encapsulation isl [vlan-id]
Enables ISL encapsulation on a subinterface for inter-VLAN routing on a router-on-a-stick configuration, assigning the subinterface to a specific VLAN.
encapsulation isl [vlan-id]When to Use This Command
- Configuring a router-on-a-stick to route between VLANs using ISL trunking on a FastEthernet interface.
- Connecting a router to a Catalyst switch that only supports ISL trunking for inter-VLAN routing.
- Setting up legacy inter-VLAN routing where the switch uses ISL encapsulation.
- Migrating from ISL to 802.1Q trunking by first configuring ISL encapsulation for testing.
Command Examples
Basic ISL encapsulation on subinterface
Router(config-subif)# encapsulation isl 10This command configures the subinterface to use ISL encapsulation and assigns it to VLAN 10. No output is displayed upon successful configuration.
Verifying ISL encapsulation configuration
Router# show running-config interface fastEthernet 0/0.10interface FastEthernet0/0.10 encapsulation isl 10 ip address 192.168.10.1 255.255.255.0
The output shows the subinterface configuration: 'encapsulation isl 10' confirms ISL encapsulation for VLAN 10, and the IP address is assigned for that VLAN.
Understanding the Output
The 'encapsulation isl [vlan-id]' command does not produce output when entered; it simply configures the subinterface. To verify, use 'show running-config' or 'show interfaces [interface]'. In the running config, look for the 'encapsulation isl' line under the subinterface. A correct configuration shows the VLAN ID after 'isl'. If missing, the subinterface may not be trunking correctly. Also check that the parent interface is not shutdown and has an IP address assigned if needed.
CCNA Exam Tips
ISL is a Cisco proprietary trunking protocol; 802.1Q is the open standard. CCNA focuses on 802.1Q, but ISL may appear in legacy questions.
ISL encapsulation uses a 30-byte header and does not support native VLANs; all VLANs are tagged.
On CCNA exam, you may need to identify that 'encapsulation isl' is used on subinterfaces for router-on-a-stick with older switches.
Remember that ISL encapsulation is configured per subinterface, not on the main interface.
Common Mistakes
Mistake: Using 'encapsulation isl' on the main interface instead of a subinterface. Consequence: The command is rejected or misapplied.
Mistake: Forgetting to assign an IP address to the subinterface after encapsulation. Consequence: The subinterface cannot route traffic.
Mistake: Using ISL encapsulation when the switch port is configured for 802.1Q trunking. Consequence: Mismatch causes no connectivity.
Related Commands
encapsulation dot1Q [vlan-id]
Configures IEEE 802.1Q VLAN encapsulation on a subinterface to enable trunking and route traffic for a specific VLAN.
show interfaces trunk
Displays trunk interface status, allowed VLANs, and pruning information for all trunk ports on a Cisco switch, used to verify trunking configuration and VLAN membership.
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