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IP Engineering > Week 6: STP > Flashcards

Week 6: STP Flashcards

1
Q

What are the two purposes for using Switching?

A

Breaks up Collision Domains (each switch port is its own separate Collision Domain)
Cost-effective way to connect workstations (switches have replaced hubs)

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2
Q

What are the advantages of using Layer 2 Switching?

A

Hardware-Based Bridging (ASICs)
Low Latency
Low Cost

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3
Q

What are the three Switch functions at Layer 2

A

Address Learning
Forward/Filter Decisions
Loop Avoidance

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4
Q

What is Switch Address Learning?

A

Layer 2 switches remember the source hardware address of each frame received on an interface and entered into the Forward/Filter MAC Database Table

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5
Q

Describe Forward/Filter Decisions for a Switch

A

Switch looks at the destination hardware address, then chooses the appropriate exit interface for it in the MAC Database

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6
Q

What is Switch Loop Avoidance?

A

If multiple connections between switches are created for redundancy purposes, network loops can occur

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7
Q

What protocol is used to prevent network loops while maintaining redundancy?

A

Spanning Tree Protocol

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8
Q

What are the three Forward/Filter Decisions for a Switch when a Frame arrives?

A
  1. If a source hardware address is not in the forwarding table -> add it
  2. if a destination hardware address is known -> Frame Filtering
  3. If the destination hardware address is not known -> frame is flooded to ALL active interfaces EXCEPT source
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9
Q

What types of links are desired between switches?

A

Redundant Links

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10
Q

What are the benefits of Redundant Links?

A

Help prevent irrecoverable network failures in case one link stops working
Helps improve a network’s fault tolerance

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11
Q

What can loops in switching lead to?

A

Broadcast Storm

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12
Q

What is a Broadcast Storm?

A

Flood of endless broadcast packets

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13
Q

What MUST switches avoid?

A

Logical Loops

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14
Q

What are the three laters in the Cisco Hierarchical Network Model?

A

Core Layer
Distribution Layer
Access Layer

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15
Q

What is the function of the Core Layer in the Cisco Hierarchical Network Model?

A

Backbone of the model

Transport data reliably & quickly

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16
Q

What is the function of the Distribution Layer in the Cisco Hierarchical Network Model?

A

Routing
filtering
WAN Access

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17
Q

What is the function of the Access Layer in the Cisco Hierarchical Network Model?

A

Switching

Separating Collision Domains

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18
Q

In which network design is Redundancy introduced?

A

Hierarchical Network Design

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19
Q

What type of loop will disable a network?

A

A Logical Loop (due to redundancy)

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20
Q

What does Spanning Tree Protocol (STP) prevent?

A

Stops loops in Layer 2 Switch Networks

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21
Q

What are the properties of the Spanning Tree Protocol?

A

Layer 2 Protocol

Uses the Spanning Tree Algorithm (STA)

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22
Q

What are the main standards of Spanning Tree Protocol (STP)?

A 
STP: IEEE802.1D
Rapid STP (RSTP): IEEE802.1W
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23
Q

What is the purpose of the Spanning Tree Algorithm?

A

To interrupt the logical loop created by physical loops in a bridged/switched environment

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24
Q

How does STP prevent Logical Loops?

A

By ensuring that certain ports on some of the bridges/switches do not forward frames

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25
Q

What is a Bridge Protocol Data Unit (BPDU)?

A

Used to exchange selection information of the root switch as well as subsequent network configuration

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26
Q

How does a BPDU function?

A

Each switch compares the parameters in the BPDU that it sends to one neighbour with the ones that it receives from other neighbours

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27
Q

What type of network hardware sends BPDU’s and how frequently?

A

Sent by Bridges periodically (usually every 2 seconds)

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28
Q

what is Port Cost?

A

The cumulative value of link costs towards the root

29
Q

What is Port Cost used for?

A

Determining the best path when multiple links are used between two switches

30
Q

How is Port Cost calculated?

A

It is calculated via the bandwidth of the links along the path

31
Q

What is a Bridge ID?

A

The combination of the Bridge Priority and the base MAC Address

32
Q

What is the default Bridge Priority for all Cisco Switches?

A

32,768

33
Q

What are Bridge ID’s used for?

A

For STP to keep track of all network-related switches

34
Q

What is a Root Bridge?

