What is usually required when configuring a router for VLAN routing?

When configuring a router for VLAN routing, the use of subinterfaces is essential. Subinterfaces allow a single physical interface on the router to be divided into multiple logical interfaces, each capable of handling traffic for a different VLAN. This setup utilizes the concept of inter-VLAN routing, where the router can facilitate communication between multiple VLANs that would otherwise be isolated from each other.

By defining subinterfaces, each one can be assigned an IP address that corresponds to a specific VLAN. This allows the router to operate as a gateway for each VLAN, enabling devices within those VLANs to communicate with one another through the router. VLAN tagging is employed, often with the 802.1Q standard, to differentiate traffic coming from different VLANs as it passes through the router.

The other choices, while related to networking concepts, do not fulfill the primary requirement for VLAN routing in this context. Physical interfaces are limited to managing a single network segment. Switch ports are part of Layer 2 functionality and do not participate in Layer 3 routing directly. Access lists, while useful in controlling traffic flow, are not fundamentally a requirement for enabling VLAN routing.