What protocol or technology requires switches to be in server mode or client mode?

The protocol or technology that requires switches to be in server mode or client mode is VTP, which stands for VLAN Trunking Protocol. VTP is a Cisco-proprietary protocol used for managing VLAN information on a network of switches. In VTP, switches can be configured in one of three modes:

1. Server Mode: In this mode, switches can create, modify, and delete VLANs. Changes made on a server are advertised to the entire VTP domain.
2. Client Mode: Switches in client mode cannot create, change, or delete VLANs. They only receive information from VTP servers and update their VLAN configuration accordingly.
3. Transparent Mode: This is the third mode, where switches do not participate in the VTP domain but will forward VTP advertisements through their trunk ports.

EtherChannel, HSRP (Hot Standby Router Protocol), and DTP (Dynamic Trunking Protocol) do not use server or client modes in their configurations. EtherChannel is a link aggregation technology, HSRP is a network redundancy protocol, and DTP is a protocol used for negotiating trunking on a link between two switches.