Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
UniDirectional Link Detection (UDLD) is a Layer 2 high-availability (HA) feature. UDLD monitors a link to verify that both ends of the link are functioning. UDLD operates by sending messages across the link.
When a port receives a UDLD message, the port responds by sending an echo message to verify that the link is bidirectional. Layer 2 HA features, such as UDLD, Spanning Tree Protocol (STP), and IEEE 802.3ad link aggregation, increase network resiliency and are often integral components in redundant topology designs.
Shortest Path First (SPF), First-Hop Redundancy Protocol (FHRP), and nonstop forwarding (NSF) are Layer 3 HA features, not Layer 2 HA features. SPF uses an efficient algorithm to determine the optimal Layer 3 path to a destination within a routing domain. FHRP provides gateway resiliency for hosts. NSF provides graceful restart provisions for common routing protocols to ensure fast convergence and uninterrupted Layer 3 forwarding during failure events, such as supervisor module failure and switchover.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 4, Virtualization Technologies, pp. 153-157 Cisco: Campus 3.0 Virtual Switching System Design Guide: VSS Architecture and Operation