Routing Protocols and Concepts, CCNA Exploration Companion Guide [NOOK Book]

Overview

Routing Protocols and Concepts

CCNA Exploration Companion Guide

Routing Protocols and Concepts, CCNA Exploration Companion Guide is the official supplemental textbook for the Routing Protocols and Concepts course in the Cisco Networking Academy® CCNA® Exploration curriculum version 4. This course describes the architecture, components, and operation of routers, and explains the principles of routing and the primary routing protocols. The ...

See more details below
Routing Protocols and Concepts, CCNA Exploration Companion Guide

Available on NOOK devices and apps  
  • NOOK Devices
  • Samsung Galaxy Tab 4 NOOK
  • NOOK HD/HD+ Tablet
  • NOOK
  • NOOK Color
  • NOOK Tablet
  • Tablet/Phone
  • NOOK for Windows 8 Tablet
  • NOOK for iOS
  • NOOK for Android
  • NOOK Kids for iPad
  • PC/Mac
  • NOOK for Windows 8
  • NOOK for PC
  • NOOK for Mac

Want a NOOK? Explore Now

NOOK Book (eBook)
$27.49
BN.com price
(Save 42%)$48.00 List Price

Overview

Routing Protocols and Concepts

CCNA Exploration Companion Guide

Routing Protocols and Concepts, CCNA Exploration Companion Guide is the official supplemental textbook for the Routing Protocols and Concepts course in the Cisco Networking Academy® CCNA® Exploration curriculum version 4. This course describes the architecture, components, and operation of routers, and explains the principles of routing and the primary routing protocols. The Companion Guide, written and edited by Networking Academy instructors, is designed as a portable desk reference to use anytime, anywhere. The book’s features reinforce the material in the course to help you focus on important concepts and organize your study time for exams.

New and improved features help you study and succeed in this course:

  • Chapter objectives–Review core concepts by answering the focus questions listed at the beginning of each chapter.
  • Key terms–Refer to the updated lists of networking vocabulary introduced and turn to the highlighted terms in context in each chapter.
  • Glossary–Consult the comprehensive glossary with more than 150 terms.
  • Check Your Understanding questions and answer key–Evaluate your readiness with the updated end-of-chapter questions that match the style of questions you see on the online course quizzes. The answer key explains each answer.
  • Challenge questions and activities–Strive to ace more challenging review questions and activities designed to prepare you for the complex styles of questions you might see on the CCNA exam. The answer key explains each answer.

Rick Graziani has been a computer science and networking instructor at Cabrillo College since 1994.

Allan Johnson works full time developing curriculum for Cisco Networking Academy. Allan also is a part-time instructor at Del Mar College in Corpus Christi, Texas.

How To–Look for this icon to study the steps you need to learn to perform certain tasks.

Packet Tracer Activities– Explore networking concepts in activities interspersed throughout some chapters using

Packet Tracer v4.1 developed by Cisco®. The files for these activities are on the accompanying CD-ROM.

Also available for the Routing Protocols and Concepts Course:

Routing Protocols and Concepts

CCNA Exploration

Labs and Study Guide

ISBN-10: 1-58713-204-4

ISBN-13: 978-1-58713-204-9

Companion CD-ROM

**See instructions within the ebook on how to get access to the files from the CD-ROM that accompanies this print book.**

The CD-ROM provides many useful tools and information to support your education:

  • Packet Tracer Activity exercise files v4.1
  • A Guide to Using a Networker’s Journal booklet
  • Taking Notes: a .txt file of the chapter objectives
  • More IT Career Information
  • Tips on Lifelong Learning in Networking

This book is part of the Cisco Networking Academy Series from Cisco Press®. The products in this series support and complement the Cisco Networking Academy online curriculum.

Read More Show Less

Product Details

  • ISBN-13: 9780132877527
  • Publisher: Pearson Education
  • Publication date: 12/20/2007
  • Series: Companion Guide
  • Sold by: Barnes & Noble
  • Format: eBook
  • Edition number: 1
  • Pages: 640
  • Sales rank: 1,185,052
  • File size: 18 MB
  • Note: This product may take a few minutes to download.

