CISCO - OSPF REVIEW QUESTIONS AND ANSWERS:

SHORT INTRODUCTION OF OSPF :

Open Shortest Path First (OSPF) Was Developed By The Internet Engineering Task Force (IETF) As A Replacement For The Problematic RIP And Is Now The IETF-Recommended Interior Gateway Protocol (IGP).

OSPF Is A Link-State Protocol That, As The Name Implies, Uses Dijkstra's Shortest Path First (SPF) Algorithm And That Is Open That Is, It Isn't Proprietary To Any Vendor Or Organization.

OSPF Has Evolved Through Several RFCs, All Of Which Were Written By John Moy. Version 1 Of The Protocol Was Specified In RFC 1131; This Version Never Progressed Beyond The Experimental Stage. Version 2, Which Is Still The Current Version For IPv4, Was First Specified In RFC 1247, And The Most Recent Specification Is RFC 2328.

OSPF HAS THE FOLLOWING FEATURES :

OSPF Has Introduced New Concepts Such As Authentication Of Routing Updates, Variable Length Subnet Masks (VLSM), Route Summarization, And So Forth.

OSPF Also Utilizes IP Multicast When Sending/Receiving The Updates. In Addition, Much Work Has Been Done To Produce A Protocol That Responds Quickly To Topology Changes, Yet Involves Small Amounts Of Routing Protocol Traffic.

◙ - ➤  FAST CONVERGENCE : OSPF Can Detect And Propagate Topology Changes Faster Than RIP. Count-To-Infinity Does Not Occur With OSPF.

◙ - ➤  LOOP-FREE ROUTES : OSPF-Calculated Routes Are Always Loop-Free.

◙ - ➤  SCALABILITY : With OSPF, An AS Can Be Subdivided Into Contiguous Groups Of Networks Called Areas. Routes Within Areas Can Be Summarized To Minimize Route Table Entries. Areas Can Be Configured With A Default Route Summarizing All Routes Outside The AS Or Outside The Area. As A Result, OSPF Can Scale To Large And Very Large Internetworks. In Contrast, RIP For IP Internetworks Cannot Be Subdivided And No Route Summarization Is Done Beyond The Summarizing For All Subnets Of A Network ID.

◙ - ➤  SUBNET MASK ADVERTISED WITH THE NETWORK : OSPF Was Designed To Advertise The Subnet Mask With The Network. OSPF Supports Variable-Length Subnet Masks (VLSM), Disjointed Subnets, And Supernetting.

◙ - ➤  SUPPORT FOR EXTERNAL ROUTES : Routes Outside Of The OSPF AS Are Advertised Within The AS So That OSPF Routers Can Calculate The Least Cost Route To External Networks.

◙ - ➤  SUPPORT FOR AUTHENTICATION : Information Exchanges Between OSPF Routes Can Be Authenticated.

THE ADVANTAGES OF OSPF ARE:

• OSPF Is Not A Cisco Proprietary Protocol.
• OSPF Always Determine The Loop Free Routes.
• If Any Changes Occur In The Network It Updates Fast.
• OSPF Minimizes The Routes And Reduces The Size Of Routing Table By Configuring Area.
• Low Bandwidth Utilization.
• Transfers And Tags External Routes Injected Into AS.
• Multiple Routes Are Supported.
• Support Variable Length Subnet Masking.
• It Is Suitable For Large Network.

HOWEVER THERE ARE FEW DISADVANTAGES :

• OSPF Is Quite CPU And Memory Intensive Due To SPF Algorithm And Maintenance Of Multiple Copies Of Routing Information;
• More Complex Protocol To Implement Compared To RIP

◙ - ►  Router OS implements OSPF Version 2 (OSPF For IPv4) (RFC 2328) And Version 3 (RFC 5340, OSPF For IPv6).

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DESIGNING OSPF AREAS

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DESIGNING AREAS :

◙ The Number Of Routers Within Each OSPF Area Is A Flexible Variable That Can Depend On A Number Of Factors.

◙ By Most Cases, The Optimal Upper Limit For OSPF In The Majority Of Networks Is Area 40 To 50 Routers Per Area, But This Is More Of A Guideline That It Is A Requirement.

◙ Your Own Implementations And Use May Vary Depending On What Types Of Routers You're Using.

◙ In Most Cases, There Should Not Be Any More Than 100 Routers Per Area, Because The Larger The Area, The Greater The Likelihood For Performance Problems Associated With OSPF Routing Recalculations, And The More Unstable The Area Becomes.

