The Open Short Path First (OSPF) Is Defined In RFC 2328.It Is An Interior Gateway Protocol Used To Distribute Routing Information Within A Single Autonomous System. It Is A Most Common Routing Protocol Using By Service Providers, Because Unlike EIGRP It Is An Open Standard Protocol.
Why We Are Calling OSPF Is A Link State Routing Protocol. Unlike RIP, OSPF Is Not Only Sending The Routing Updates To Its Neighbors. It Is Sending All The Information About The Link Like IP Address Of The Interface And Subnet Mask, The Type Of Network It Is Connected To (P2P Or P2Multi-Point Or FR) And The Routers Which Is Connected To It. The Collection Of These Link States Will Form A Link State Database.
OSPF Vs RIP: Comparison between RIP Vs OSPF is given below. RIP: ◙ - ➤ RIP Has Limited HOP Counts. It Is 16.A RIP Network Spans More Than 15 HOPS, Considered As Unreachable.
◙ - ➤ RIP Doesn’t Support For VLSM.
◙ - ➤ Periodic Update Of Routing Table Consumes Lots Of Bandwidth Especially On WAN Clouds.
◙ - ➤ RIP Converges Slower Than OSPF Does.
◙ - ➤ RIP Network Is A FLAT Network. Here No Concept Of Areas & Boundaries & Summarization.
OSPF ◙-◙ No Limitations On The Hop Count.
◙-◙ Can Use VLSM
◙-◙ Converges Quickly
◙-◙ Can Divide Into Areas. This Will Help Us To Use Summarization.
◙-◙ Allows Authentication.
◙-◙ It Uses Dijkstra’s Algorithm (SPF Algorithm) Reducing The Usage Of BW, By Sending Triggered Updates To Announce The Network Changes.
◙-◙ Sending Periodic Updates On Long Intervals (30 Mins).
Unlike RIP, OSPF Doesn’t Send Any Routing Updates On Periodic Intervals. It Will Only Send Triggered Updates. It Means Every Time It Doesn’t Send Full Routing Table To Its Neighbors. Whenever Any Changes In Network, Like New Router Added Or A Router Removed From The Network, It Will Send Information About That Particular Network To Its Neighbor.
✓ TRANSIT AREA: Area 0 (Also Called Backbone Area)
✓ NORMAL AREAS: Non-Transit Areas
◙ - ► TYPES OF OSPF ROUTERS:
✓ BACKBONE ROUTER: A Router With All Interfaces In Area 0
✓ INTERNAL ROUTER: A Router With All Interfaces In A Non-Backbone Area
✓ ABR: Area Border Router – A Router That Connects Area 0 To Non-Backbone Area
✓ ASBR: Connects Any OSPF Area To A Different Routing Administration. The ASBR Is Where External Routes Can Be Redistributed Into OSPF
◙ - ► OSPF USES COST AS A METRIC. COST IS CALCULATED PER LINK BY THE FORMULA 10 TO THE 8th / BANDWIDTH
✓ This Is Configurable With The Auto-Cost Reference-Bandwidth Command (Config-Router)
◙ - ► OSPF TABLES:
✓ Neighbor Table
✓ Link State Database
✓ Routing Table
◙ - ➤ NEIGHBOR TABLE: The Router Tracks All The Neighbors Which Is Running OSPF As A Routing Protocol And Put That Information In This Table. It Contains All The Information About The Directly Connected Neighbors. Like Their Router Id, To Which Network They Are Connected, And Which Network They Are Advertising. It Will Exchange Routing Information With Routers Which Is In This Table.
◙ - ➤ TOPOLOGY TABLE: This Is The One Of The Big Difference Between Distance Vector And Link-State Protocols. Distance Vector Protocol Doesn’t Have This Topology Table. They Only Know About The Directly Connected Neighbors. This Table Is A Road Map For Each And Every Single Network Which Is Available In A Particular Area. All The Routers In A Particular Area Will Be Having The Same Type Of Topology Table.
◙ - ➤ ROUTING TABLE: This Table Contains All The Best Routes To Reach A Particular Network. Based On The Topology Table It Will Be Having Multiple Paths For A Single Destination Network. It Will Run The Spf Algorithm To Find The Best Routes For Each And Every Network.
The Algorithm Places Each Router At The Root Of A Tree And Calculates The Shortest Path To Each Destination Based On The Cumulative Cost Required To Reach That Destination. Each Router Will Have Its Own View Of The Topology Even Though All The Routers Will Build A Shortest Path Tree Using The Same Link-State Database.