Meet the Author

Rick Graziani teaches computer science and computer networking courses at Cabrillo College in Aptos, California. Rick has worked and taught in the computer networking and information technology field for almost 30 years. Prior to teaching, Rick worked in IT for various companies including Santa Cruz Operation, Tandem Computers, and Lockheed Missiles and Space Corporation. He holds an M.A. in computer science and systems theory from California State University Monterey Bay. Rick also does consulting work for Cisco and other companies. When Rick is not working, he is most likely surfing. Rick is an avid surfer who enjoys longboarding at his favorite Santa Cruz surf breaks.

Allan Johnson entered the academic world in 1999 after 10 years as a business owner/operator to dedicate his efforts to his passion for teaching. He holds both an M.B.A. and an M.Ed. in occupational training and development. He is an information technology instructor at Del Mar College in Corpus Christi, Texas. In 2003, Allan began to commit much of his time and energy to the CCNA Instructional Support Team, providing services to Networking Academy instructors worldwide and creating training materials. He now works full time for the Academy in Learning Systems Development.

Read More Show Less

Table of Contents

Introduction

Chapter 1 Introduction to Routing and Packet Forwarding

Objectives

Key Terms

Inside the Router

Routers Are Computers

Routers Are at the Network Center

Routers Determine the Best Path

Router CPU and Memory

CPU

RAM

ROM

Flash Memory

NVRAM

Internetwork Operating System (IOS)

Router Bootup Process

Bootup Process

Command-Line Interface

Verifying Router Bootup Process

IOS Version

ROM Bootstrap Program

Location of IOS

CPU and Amount of RAM

Interfaces

Amount of NVRAM

Amount of Flash

Configuration Register

Router Ports and Interfaces

Management Ports

Router Interfaces

Interfaces Belong to Different Networks

Example of Router Interfaces

Routers and the Network Layer

Routing Is Forwarding Packets

Routers Operate at Layers 1, 2, and 3

CLI Configuration and Addressing

Implementing Basic Addressing Schemes

Populating an Address Table

Basic Router Configuration

Host Name and Passwords

Configuring a Banner

Router Interface Configuration

Each Interface Belongs to a Different Network

Verifying Basic Router Configuration

Building the Routing Table

Introducing the Routing Table

show ip route Command

Directly Connected Networks

Static Routing

When to Use Static Routes

Dynamic Routing

Automatic Network Discovery

Maintaining Routing Tables

IP Routing Protocols

Routing Table Principles

Asymmetric Routing

Path Determination and Switching Functions

Packet Fields and Frame Fields

Internet Protocol (IP) Packet Format

MAC Layer Frame Format

Best Path and Metrics

Best Path

Comparing Hop Count and Bandwidth Metrics

Equal-Cost Load Balancing

Equal-Cost Paths Versus Unequal-Cost Paths

Path Determination

Switching Function

Path Determination and Switching Function Details

Path Determination and Switching Function Summary

Summary

Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

End Notes

Chapter 2 Static Routing

Objectives

Key Terms

Routers and the Network

Role of the Router

Introducing the Topology

Examining the Connections of the Router

Router Connections

Serial Connectors

Ethernet Connectors

Router Configuration Review

Examining Router Interfaces

Interfaces and Their Statuses

Additional Commands for Examining Interface Status

Configuring an Ethernet Interface

Configuring an Ethernet Interface

Unsolicited Messages from IOS

Reading the Routing Table

Routers Usually Store Network Addresses

Verifying Ethernet Addresses

Commands to Verify Interface Configuration

Ethernet Interfaces Participate in ARP

Configuring a Serial Interface

Examining Serial Interfaces

Physically Connecting a WAN Interface

Configuring Serial Links in a Lab Environment

Verifying the Serial Interface Configuration

Exploring Directly Connected Networks

Verifying Changes to the Routing Table

Routing Table Concepts

Observing Routes as They Are Added to the Routing Table

Changing an IP Address

Devices on Directly Connected Networks

Accessing Devices on Directly Connected Networks

Pings from R2 to 172.16.3.1

Pings from R2 to 192.168.1.1

Cisco Discovery Protocol (CDP)

