A, STP protocol
The STP: Spanning Tree Protocol. When a plurality of loops form a physical switch, it will cause broadcast storms, using the STP to avoid loops at the broadcast storm.
STP protocol is a loop switch through (relatively) negotiation, the logic goes down an interface, thereby breaking the loop, but when the other line loop problems, before the interface is down immediately reopen work.
Two, PVST agreement
PVST protocol is an upgrade for STP, vlan is taken into account, that is, all switches in the loop for each vlan VLAN negotiate a separate decision is down an interface, which is a virtual local area network is down different for different vlan interface vlan will each take a different route, to achieve load balancing.
PVST is a Cisco proprietary protocol agreement, the agreement is a public MST protocol.