◙ But With A More Stable Network Infrastructure In Place, You Can Most Likely Run Many More Routers Per Area.

◙ One Important Factor To Consider Before It Becomes A Problem Is Making Sure That The Backbone Area Does Not Grow Too Big.

◙ It Is Not A Requirement That Most Routes Should Belong In Area 0.

◙ Be Sure To Outline The Logical View Of The Network From The Beginning, And Focus On Which Routes, Routers, And Networks Belong In What Area.

◙ Also Be Sure To Start Creating Non-Required Areas Before They Are Needed.

◙ It Is Always A Good Idea To Plan For Maximum Growth Coupled With Long-Term Planning At The Beginning Of The Network Design Process.

◙ Remember That Planning For Too Much Is Not A Bad Thing To Do.

◙ Below Are Cisco's Own Recommendations In Regards To OSPF Network Size Parameters.

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OSPF NETWORK SIZE RECOMMENDATIONS

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Below Are Cisco's Own Recommendations In Regards To OSPF Network Size Parameters, Came From The “IETF OSPF Standard Report”.

NETWORK PARAMETER	MINIMUM	AVERAGE	MAXIMUM
ROUTERS PER DOMAIN	1	500	1000
ROUTERS PER AREA	1	100	350
AREAS PER DOMAIN	1	25	75
NEIGHBORS PER ROUTER	1	50	100
AREAS PER ROUTER	1	3	5

◙ - ►  For More About - > OSPF STANDARDIZATION REPORT:

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OSPF REVIEW QUESTIONS AND ANSWERS

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OSPF REVIEW QUESTIONS :

1.	What Is An OSPF Neighbor?
2.	What Is An OSPF Adjacency?
3.	What Are The Five OSPF Packet Types? What Is The Purpose Of Each Type?
4.	What Is An LSA? How Does An LSA Differ From An OSPF Update Packet?
5.	What Are LSA Types 1 To 5 And LSA Type 7? What Is The Purpose Of Each Type?
6.	What Is A Link-State Database? What Is Link-State Database Synchronization?
7.	What Is The Default Hello Interval?
8.	What Is The Default Router Dead Interval?
9.	What Is A Router ID? How Is A Router ID Determined?
10.	What Is An Area?
11.	What Is The Significance Of Area 0?
12.	What is Max Age?
13.	What Are The Four OSPF Router Types?
14.	What Are The Four OSPF Path Types?
15.	What Are The Five OSPF Network Types?
16.	What Is A Designated Router?
17.	How Does A Cisco Router Calculate The Outgoing Cost Of An Interface?
18.	What Is A Partitioned Area?
19.	What Is A Virtual Link?
20.	What Is The Difference Between A Stub Area, A Totally Stubby Area, And A Not-So-Stubby Area?
21.	What Is The Difference Between OSPF Network Entries And OSPF Router Entries?
22.	Why Is Type 2 Authentication Preferable Over Type 1 Authentication?
23.	Which Three Fields In The LSA Header Distinguish Different LSAS? Which Three Fields In The LSA Header Distinguish Different Instances Of The Same LSA? CISCO - OSPF COMMAND REFERENCE Command Description area area-id authentication [message-digest] Enables type 1 or type 2 authentication for an area. area area-id default-cost cost Specifies a cost for the default route sent into a stub area by an ABR. area area-id filter-list prefix prefix_list_name [out | in] Defines an LSA type 3 filter list. area area-id nssa [no-redistribution] [default-information-originate] [no-summary] [translate type7 suppress-fa] Configures an area as not-so-stubby (NSSA). area area-id range address mask[advertise | not-advertise] [cost] Summarizes addresses into or out of an area. The cost of the summary address can be specified. area area-id stub [no-summary] Configures an area as a stub or totally stubby area. area area-id virtual-link router-id Defines a virtual link between