Network Discovery with CDP

Layer 3 Neighbors

Layer 2 Neighbors

CDP Operation

Using CDP for Network Discovery

CDP show Commands

Disabling CDP

Static Routes with “Next-Hop” Addresses

Purpose and Command Syntax of the ip route Command

ip route Command

Configuring Static Routes

Verifying the Static Route

Configuring Routes to Two More Remote Networks

Routing Table Principles and Static Routes

Applying the Principles

Resolving to an Exit Interface with a Recursive Route Lookup

Exit Interface Is Down

Static Routes with Exit Interfaces

Configuring a Static Route with an Exit Interface

Static Route and an Exit Interface

Static Routes and Point-to-Point Networks

Modifying Static Routes

Verifying the Static Route Configuration

Verifying Static Route Changes

Static Routes with Ethernet Interfaces

Ethernet Interfaces and ARP

Sending an ARP Request

Static Routes and Ethernet Exit Interfaces

Advantages of Using an Exit Interface with Static Routes

Summary and Default Static Routes

Summary Static Routes

Summarizing Routes to Reduce the Size of the Routing Table

Route Summarization

Calculating a Summary Route

Configuring a Summary Route

Default Static Route

Most Specific Match

Configuring a Default Static Route

Verifying a Default Static Route

Managing and Troubleshooting Static Routes

Static Routes and Packet Forwarding

Static Routes and Packet Forwarding

Troubleshooting a Missing Route

Troubleshooting a Missing Route

Solving the Missing Route

Summary

Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

Floating Static Routes

Discard Route

Further Reading on Static Routing

End Notes

Chapter 3 Introduction to Dynamic Routing Protocols

Objectives

Key Terms

Introduction to Dynamic Routing Protocols

Perspective and Background

Evolution of Dynamic Routing Protocols

Role of Dynamic Routing Protocol

Network Discovery and Routing Table Maintenance

Purpose of Dynamic Routing Protocols

Dynamic Routing Protocol Operation

Dynamic Routing Protocol Advantages

Static Routing Usage, Advantages, and Disadvantages

Dynamic Routing Advantages and Disadvantages

Classifying Dynamic Routing Protocols

IGP and EGP

Distance Vector and Link-State Routing Protocols

Distance Vector Routing Protocol Operation

Link-State Protocol Operation

Classful and Classless Routing Protocols

Classful Routing Protocols

Classless Routing Protocols

Dynamic Routing Protocols and Convergence

Metrics

Purpose of a Metric

Metrics and Routing Protocols

Metric Parameters

Metric Field in the Routing Table

Load Balancing

Administrative Distance

Purpose of Administrative Distance

Multiple Routing Sources

Purpose of Administrative Distance

Dynamic Routing Protocols and Administrative Distance

Static Routes and Administrative Distance

Directly Connected Networks and Administrative Distance

Summary

Activities and Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

Chapter 4 Distance Vector Routing Protocols

Objectives

Key Terms

Introduction to Distance Vector Routing Protocols

Distance Vector Technology

Meaning of Distance Vector

Operation of Distance Vector Routing Protocols

Routing Protocol Algorithms

Routing Protocol Characteristics

Comparing Routing Protocol Features

Network Discovery

Cold Start

Initial Exchange of Routing Information

Exchange of Routing Information

Convergence

Routing Table Maintenance

Periodic Updates

Maintaining the Routing Table

RIP Timers

Bounded Updates

Triggered Updates

Random Jitter

Routing Loops

Defining a Routing Loop

Implications of Routing Loops

Count-to-Infinity Condition

Preventing Routing Loops by Setting a Maximum Metric Value

Preventing Routing Loops with Hold-Down Timers

Preventing Routing Loops with the Split Horizon Rule

Route Poisoning

Split Horizon with Poison Reverse

Preventing Routing Loops with IP and TTL

Distance Vector Routing Protocols Today

RIP and EIGRP

RIP

EIGRP

Summary

Activities and Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

Chapter 5 RIP Version 1

Objectives

Key Terms

RIPv1: Distance Vector, Classful Routing Protocol

Background and Perspective

RIPv1 Characteristics and Message Format

RIP Characteristics

RIP Message Format: RIP Header

RIP Message Format: Route Entry

Why Are So Many Fields Set to Zero?

