CISCO CCNA 200-301 Q178

What port types are available for Rapid Spanning Tree Protocol (RSTP) but NOT available in Spanning Tree Protocol (STP)? (Choose two.)

A. Root port
B. Backup port
C. Alternate port
D. Designated port
E. Learning port

Correct Answer: B, C

RSTP was developed to reduce the high convergence times required in STP, and introduces the alternate port and backup port roles. RSTP is an Institute of Electrical and Electronics Engineers (IEEE) standard, 802.1w, and is interoperable with 802.1d (STP). It operates on the Data Link layer of the OSI model.

An alternate port is a port that has an alternative path or paths to the root bridge, but is currently in a discarding state. A backup port is a port on a segment that could be used to reach the root port, but there is already an active designated port for the segment. An alternate port can also be described as a secondary, unused root port, and a backup port as a secondary, unused designated port.

A root port is a port on non-root switches used to reach the root switch. There can be only one root port on a switch, and it is determined by the least path cost to the root switch. Root ports are used in STP and RSTP.

A designated port is the port used by a network segment to reach the root switch. Designated ports lead away (downstream) from the root switch, and are determined by the lowest path cost to the root switch. While a switch can only have one root port, every other port could potentially be a designated port. Whenever a network segment could be serviced by more than one switch, STP will elect one switch as designated for the segment, and the other(s) will be blocking. This is a core function of the STP protocol, in that only one active Layer 2 path can exist between any two network segments. This port type is available in STP.

A learning port is not a valid port type in STP or RSTP. Learning is one of the possible port states in STP and RSTP. STP has five port states; blocked, listening, learning, forwarding, and disabled. There are only three port states in RSTP; discarding, learning, and forwarding.