HOW TO CALCULATE OSPF COST? Interface Cost Is Derived From The Bandwidth. Formula Is: OSPF’S COST= 10000 0000/Bandwith In BPS
Cost = Reference / Bandwidth.
By default, Reference is 100000 [ Kb/s ].
So, For Your Bandwidth Of 5120, Cost Should Be:
Cost = 100000 / 5120 = 19.53. Rounded Down To The Closest Integer, It's 19.
If You Want To Have Cost Of 51, You Would Need To Recalculate Your Bandwidth:
51 = 100000 / x => x= 100000 / 51 = 1960.78. Round it down to 1960.
◙ - ► OSPF ADJACENCIES:
✓ Routing Updates And Topology Information Are Passed Only Between Adjacent Routers
✓ Forming OSPF Adjacencies On Lan Links Is Different From Forming Them On Point To Point Links
◙ - ► BUILDING THE LINKSTATE DATABASE (LSDB):
✓ Hello Protocol Is Used To Define Neighbors
✓ Adjacency Is Established
✓ Adjacent Routers Exchange Link State Advertisements (LSA)
✓ Each Router Builds A LSDB Using LSAS
◙ - ► OSPF PACKET TYPES:
✓ HELLO: Discovers Neighbors And Builds Adjacencies between Them
✓ DATABASE DESCRIPTION (DBD): Checks For Db Synchronization Between Routers By Sending A Summary List Of All Routes In DB
✓ LINKS STATE REQUEST (LSR): Requests Specific Link State Records From Another Router
✓ LINK STATE UPDATE (LSU): Sends Specifically Requested Link State Requests
✓ LINK STATE ACKNOWLEDGEMENT (LSACK): Acknowledges The Other Packet Types
◙ - ► OSPF HELLO PARAMETERS THAT MUST MATCH FOR NEIGHBORS TO BECOME ADJACENT:
✓ Area ID
✓ Hello And Dead Intervals
✓ DR IP Address
✓ MTU Size
✓ Authentication (If Authentication Used)
✓ Stub Area Flag (If Area Is Stubbed)
◙ - ► STATES AN INTERFACE GOES THROUGH BEFORE BECOMING ADJACENT TO ANOTHER ROUTER:
◙ DOWN:Router Has Not Exchanged Information With Any Other Router
◙ INIT: All Directly Connected Routers That Are Running OSPF Receive A Hello Packet And Add Each Other To Their List Of Neighbors
◙ TWO-WAY: Each Router Can See Its Own Router Id In Their Neighbor’s Hello Packet
◙ EXSTART: The DR And BDR Establish Adjacencies With Each Other And With Each Router In The Network. During This Process A Master-Slave Relationship Is Established Between Each Router And Its’ Adjacent DR And BDR.
◙ EXCHANGE: The Master And Slave Routers Exchange One Or More DBD Packets
◙ LOADING: When Router Receives A DBD Packet, It Acknowledges It. It Then Compares The Information It Received With Its Own LSDB. If It Has A More Recent Link State Entry, It Sends An LSR To The Other Router. The Other Router Sends The Complete Information About The Requested LSU Packet. When The Other Router Receives The LSU, It Sends An LSACK.