RIP Operation

RIP Request/Response Process

IP Address Classes and Classful Routing

Administrative Distance

Basic RIPv1 Configuration

RIPv1 Scenario A

Enabling RIP: router rip Command

Specifying Networks

Verification and Troubleshooting

Verifying RIP: show ip route Command

Verifying RIP: show ip protocols Command

Verifying RIP: debug ip rip Command

Passive Interfaces

Unnecessary RIP Updates Impact Network

Stopping Unnecessary RIP Updates

Automatic Summarization

Modified Topology: Scenario B

Boundary Routers and Automatic Summarization

Processing RIP Updates

Rules for Processing RIPv1 Updates

Example of RIPv1 Processing Updates

Sending RIP Updates: Using debug to View Automatic

Summarization

Advantages and Disadvantages of Automatic Summarization

Advantages of Automatic Summarization

Disadvantage of Automatic Summarization

Discontiguous Topologies Do Not Converge with RIPv1

Default Route and RIPv1

Modified Topology: Scenario C

Propagating the Default Route in RIPv1

Summary

Activities and Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

Chapter 6 VLSM and CIDR

Objectives

Key Terms

Classful and Classless Addressing

Classful IP Addressing

High-Order Bits

IPv4 Classful Addressing Structure

Classful Routing Protocol

Classless IP Addressing

Moving Toward Classless Addressing

CIDR and Route Summarization

Classless Routing Protocol

VLSM

VLSM in Action

VLSM and IP Addresses

CIDR

Route Summarization

Calculating Route Summarization

Summary

Activities and Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

Chapter 7 RIPv2

Objectives

Key Terms

RIPv1 Limitations

Summary Route

VLSM

RFC 1918 Private Addresses

Cisco Example IP Addresses

Loopback Interfaces

RIPv1 Topology Limitations

Static Routes and Null Interfaces

Route Redistribution

Verifying and Testing Connectivity

RIPv1: Discontiguous Networks

Examining the Routing Tables

How Classful Routing Protocols Determine Subnet Masks

RIPv1: No VLSM Support

RIPv1: No CIDR Support

192.168.0.0/16 Static Route

Configuring RIPv2

Enabling and Verifying RIPv2

Auto-Summary and RIPv2

Disabling Auto-Summary in RIPv2

Verifying RIPv2 Updates

VLSM and CIDR

RIPv2 and VLSM

RIPv2 and CIDR

Verifying and Troubleshooting RIPv2

Verification and Troubleshooting Commands

show ip route Command

show ip interface brief Command

show ip protocols Command

debug ip rip Command

ping Command

show running-config Command

Common RIPv2 Issues

Authentication

Summary

Activities and Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

Chapter 8 The Routing Table: A Closer Look

Objectives

Key Terms

The Routing Table Structure

Lab Topology

Routing Table Entries

Level 1 Routes

Parent and Child Routes: Classful Networks

Level 1 Parent Route

Level 2 Child Route

Parent and Child Routes: Classless Networks

Routing Table Lookup Process

Steps in the Route Table Lookup Process

The Route Lookup Process

Longest Match: Level 1 Network Routes

Longest Match

Example: Level 1 Ultimate Route

Longest Match: Level 1 Parent and Level 2 Child Routes

Example: Level 1 Parent Route and Level 2 Child Routes

Example: Route Lookup Process with VLSM

Routing Behavior

Classful and Classless Routing Behavior

Topology Changes

Classful Routing Behavior: no ip classless

Classful Routing Behavior: Search Process

Example: R2 Operating with Classful Routing Behavior

Classless Routing Behavior: ip classless

The Route Lookup Process

Classless Routing Behavior: Search Process

Example: R2 Operating with Classless Routing Behavior

Classful Route on R3

Classful vs. Classless Routing Behavior in the Real World

Summary

Activities and Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

End Notes

Chapter 9 EIGRP

Objectives

Key Terms

Introduction to EIGRP

EIGRP: An Enhanced Distance Vector Routing Protocol

Roots of EIGRP: IGRP

The Algorithm

Path Determination

Convergence

EIGRP Message Format

Protocol-Dependent Modules

RTP and EIGRP Packet Types

EIGRP Packet Types

Hello Protocol

EIGRP Bounded Updates

DUAL: An Introduction

Administrative Distance

Authentication

Basic EIGRP Configuration

EIGRP Network Topology

Autonomous Systems and Process IDs

Autonomous System

Process ID

The router eigrp Command

