assigning the name management to vlan 3)

SwitchB(config)#exit                                            (exit from vlan 3)

Now assigning the ports 2 and 3 to VLAN 2, it must be done from the interface mode. Enter the following commands to add port 2 and 3 to VLAN 2.

SwitchB(config)#configure terminal                                                (enter in global configuration mode) 

SwitchB(config)#interface fastethernet 0/2                    (select the Ethernet 0 of port 2) 

SwitchB(config-if)#switchport access vlan 2                 (allot the membership of vlan 2)

SwitchB(config-if)#exit                                                        (exit from interface 2) 

SwitchB(config)#interface fastethernet 0/3                    (select the Ethernet 0 of port 3) 

SwitchB(config-if)#switchport access vlan 2                 (allot the membership of vlan 2)

SwitchB(config-if)#exit                                                        (exit from interface 3) 

Now assigning the ports 4 and 5 to VLAN 3, enter the following commands to add port 4 and 5 to VLAN 3. 

SwitchB(config)#configure terminal                                                (enter in global configuration mode) 

SwitchB(config)#interface fastethernet 0/4                    (select the Ethernet 0 of port 4) 

SwitchB(config-if)#switchport access vlan 3                 (allot the membership of vlan 3)

SwitchB(config-if)#exit                                                        (exit from interface 4) 

SwitchB(config)#interface fastethernet 0/5                    (select the Ethernet 0 of port 5) 

SwitchB(config-if)#switchport access vlan 3                 (allot the membership of vlan 3)

SwitchB(config-if)#exit                                                        (exit from interface 5) 

Trunking with ISL 

Now create ISL trunk link between the two switches to allow communication between VLANs. 

On both switches, SwitchA and SwitchB type the following command with ISL at the fastethernet 0/1 interface. 

SwitchA(config)#configure terminal                                 (enter in global configuration mode) 

SwitchA(config)#interface fastethernet 0/1                     (select the Ethernet 0 of port 1) 

SwitchA(config-if)#switchport mode trunk                (set port 1 as trunk port) 

SwitchA(config-if)#switchport trunk encapsulation isl 

SwitchA(config-if)#end                                                       (exit from interface 1) 

SwitchB(config)#interface fastethernet 0/1                    (select the Ethernet 0 of port 1) 

SwitchB(config-if)#switchport mode trunk                (set port 1 as trunk port) 

SwitchB(config-if)#switchport trunk encapsulation isl 

SwitchB(config-if)#end                                                       (exit from interface 1) 

To verify that fastethernet 0/1 has been established as trunk port, type the show interface fastethernet 0/1 switchport at the privileged EXEC mode.

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