STP Terms

BRIDGE: 

• STP is oldest protocol
• at that time no switches(or may be switches are called bridges), so throughout STP, Switch is called as Bridge.

Root Bridge:

• Instead of broadcast, all bridges are elected one bridge as Root,
• then send information to that root bridge, root will send to destination
• Other than root bridge remaining all are called Designated Bridges

Root Port:

• Present on designated bridge
• By using this port only Designated Bridges connect to Root Bridge, 
• Remaining ports are Alternative Ports which are in blocked state.
• Any time only one Root Port per switch towards Root Bridge

Designated Port:

• Present on both Root & Designated Bridge
• Designated Port is a Port which is connected to Root port, 
• It sends information away from Root Bridge

Path Cost:

• Based on path cost we will elect the Root/Designated Port.
• Path Cost  is numerical value, 
• Based on the type of connecting media (Fast Ethernet, gigabit Ethernet, etc) path cast value will be changed

Data rate	STP Cost (802.1D-1998)	RSTP Cost (802.1D-2004 / 802.1w)
4 Mbit/s	250	5,000,000
10 Mbit/s	100	2,000,000
16 Mbit/s	62	1,250,000
100 Mbit/s	19	200,000
1 Gbit/s	4	20,000
2 Gbit/s	3	10,000
10 Gbit/s	2	2,000

Source: wikipedia