The network Command

The network Command with a Wildcard Mask

Verifying EIGRP

Examining the Routing Table

Introducing the Null0 Summary Route

R3 Routing Table

EIGRP Metric Calculation

EIGRP Composite Metric and the K Values

The Composite Metric

Verifying the K Values

EIGRP Metrics

Examining the Metric Values

Bandwidth

Delay

Reliability

Load

Using the bandwidth Command

Calculating the EIGRP Metric

Bandwidth

Delay

Adding Bandwidth and Delay

DUAL

DUAL Concepts

Successor and Feasible Distance

Feasible Successors, Feasibility Condition, and Reported Distance

Topology Table: Successor and Feasible Successor

Topology Table: No Feasible Successor

Finite State Machine

DUAL FSM

No Feasible Successor

More EIGRP Configurations

The Null0 Summary Route

Disabling Automatic Summarization

Manual Summarization

Determining the Summary EIGRP Route

Configure EIGRP Manual Summarization

EIGRP Default Route

Fine-Tuning EIGRP

EIGRP Bandwidth Utilization

Configuring Hello Intervals and Hold Times

Summary

Activities and Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

Chapter 10 Link-State Routing Protocols

Objectives

Key Terms

Link-State Routing

Link-State Routing Protocols

Introduction to the SPF Algorithm

Link-State Routing Process

Step 1: Learning About Directly Connected Networks

Links

Link States

Step 2: Sending Hello Packets to Neighbors

Step 3: Building the Link-State Packet

Step 4: Flooding Link-State Packets to Neighbors

Step 5: Constructing a Link-State Database

Shortest Path First (SPF) Tree

Building the SPF Tree

Determining the Shortest Path

Generating a Routing Table from the SPF Tree

Implementing Link-State Routing Protocols

Advantages of a Link-State Routing Protocol

Builds a Topological Map

Fast Convergence

Event-Driven Updates

Hierarchical Design

Requirements of a Link-State Routing Protocol

Memory Requirements

Processing Requirements

Bandwidth Requirements

Comparison of Link-State Routing Protocols

Summary

Activities and Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

Chapter 11 OSPF

Objectives

Key Terms

Introduction to OSPF

Background of OSPF

OSPF Message Encapsulation

OSPF Packet Types

Hello Protocol

Neighbor Establishment

OSPF Hello and Dead Intervals

Electing a DR and BDR

OSPF LSUs

OSPF Algorithm

Administrative Distance

Authentication

Basic OSPF Configuration

Lab Topology

The router ospf Command

The network Command

OSPF Router ID

Determining the Router ID

Highest Active IP Address

Verifying the Router ID

Loopback Address

OSPF router-id Command

Modifying the Router ID

Duplicate Router IDs

Verifying OSPF

Examining the Routing Table

The OSPF Metric

OSPF Metric

Reference Bandwidth

OSPF Accumulates Cost

Default Bandwidth on Serial Interfaces

Modifying the Cost of the Link

The bandwidth Command

The ip ospf cost Command

The bandwidth Command vs. the ip ospf cost Command

OSPF and Multiaccess Networks

Challenges in Multiaccess Networks

Multiple Adjacencies

Flooding of LSAs

Solution: Designated Router

DR/BDR Election Process

Topology Change

DR/BDR Election

Timing of DR/BDR Election

OSPF Interface Priority

More OSPF Configuration

Redistributing an OSPF Default Route

Topology

Fine-Tuning OSPF

Reference Bandwidth

Modifying OSPF Intervals

Summary

Activities and Labs

Check Your Understanding

Challenge Questions and Activities

To Learn More

Appendix Check Your Understanding and Challenge Questions Answer Key

Glossary of Key Terms

Index

1587132060 TOC 11/9/2007

Read More Show Less

Customer Reviews

Be the first to write a review
( 0 )
Rating Distribution

5 Star

(0)

4 Star

(0)

3 Star

(0)

2 Star

(0)

1 Star

(0)

Your Rating:

Your Name: Create a Pen Name or

Barnes & Noble.com Review Rules

Our reader reviews allow you to share your comments on titles you liked, or didn't, with others. By submitting an online review, you are representing to Barnes & Noble.com that all information contained in your review is original and accurate in all respects, and that the submission of such content by you and the posting of such content by Barnes & Noble.com does not and will not violate the rights of any third party. Please follow the rules below to help ensure that your review can be posted.

