Redundancy Protocols

are designed to ensure high availability for network devices or services, usually by providing backup paths or failover mechanisms
most operate at Layer 2 or Layer 3

Spanning Tree Protocols (STP/RSTP/MSTP)

These operate at Layer 2 to provide loop-free redundant paths in Ethernet networks.

Protocol	Layer	Notes
Spanning Tree Protocol (STP)	2	Original IEEE 802.1D; disables loops by blocking redundant links.
Rapid Spanning Tree Protocol (RSTP)	2	IEEE 802.1w; faster convergence than STP.
Multiple Spanning Tree Protocol (MSTP)	2	IEEE 802.1s; allows multiple VLANs to use different spanning trees.

Link Aggregation / NIC Teaming

Provides redundant links for bandwidth and failover.

Protocol	Layer	Notes
Link Aggregation Control Protocol (LACP)	2	IEEE 802.3ad; multiple physical links appear as one logical link.
Active/Backup NIC Teaming	2	One link active, others take over if it fails.

First Hop Redundancy Protocols (FHRP)

These provide a highly available default gateway.

Protocol	Layer	Notes
Virtual Router Redundancy Protocol (VRRP)	3	Open standard; elects a master router for a virtual IP.
Hot Standby Router Protocol (HSRP)	3	Cisco proprietary; similar to VRRP.
Gateway Load Balancing Protocol (GLBP)	3	Cisco proprietary; provides both redundancy and load balancing.
Common Address Redundancy Protocol (CARP)	3	OpenBSD/FreeBSD; similar to VRRP.