ABRs. debug ip ospf adj Shows the events involved in the building or breaking of an OSPF adjacency. [no] discard-route {internal | external} No discard-route removes the automatically created static route to the Null interface. ip ospf authentication-key password Assigns a password to an OSPF interface for use with type 1 authentication. ip ospf cost cost Specifies the outgoing cost of an OSPF interface. ip ospf dead-interval seconds Specifies the OSPF RouterDeadInterval for an interface. ip ospf demand-circuit Configures an interface as an OSPF demand circuit. ip ospf hello-interval seconds Specifies the OSPF HelloInterval for an interface. ip ospf message-digest-key key-id md5 key Specifies an interface's key ID and key (password) for use with type 2 authentication. ip ospf name-lookup Enables the reverse DNS lookup of names to match Router IDs in certain show commands. ip ospf network [broadcast] [nonbroadcast] [point-to-multipoint] Configures the OSPF network type. ip ospf priority number Sets the router priority of an interface for use in the DR/BDR election process. ip ospf retransmit-interval seconds Sets an interface's OSPF RxmtInterval. ip ospf transmit-delay seconds Sets an interface's OSPF InfTransDelay. ip prefix-list prefix_list_name [seq num] {deny | permit} address/length Defines which addresses to permit or deny in a prefix list. log-adjacency-changes [detail] Logs neighbor state changes. maximum-paths Sets the number of paths over which OSPF performs load balancing. neighbor ip-address[priority number] [poll-interval seconds] [cost cost] Manually informs a router of its neighbors on a non-broadcast network. network address inverse-mask area area-id Specifies the interfaces on which OSPF is to run and specifies the area to which the interface is connected. ospf auto-cost reference-bandwidth reference-bandwidth Changes the default OSPF reference bandwidth used for the calculation of link costs. router ospf process-id Enables an OSPF routing process. show ip ospf [process-id] Displays general information about an OSPF routing process. show ip ospf border-routers Displays a router's internal OSPF route table. show ip ospf [process-id area-id] database Displays all entries in the OSPF link-state database. show ip ospf [process-id area-id] database router [link state-id] Displays type 1 LSAs in the OSPF link-state database. show ip ospf [process-id area-id] database network [link state-id] Displays type 2 LSAs in the OSPF link-state database. show ip ospf [process-id area-id] database summary [link state-id] Displays type 3 LSAs in the OSPF link-state database. show ip ospf [process-id area-id] database asbr-summary [link state-id] Displays type 4 LSAs in the OSPF link-state database. show ip ospf [process-id area-id] database nssa-external [link state-id] Displays type 7 LSAs in the OSPF link-state database. show ip ospf [process-id] database external [link state-id] Displays type 5 LSAs in the OSPF link-state database. show ip ospf [process-id area-id] database database-summary Displays the number of LSAs in the OSPF link-state database by type and by area ID. show ip ospf interface [type number] Displays OSPF-specific information about an interface. show ip ospf neighbor [type number] [neighbor-id] [detail] Displays information from the OSPF neighbor table. show ip ospf virtual-links Displays information about OSPF virtual links. timer lsa-group-pacing seconds or timer pacing lsa-group seconds Sets the minimum pacing time between two groups of LSAs whose refresh timers have expired.