Reviews by Our Customers Under the Age of 13

We highly value and respect everyone's opinion concerning the titles we offer. However, we cannot allow persons under the age of 13 to have accounts at BN.com or to post customer reviews. Please see our Terms of Use for more details.

What to exclude from your review:

Please do not write about reviews, commentary, or information posted on the product page. If you see any errors in the information on the product page, please send us an email.

Reviews should not contain any of the following:

  • - HTML tags, profanity, obscenities, vulgarities, or comments that defame anyone
  • - Time-sensitive information such as tour dates, signings, lectures, etc.
  • - Single-word reviews. Other people will read your review to discover why you liked or didn't like the title. Be descriptive.
  • - Comments focusing on the author or that may ruin the ending for others
  • - Phone numbers, addresses, URLs
  • - Pricing and availability information or alternative ordering information
  • - Advertisements or commercial solicitation

Reminder:

  • - By submitting a review, you grant to Barnes & Noble.com and its sublicensees the royalty-free, perpetual, irrevocable right and license to use the review in accordance with the Barnes & Noble.com Terms of Use.
  • - Barnes & Noble.com reserves the right not to post any review -- particularly those that do not follow the terms and conditions of these Rules. Barnes & Noble.com also reserves the right to remove any review at any time without notice.
  • - See Terms of Use for other conditions and disclaimers.
Search for Products You'd Like to Recommend

Recommend other products that relate to your review. Just search for them below and share!

Create a Pen Name

Your Pen Name is your unique identity on BN.com. It will appear on the reviews you write and other website activities. Your Pen Name cannot be edited, changed or deleted once submitted.

 
Your Pen Name can be any combination of alphanumeric characters (plus - and _), and must be at least two characters long.

Continue Anonymously
Sort by: Showing all of 2 Customer Reviews
  • Posted April 29, 2013

    Decent book

    The book is o.k. but not great. It does follow along with the online course. Good book to have so that you can continue reading when access to a computer is not available. The physical aspect of the book was not very good. The pages became unglued from the binding just a week after use. Now all the pages keep falling out the book. This is the cause of my 3 star rating.

    Was this review helpful? Yes  No   Report this review
  • Posted April 5, 2012

    more from this reviewer

    Another reviewer remarked negatively about the so-called poor qu

    Another reviewer remarked negatively about the so-called poor quality of the paper in the book. I simply do not see this with my copy. The pages are high quality and quite reflective, without being actually glossy. In the printing business, there are grades of paper quality, and while I'm not in that field, the pages are certainly not low grade.

    As to the actual contents on those pages :) -

    It looks like comprehensive coverage. Remember that this is a Cisco produced book, for a Cisco certificate and where the hardware used is Cisco. So by definition, you are getting an authoritative reference for exam preparation. Unlike a third party offering [I don't know if these actually exist for this Cisco exam/certificate]. So to play it safe, if you are serious about getting the qualification, this text may be the best choice.

    The book has many examples of how to configure and otherwise manage routing. Instead of merely passively reading the text, you should manually try all the examples. Well, to save you time, these have been included in the accompanying CD. Related to this is the single most important point in the book. You need the Packet Tracer simulator. This does not come with the book, which advises you to ask your instructor, who has to be part of the Cisco Networking Academies. If you are trying to learn the material strictly on your own, then this will be harder to assimilate, short of you having access to another simulator or an actual network where you have administrative permissions.

    Having said that, a subsidiary role is possible for the book. If you already run a network, which might have Cisco hardware, and you need a solid reference on routing protocols, try using this.

    Was this review helpful? Yes  No   Report this review
Sort by: Showing all of 2 Customer Reviews

If you find inappropriate content, please report it to Barnes & Noble
Why is this product inappropriate?
Comments (optional)