Which protocol is recognized as the industry standard for VLAN trunking?

The correct answer is IEEE 802.1Q, which is widely recognized as the industry standard for VLAN trunking. This protocol enables the tagging of Ethernet frames to identify which VLAN they belong to. By adding a VLAN tag to frames, IEEE 802.1Q allows multiple VLANs to coexist on a single physical link or trunk, facilitating efficient bandwidth utilization and separation of traffic for different VLANs.

This standard is essential for proper handling of VLAN traffic, allowing switches and routers to distinguish frames from different VLANs while passing through a trunk link. It ensures that VLAN segregation is maintained across the network, providing isolation for different types of traffic, which enhances overall network security and performance.

ISL (Inter-Switch Link) is a Cisco-proprietary protocol that was previously used for trunking but is not as widely accepted or supported as IEEE 802.1Q, thus not being the industry standard. Frame Relay and PPP (Point-to-Point Protocol) are not related to VLAN trunking, as they serve different purposes in networking, such as providing WAN connectivity and encapsulating datagrams, respectively.