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ANSWERS FOR ABOVE OSPF REVIEW QUESTIONS

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1.	From The Perspective Of An OSPF Router, A Neighbor Is Another OSPF Router That Is Attached To One Of The First Router's Directly Connected Links.
2.	An OSPF Adjacency Is A Conceptual Link To A Neighbor Over Which LSAs Can Be Sent.
3.	The Five OSPF Packet Types, And Their Purposes, Are: Hellos:, Which Are Used To Discover Neighbors, And To Establish And Maintain Adjacencies. Updates:, Which Are Used To Send LSAs Between Neighbors. Database Description Packets:, Which A Router Uses To Describe Its Link State Database To A Neighbor During Database Synchronization. Link State Requests:, Which A Router Uses To Request One Or More LSAs From A Neighbor's Link-State Database. Link State Acknowledgments:, Used To Ensure Reliable Delivery Of LSAs.
4.	A Router Originates A Link-State Advertisement To Describe One Or More Destinations. An OSPF Update Packet Transports LSAs From One Neighbor To Another. Although LSAs Are Flooded Throughout An Area Or OSPF Domain, Update Packets Never Leave A Data Link.
5.	The Most Common LSA Types And Their Purposes Are: Type 1 (Router LSAs): Are Originated By Every Router And Describe The Originating Router, The Router's Directly Connected Links And Their States, And The Router's Neighbors. Type 2 (Network LSAs) :Are Originated By Designated Routers On Multiaccess Links And Describe The Link And All Attached Neighbors. Type 3 (Network Summary LSAs): Are Originated By Area Border Routers And Describe Inter-Area Destinations. Type 4 LSAs (ASBR Summary LSAs): Are Originated By Area Border Routers To Describe Autonomous System Boundary Routers Outside The Area. Type 5 (AS External LSAs): Are Originated By Autonomous System Boundary Routers To Describe Destinations External To The OSPF Domain. Type 7 (NSSA External LSAs): Are Originated By Autonomous System Boundary Routers Within Not-So-Stubby Areas.
6.	The Link State Database Is Where A Router Stores All The OSPF LSAs It Knows Of, Including Its Own. Database Synchronization Is The Process Of Ensuring That All Routers Within An Area Have Identical Link-State Databases.
7.	The Default OSPF Hello Interval Is 10 Seconds.
8.	The Default Router Dead Interval Is Four Times The Hello Interval.
9.	A Router ID Is An Address By Which An OSPF Router Identifies Itself. It Is Either The Numerically Highest IP Address Of All The Router's Loopback Interfaces, Or If No Loopback Interfaces Are Configured, It Is The Numerically Highest IP Address Of All The Router's LAN Interfaces. It Can Also Be Manually Configured.
10.	An Area Is An OSPF Sub-Domain, Within Which All Routers Have An Identical Link-State Database.
11.	Area 0 Is The Backbone Area. All Other Areas Must Send Their Inter-Area Traffic Through The Backbone.
12.	Maxage, One Hour, Is The Age At Which An LSA Is Considered To Be Obsolete.
13.	The Four OSPF Router Types Are: Internal Routers:, Whose OSPF Interfaces All Belong To The Same Area. Backbone Routers:, Which Are Internal Routers In Area 0. Area Border Routers:, which Have OSPF Interfaces In More Than One Area. Autonomous System Boundary Routers:, which Advertise External Routes Into The OSPF Domain.
14.	The Four OSPF Path Types Are: Intra-Area Paths. Inter-Area Paths. Type 1 External Paths. Type 2 External Paths.
15.	What Are The Five OSPF Network Types? The Five OSPF Network Types Are: Point-To-Point Networks. Broadcast Networks. Non-Broadcast Multiaccess (NBMA) Networks. Point-To-Multipoint Networks. Virtual Links.
16.	A Designated Router Is A Router That Represents A Multiaccess Network, And The Routers Connected To The Network, To The Rest Of The OSPF Domain.
17.	Cisco IOS Calculates The Outgoing Cost Of An Interface As 108/BW, Where BW Is The Configured Bandwidth Of The Interface. 108 Can Be Changed With The OSPF Command Auto-Cost Reference-Bandwidth.
18.	An Area Is Partitioned If One Or More Of Its Routers Cannot Send A Packet To The Area's Other Routers Without Sending The Packet Out Of The Area.
19.	A Virtual Link Is A Tunnel That Extends An OSPF Backbone Connection Through A Non-Backbone Area.
20.	A Stub Area Is An Area Into Which No Type 5 LSAs Are Flooded. A Totally Stubby Area Is An Area Into Which No Type 3, 4, Or 5 LSAs Are Flooded, With The Exception Of Type 3 LSAs To Advertise A Default Route. Not-So-Stubby Areas Are Areas Through Which External Destinations Are Advertised Into The OSPF Domain, But Into Which No Type 5 LSAs Are Sent By The ABR.
21.	OSPF Network Entries Are Entries In The Route Table, Describing IP Destinations. OSPF Router Entries Are Entries In A Separate Route Table That Record Only Routes To ABRs And ASBRs.
22.	Type 2 Authentication Uses MD5 Encryption, Whereas Type 1 Authentication Uses Clear-Text Passwords.
23.	The Three Fields In The LSA Header That Distinguish Different Lsas Are The Type, Advertising Router, And The Link State ID Fields. The Three Fields In The LSA Header That Distinguish Different Instances Of The Same LSA Are The Sequence Number, Age, And Checksum Fields.

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SUMMARY OF OSPF:

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All Of These Terms Are Important For Understanding The Operation Of The OSPF And They Are Used Throughout The Article.

OSPF Is Interior Gateway Protocol (IGP) And Distributes Routing Information Only Between Routers Belonging To The Same Autonomous System (AS).

• NEIGHBOR : Connected (Adjacent) Router That Is Running Ospf With The Adjacent Interface Assigned To The Same Area. Neighbors Are Found By Hello Packets.

• ADJACENCY : Logical Connection Between Router And Its Corresponding DR And BDR. No Routing Information Is Exchanged Unless Adjacencies Are Formed.

• LINK : Link Refers To A Network Or Router Interface Assigned To Any Given Network.

• INTERFACE : Physical Interface On The Router. Interface Is Considered As Link, When It Is Added To OSPF. Used To Build Link Database.

