Cisco Certified Network Professional Practice Test

What is ISL in relation to VLANs?

1. A standard for IP routing

2. A Cisco proprietary protocol for VLAN tagging

3. A method for managing subnet traffic

4. A non-proprietary switching standard

The correct answer is: A Cisco proprietary protocol for VLAN tagging

ISL, which stands for Inter-Switch Link, is a Cisco proprietary protocol that is specifically used for VLAN tagging. This means that it allows the identification and separation of traffic belonging to different VLANs as it traverses switches. By tagging frames with a VLAN ID, ISL enables switches to understand which VLAN the data belongs to, thus facilitating proper delivery and maintaining the integrity of VLAN segregation across a network. ISL encapsulates the original Ethernet frame and adds VLAN information, allowing for effective communication between switches in a network that supports multiple VLANs. This capability is essential in environments where traffic isolation between different segments of the network is required for security, performance, or management purposes. Other options fail to represent the specific function and nature of ISL. For instance, while there are standards for IP routing, ISL does not pertain to routing but specifically to the management and tagging of VLANs. Similarly, ISL is unique to Cisco and not a non-proprietary switching standard, which differentiates it from protocols that are widely accepted across various vendors. Lastly, although managing subnet traffic is vital in network design, this function is not the specific role of ISL in VLAN management.