Week 6: STP Flashcards by Mark Philips

What are the two purposes for using Switching?

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

How well did you know this?

Not at all

Perfectly

What are the advantages of using Layer 2 Switching?

Hardware-Based Bridging (ASICs)
Low Latency
Low Cost

How well did you know this?

Not at all

Perfectly

What are the three Switch functions at Layer 2

Address Learning
Forward/Filter Decisions
Loop Avoidance

How well did you know this?

Not at all

Perfectly

What is Switch Address Learning?

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

How well did you know this?

Not at all

Perfectly

Describe Forward/Filter Decisions for a Switch

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

How well did you know this?

Not at all

Perfectly

What is Switch Loop Avoidance?

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

How well did you know this?

Not at all

Perfectly

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

Spanning Tree Protocol

How well did you know this?

Not at all

Perfectly

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

If a source hardware address is not in the forwarding table -> add it
if a destination hardware address is known -> Frame Filtering
If the destination hardware address is not known -> frame is flooded to ALL active interfaces EXCEPT source

How well did you know this?

Not at all

Perfectly

What types of links are desired between switches?

Redundant Links

How well did you know this?

Not at all

Perfectly

What are the benefits of Redundant Links?

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

How well did you know this?

Not at all

Perfectly

What can loops in switching lead to?

Broadcast Storm

How well did you know this?

Not at all

Perfectly

What is a Broadcast Storm?

Flood of endless broadcast packets

How well did you know this?

Not at all

Perfectly

What MUST switches avoid?

Logical Loops

How well did you know this?

Not at all

Perfectly

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

Core Layer
Distribution Layer
Access Layer

How well did you know this?

Not at all

Perfectly

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

Backbone of the model

Transport data reliably & quickly

How well did you know this?

Not at all

Perfectly

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

Routing
filtering
WAN Access

How well did you know this?

Not at all

Perfectly

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

Switching

Separating Collision Domains

How well did you know this?

Not at all

Perfectly

In which network design is Redundancy introduced?

Hierarchical Network Design

How well did you know this?

Not at all

Perfectly

What type of loop will disable a network?

A Logical Loop (due to redundancy)

How well did you know this?

Not at all

Perfectly

What does Spanning Tree Protocol (STP) prevent?

Stops loops in Layer 2 Switch Networks

How well did you know this?

Not at all

Perfectly

What are the properties of the Spanning Tree Protocol?

Layer 2 Protocol

Uses the Spanning Tree Algorithm (STA)

How well did you know this?

Not at all

Perfectly

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

STP: IEEE802.1D
Rapid STP (RSTP): IEEE802.1W

How well did you know this?

Not at all

Perfectly

What is the purpose of the Spanning Tree Algorithm?

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

How well did you know this?

Not at all

Perfectly

How does STP prevent Logical Loops?

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

How well did you know this?

Not at all

Perfectly

What is a Bridge Protocol Data Unit (BPDU)?

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

How does a BPDU function?

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

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

Sent by Bridges periodically (usually every 2 seconds)

what is Port Cost?

The cumulative value of link costs towards the root

What is Port Cost used for?

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

How is Port Cost calculated?

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

What is a Bridge ID?

The combination of the Bridge Priority and the base MAC Address

What is the default Bridge Priority for all Cisco Switches?

32,768

What are Bridge ID's used for?

For STP to keep track of all network-related switches

What is a Root Bridge?

The Bridge with the lowest Bridge ID

What is a Non-root Bridge?

All bridges that are not the Root Bridge

What is the main function of Non-Root Bridges?

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

What is a Root Port?

The port that provides the best path to the root bridge

What are two properties of the Root Port?

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

What is a Designated Port?

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

What can a Designated Port be marked as?

A Forwarding Port

What is a Non-Designated Port?

A port with a higher cost than the Designated Port

What is a Non-Designated Port marked as?

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

What is a Forwarding Port?

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

What is a Blocked Port?

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

What is an exception to a Blocked Port?

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

How do Switches & Bridges configure a single logical path?

via election

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

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

What process is triggered when there is a Root Failure?

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

What is a Stable State?

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

What is a Transitory State?

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

What are the three Stable States?

Blocking Forwarding Disabled

What are the two Transitory States?

Listening | Learning

When are Transitory States used?

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

What is a Disabled State?

Virtually non-operational port

What is a Listening State?

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

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

Does not learn MAC Address | Does not forward Frames

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

Processes BPDUs Learns MAC Address Does not forward packets

What is a Forwarding State?

Normal port operation; sending and receiving data

What is a Blocking State?

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

What is one property of a Blocking State?

BPDU data is still received

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

1. Other links in use were to fail | 2. STA determines the port may transition to the Forwarding State

How many bytes long is the Bridge ID?

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

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

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

System ID Extension is what?

The VLAN Number

What are the three STP Timers?

Hello Time Forward Delay Maximum Age

What is the Hello Time STP timer?

The time between each BPDU Frame that is sent on a port | 2 seconds by default, can be 1 - 10 seconds

What is the Forward Delay STP timer?

The time spent in the listening & learning state | 15 seconds for each state by default, can be 4 - 30 seconds

What is the Maximum Age STP Timer?

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