A

The Bridge with the lowest Bridge ID

35
Q

What is a Non-root Bridge?

A

All bridges that are not the Root Bridge

36
Q

What is the main function of Non-Root Bridges?

A

To exchange BPDU’s with all bridges & update the STP Topology database on all switches

37
Q

What is a Root Port?

A

The port that provides the best path to the root bridge

38
Q

What are two properties of the Root Port?

A

Root Port is always the link directly connected to the root bridge
OR
The Lowest Path Cost to the root bridge

39
Q

What is a Designated Port?

A

A port that has been determined as having the best (lowest) cost to the Root Bridge via the Root Port

40
Q

What can a Designated Port be marked as?

A

A Forwarding Port

41
Q

What is a Non-Designated Port?

A

A port with a higher cost than the Designated Port

42
Q

What is a Non-Designated Port marked as?

A

They are marked with Blocking Mode - they are not forwarding ports

43
Q

What is a Forwarding Port?

A

A port that forwards frames; can be a Root Port or a Designated Port

44
Q

What is a Blocked Port?

A

A port that, in order to prevent loops, will NOT forward Frames

45
Q

What is an exception to a Blocked Port?

A

A Blocked Port will always listen to BPDU Frames but drop any & all other Frames

46
Q

How do Switches & Bridges configure a single logical path?

A

via election

47
Q

What is the election process that Switches & Bridges undergo to configure a single logical path?

A
  1. A Root Bridge (root device) is selected; all other Bridges/Switches configure their ports using the root Bridge as a point of reference
  2. Bridges use STP (BPDUs) to transfer the information about each Bridge’s MAC Address & Priority Number
  3. Each Bridge or Switch determines which of its own ports offers the best path to the Root Bridge
48
Q

What process is triggered when there is a Root Failure?

A

The device in an STP-enabled network (that has stopped receiving BPDUs) will assume:

i. The Root Bridge has failed
ii. Claim to be the Root Bridge
iii. Will begin sending BPDUs describing itself as such
iv. Begin Root Bridge re-election

49
Q

What is a Stable State?

A

The normal operating state of all ports when the Root Bridge is available and all paths are functioning as expected
Normal Data Transfer occurs

50
Q

What is a Transitory State?

A

Prevents logical loops during a period of transition from one root bridge to another
NO DATA PACKET TRANSFER OCCURS

51
Q

What are the three Stable States?

A

Blocking
Forwarding
Disabled

52
Q

What are the two Transitory States?

A

Listening

Learning

53
Q

When are Transitory States used?

A

When an STP devices are waiting on a new bridge to be elected

54
Q

What is a Disabled State?

A

Virtually non-operational port

55
Q

What is a Listening State?

A

Switch processe BPDUs & awaits possible new information that would cause it to return to the blocking state

56
Q

What are two things are ignored by an STP-device in Listening State?

A

Does not learn MAC Address

Does not forward Frames

57
Q

What are the properties of an STP-device in Learning State?

A

Processes BPDUs
Learns MAC Address
Does not forward packets

58
Q

What is a Forwarding State?

A

Normal port operation; sending and receiving data

59
Q

What is a Blocking State?

A

A port that would cause a Switching Loop; no user data is sent or received

60
Q

What is one property of a Blocking State?

A

BPDU data is still received

61
Q

Under what conditions will a Blocking State go into Forwarding Mode?

A
  1. Other links in use were to fail

2. STA determines the port may transition to the Forwarding State

62
Q

How many bytes long is the Bridge ID?

A

8 Bytes (2 Byte Priority, 6 Byte MAC Address)

63
Q

What is used in addition to the Bridge ID in the case of multiple VLANs in a Switch?

A

VLAN Number is used in the Extended System ID field for distinction

64
Q

System ID Extension is what?

A

The VLAN Number

65
Q

What are the three STP Timers?

A

Hello Time
Forward Delay
Maximum Age

66
Q

What is the Hello Time STP timer?

A

The time between each BPDU Frame that is sent on a port

2 seconds by default, can be 1 - 10 seconds

67
Q

What is the Forward Delay STP timer?

A

The time spent in the listening & learning state

15 seconds for each state by default, can be 4 - 30 seconds

68
Q

What is the Maximum Age STP Timer?

A

The maximum amount of time a switch saves the configuration BPDU information
20 seconds by default, can be 6 - 40 seconds

IP Engineering (15 decks)

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