In a Cisco SD-Access solution, which protocol is used by an extended node to connect to a single edge node?

VXLAN is the encapsulation protocol used for the data plane overlay *within* the SD-Access fabric between fabric nodes, not for connecting an extended node to a fabric edge.

IS-IS (Intermediate System to Intermediate System) is commonly used as the underlay routing protocol within the SD-Access fabric to establish IP connectivity between fabric nodes, but it is not used for direct connectivity between an extended node and a fabric edge.

An SD-Access extended node, which operates as a traditional switch, connects to a fabric edge node using standard 802.1Q trunking to extend VLANs from the fabric edge to the extended node, enabling host connectivity within the fabric overlay.

CTS (Cisco TrustSec) is a security framework used for policy enforcement (e.g., SGT propagation), not a direct network connectivity protocol between an extended node and an edge node.