Ch. 9: Spanning Tree Protocol Concepts

Question 1

blocking state

Answer 1

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 2

BPDU Guard

Answer 2

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 3

bridge ID (BID)

Answer 3

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 4

BPDU

Answer 4

Bridge protocol data unit. The generic name for Spanning Tree Protocol messages.

Question 5

designated port

Answer 5

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 6

EtherChannel

Answer 6

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 7

forward delay

Answer 7

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 8

forwarding state

Answer 8

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

Question 9

Hello BPDU

Answer 9

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 10

learning state

Answer 10

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 11

listening state

Answer 11

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 12

MaxAge

Answer 12

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 13

PortFast

Answer 13

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 14

root port

Answer 14

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 15

root switch

Answer 15

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 16

root cost

Answer 16

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 17

Spanning Tree Protocol (STP)

Answer 17

A protocol defined by IEEE standard 802.ID. Allows switches and bridges to create a redundant LAN, with the protocol dynamically causing some ports to block traffic, so that the bridge/switch forwarding logic will not cause frames to loop indefinitely around the LAN.

Question 18

Rapid Spanning Tree Protocol (RSTP)

Answer 18

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

Question 19

alternate port

Answer 19

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 20

backup port

Answer 20

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 21

disabled port

Answer 21

In STP, a port role for nonworking interfaces—in other words, interfaces that are not in a connect or up/up interface state.

Question 22

discarding state

Answer 22

An RSTP interface state in which no received frames are processed and the switch forwards no frames out the interface, with the exception of RSTP messages.