• LSA : Link State Advertisement, Data Packet Contains Link-State And Routing Information, That Is Shared Among OSPF Neighbors.

• DR : Designated Router, Chosen Router To Minimize The Number Of Adjacencies Formed. Option Is Used In Broadcast Networks.

• BDR : Backup Designated Router, Hot Standby For The DR. BDR Receives All Routing Updates From Adjacent Routers, But It Does Not Flood LSA Updates.

• AREA : Areas Are Used To Establish A Hierarchical Network.

• ABR : Area Border Router, Router Connected To Multiple Areas.

• ASBR : Autonomous System Boundary Router, Router Connected To An External Network (In A Different AS).

• NBMA : Non-Broadcast Multi-Access, Networks Allow Multi-Access But Have No Broadcast Capability (For Example X.25, Frame Relay). Additional OSPF Neighbor Configuration Is Required For Those Networks.

• BROADCAST : Network That Allows Broadcasting, For Example Ethernet.

• POINT-TO-POINT : Network Type Eliminates The Need For Drs And BDRS

• ROUTER-ID: IP Address Used To Identify OSPF Router. If The OSPF Router-ID Is Not Configured Manually, Router Uses One Of The IP Addresses Assigned To The Router As Its Router-ID.

• LINK STATE : The Term Link State Refers To The Status Of A Link Between Two Routers. It Defines The Relationship Between A Router's Interface And Its Neighboring Routers.

• COST Link-State Protocols Assign A Value To Each Link Called Cost. The Cost Value Is Depend To Speed Of Media. A Cost Is Associated With The Outside Of Each Router Interface. This Is Referred To As Interface Output Cost.

• AUTONOMOUS SYSTEM : An Autonomous System Is A Group Of Routers That Use A Common Routing Protocol To Exchange Routing Information.

OSPF ROUTER TYPES :

ROUTER TYPE>	DESCRIPTION
INTERNAL ROUTER	A Router With All Interfaces Connected To The Same Area. Internal Routers Each Have A Single LSDB.
AREA BORDER ROUTER (ABR)	A Router With Interfaces Connected To Different Areas. ABRs Have Multiple LSDBs, One For Each Attached Area.
BACKBONE ROUTER	A Router With An Interface On The Backbone Area. This Includes All ABRs And Internal Routers Of The Backbone Area.
AS BOUNDARY ROUTER (ASBR)	A Router That Exchanges Routes With Sources Outside Of The OSPF AS. ASBRs Advertise External Routes Throughout The OSPF AS.

OSPF REFERENCES :

◙ - ►  For More About - > OSPF QUICK REFERENCE:

◙ - ►  For More About - > OSPF QUICK REFERENCE TABLE :

◙ - ►  For More About - > OSPF SHORT NOTES:

◙ - ►  For More About - > DIFFERENCES IN BETWEEN OSPF v2 (IPv4) & OSPF v3 (IPv6):

◙ - ►  For More About - > CISCO - OSPF COMMANDS REFERENCE:

◙ - ►  For More About - > CISCO - IMPLEMENTING OSPF FOR IPV6:

◙ - ►  For More About - > OSPF CONFIGURATION EXAMPLES:

◙ - ►  For More About - > OSPF VS BGP & EIGRP:

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CONCLUSION:

The Goal Of This Article Is To Give An Easy Way To Understand The “CISCO - OSPF Command Review & OSPF Review Questions And Answers: ". Hope This Article Will Help Every Beginners Who Are Going To Start Cisco Lab Practice Without Any Doubts.

Some Topics That You Might Want To Pursue On Your Own That We Did Not Cover In This Article Are Listed Here, Thank You And Best Of Luck.

This Article Written Author By: Premakumar Thevathasan. CCNA, CCNP, CCIP, MCSE, MCSA, MCSA - MSG, CIW Security Analyst, CompTIA Certified A+.

DISCLAIMER:

This Document Carries No Explicit OR Implied Warranty. Nor Is There Any Guarantee That The Information Contained In This Document Is Accurate. Every Effort Has Been Made To Make All Articles As Complete And As Accurate As Possible.

It Is Offered In The Hopes Of Helping Others, But You Use It At Your Own Risk. The Author Will Not Be Liable For Any Special, Incidental, Consequential Or Indirect Any Damages Due To Loss Of Data Or Any Other Reason That Occur As A Result Of Using This Document. But No Warranty Or Fitness Is Implied. The Information Provided Is On An "As Is" Basic. All Use Is Completely At Your Own Risk.

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