9 - Spanning Tree Protocol

Question 1

alternate port (role)

Answer 1

alternate port (role)
With RSTP, a port role in which the port acts as an alternative to a switch’s root port, so that when the switch’s root port fails, the alternate port can immediately take over as the root port.

Question 2

backup port (role)

Answer 2

With RSTP, a port role in which the port acts as a backup to one of the switch’s ports acting as a designated port. If the switch’s designated port fails, the switch will use the backup port to immediately take over as the designated port.

Question 3

blocking state

Answer 3

In STP, a port state in which no received frames are processed and the switch forwards no frames out the interface, with the exception of STP messages.

Question 4

BPDU Guard

Answer 4

A Cisco switch feature that listens for incoming STP BPDU messages, disabling the interface if any are received. The goal is to prevent loops when a switch connects to a port expected to only have a host connected to it.

Question 5

BPDU Filter

Answer 5

A Cisco switch feature that uses the monitoring and filtering (discarding) of STP BPDUs to achieve goals, such as protecting against forwarding loops on PortFast ports and disabling STP by filtering all BPDU messages.

Question 6

bridge ID

Answer 6

bridge ID
An 8-byte identifier for bridges and switches used by STP and RSTP. It is composed of a 2-byte priority field followed by a 6-byte System ID field that is usually filled with a MAC address.

Question 7

bridge protocol data unit (BPDU)

Answer 7

bridge protocol data unit (BPDU)
The generic name for Spanning Tree Protocol messages.

Question 8

broken state

Answer 8

broken state
An STP port state on Cisco switches, used by Root Guard and Loop Guard as a method for STP to disable the use of the port. A port in the broken state does not forward or process received frames.

Question 9

designated port

Answer 9

designated port
In both STP and RSTP, a port role used to determine which of multiple interfaces on multiple switches, each connected to the same segment or collision domain, should forward frames to the segment. The switch advertising the lowest-cost Hello BPDU onto the segment becomes the DP.

Question 10

disabled port (role)

Answer 10

disabled port (role)
In STP, a port role for nonworking interfaces—in other words, interfaces that are not in a connect or up/up interface state. The reason can be due to administrative setting (shutdown) or interface failure.

Question 11

disabled state

Answer 11

disabled state
In STP but not RSTP, the state to be used for interfaces in the disabled port role.

Question 12

discarding state

Answer 12

An RSTP interface state, which primarily replaces the STP blocking state, as used for interfaces with port roles other than designated or root ports. In this state, the switch does not forward data frames out the interface, nor does it process received frames other than RSTP messages.

Question 13

EtherChannel

Answer 13

EtherChannel
A feature in which up to eight parallel Ethernet segments exist between the same two devices, each using the same speed. May be a Layer 2 EtherChannel, which acts like a single link for forwarding and Spanning Tree Protocol logic, or a Layer 3 EtherChannel, which acts like a single link for the switch’s Layer 3 routing logic.

Question 14

forward delay

Answer 14

forward delay
An STP timer, defaulting to 15 seconds, used to dictate how long an interface stays in the listening state and the time spent in learning state. Also called the forward delay timer.

Question 15

forwarding state

Answer 15

forwarding state
An STP and RSTP port state in which an interface operates unrestricted by STP.

Question 16

Hello BPDU

Answer 16

Hello BPDU
The STP and RSTP message used for the majority of STP communications, listing the root’s bridge ID, the sending device’s bridge ID, and the sending device’s cost with which to reach the root.

Question 17

learning state

Answer 17

learning state
In STP, a temporary port state in which the interface does not forward frames, but it can begin to learn MAC addresses from frames received on the interface.

Question 18

listening state

Answer 18

listening state
A temporary STP port state that occurs immediately when a blocking interface must be moved to a forwarding state. The switch times out MAC table entries during this state. It also ignores frames received on the interface and doesn’t forward any frames out the interface.

Question 19

Loop Guard

Answer 19

Loop Guard
A complex Cisco switch mechanism that protects against STP loops in a specific common case. For switches whose switch-to-switch links settle into either a root port or an alternate port role, as expected per the STP design, Loop Guard disables use of those ports if normal STP operation attempts to assign them the designated port role.

Question 20

MaxAge

Answer 20

MaxAge
In STP, a timer that states how long a switch should wait when it no longer receives Hellos from the root switch before acting to reconverge the STP topology. Also called the MaxAge timer.

Question 21

PortFast

Answer 21

A switch STP feature in which a port is placed in an STP forwarding state as soon as the interface comes up, bypassing the listening and learning states. This feature is meant for ports connected to end-user devices.

Question 22

Rapid Spanning Tree Protocol (RSTP)

Answer 22

Rapid Spanning Tree Protocol (RSTP)
Defined in IEEE 802.lw. Defines an improved version of STP that converges much more quickly and consistently than STP (802.Id).

Question 23

root cost

Answer 23

root cost
The STP cost from a nonroot switch to reach the root switch, as the sum of all STP costs for all ports out which a frame would exit to reach the root.

Question 24

Root Guard

Answer 24

Root Guard
A Cisco switch feature that protects against unexpected new root switches. When enabled on an interface, IOS uses normal STP rules except to disable the use of the port after receiving a superior BPDU.

Question 25

root port (role)

Answer 25

root port (role)
In STP and RSTP, the one port on a nonroot switch in which the least-cost Hello is received. Switches put root ports in a forwarding state.

Question 26

root switch

Answer 26

root switch
In STP and RSTP, the switch that wins the election by virtue of having the lowest bridge ID and, as a result, sends periodic Hello BPDUs (default, 2 seconds).

Question 27

superior BPDU

Answer 27

superior BPDU
An STP bridge protocol data unit (BPDU) that lists a better (lower) root bridge ID (BID) as compared to the current bridge. A switch that receives a superior BPDU under normal conditions would begin using the new switch with the lower BID as the root switch.

Question 28

unidirectional link

Answer 28

unidirectional link
A condition on fiber optic links in which one of the two required fibers fails in a way that also results in both attached switches leaving their interfaces in a working (connected) state—when, in reality, the link can pass frames only in a single direction.