◙ FULL: When All LSRs Have Been Exchanged For A Given Router, The Adjacent Routers Are Considered Synchronized
◙ - ► ADJACENCY BEHAVIOR IN POINT-TO-POINT LINKS:
✓ OSPF Packets Sent Using A Destination Address Of 224.0.0.5
◙ - ► ADJACENCY BEHAVIOR IN BROADCAST MULTIACCESS NETWORKS
• DR And BDR Selection Are Required
• All Neighbor Routers Form Adjacencies With Dr And Bdr Only
• Packets To The DR And BDR Are Addressed To 224.0.0.6
• Packets From Dr To All Other Routers Is 224.0.0.5
◙ - ► OSPF OVER FRAME-RELAY – FIVE MODES OF OSPF OPERATION ARE AVAILABLE:
◙< - >◙ BROADCAST – CISCO EXTENSION:
✓ Has One IP Subnet
✓ Uses Multicast OSPF Hello Packets To Discover Neighbors
✓ Elects DR And BDR
✓ Requires A Full-Mesh Or Partial-Mesh Topology
◙< - >◙ NONBROADCAST (NBMA) – RFC 2328:
✓ Has One IP Subnet
✓ Requires Neighbors To Be Manually Configured
✓ Elects DR And BDR
✓ Requires That The DR And Br Have Full Connectivity With All Other Routers
✓ Typically Used In A Full-Mesh Or Partial-Mesh Topology
◙< - >◙ POINT-TO-MULTIPOINT – RFC 2328:
✓ Has One IP Subnet
✓ Uses Multicast OSPF Hello Packets To Discover Neighbors
✓ Does Not Require DR And BDR
✓ Typically Used In A Partial-Mesh Or Star Topology
◙< - >◙ POINT-TO-MULTIPOINT NONBROADCAST – CISCO EXTENSION:
✓ Used In Place Of RFC Compliant Point-To-Multipoint If Multicast And Broadcast Are Not Enabled On The Virtual Circuit
✓ Requires Neighbors To Be Manually Configured
✓ Does Not Require DR And BDR
◙< - >◙ POINT-TO-POINT – CISCO EXTENSION:
✓ Unique Subnet On Each Subinterface
✓ Does Not Have DR And BDR
✓ Used When Only Two Routers Need To Form An Adjacency On A Pair Of Interfaces
✓ Can Be Used With Either LAN Or Wan Interfaces
◙ - ► LSA TYPES:
✓ TYPE 1 – ROUTER LSAS: Generated By Every Router In An Area And Does Not Cross An ABR
✓ TYPE 2 – NETWORK LSAS: Advertised By Dr And Does Not Cross An ABR
✓ TYPE 3 – SUMMARY LSAS: Advertised By The ABR Of The Originating Area
✓ TYPE 4 – SUMMARY LSAS: Used To Advertise A Metric To The ASBR And Advertised By The ABR Of The Originating Area
✓ TYPE 5 – AS EXTERNAL LSAS: Used To Advertise Network From Other Autonomous Systems And Is Advertised And Owned By The Originating ASBR (Need Type 4 To Find The ASBR)
✓ TYPE 6 – MULTICAST OSPF LSAS
✓ TYPE 7 – LSAS DEFINED FOR NOT-SO-STUBBY AREAS (NSSA): Used To Advertise Networks From Other Autonomous Systems Injected Into An NSSA Area And Is Advertised And Owned By The Originating ASBR. Translated To Type 5 By The NSSA ABR
✓ TYPE 8 – EXTERNAL ATTRIBUTE LSAS: For BGP
✓ TYPES 9, 10 & 11 – OPAQUE LSAS
◙ - ► TYPES OF OSPF ROUTES:
◙ OSPF Intra-Area Routes (Router LSA And Network LSA)
✓ O Ia: – OSPF Interarea Routes (Summary LSA)
✓ O E1: – OSPF Type 1 External Routes (Advertised By External LSA)
◙ E2: – OSPF Type 2 External Routes (Advertised By External LSA) – This Is The Default For Redistributed Routes
◙ E1 – External Routes Calculate The Cost By Adding The External Cost To The Internal Cost Of Each Link That The Packet Crosses
◙ E2 – The External Cost Of O E2 Packets Routes Is Always The External Cost Only
◙ - ► OSPF PASSIVE INTERFACE:
✓ The Sending And Receiving Of Routing Updates Is Disabled
✓ The Specified Interface Address Appears As A Stub Network In The OSPF Domain
◙ - ► DESIGN LIMITATIONS OF OSPF:
✓ If More Than One Area Is Configured, One Must Be Area 0, The Backbone Area
✓ All Areas Must Be Connected To Area 0
✓ Area 0 Must Be Contiguous
◙ - ► VIRTUAL LINKS:
✓ An Extension Of The Backbone
✓ Carried By Nonbackbone Area
✓ Cannot Be Created Across A Stub Or Nssa Area
✓ Used To Allow Areas To Connect To Areas Other Than Area 0
✓ Used To Repair A Discontiguous Area 0
◙ - ► AREA TYPES:
✓ BACKBONE AREA: Connects All Other Areas
✓ NORMAL AREA: Contains All Internal And External Routing Information
✓ STUB AREA: Contains Internal And Area Routing Information, But Not External Routing Information
✓ TOTALLY STUBBY AREA: Contains Area Routing Information Only. Cisco Proprietary
✓ NSSA: Contains Area And External Routing Information
◙ - ► OSPF AUTHENTICATION:
✓ Simple Password Authentication
✓ MD5 Authentication
✓ Router Generates And Checks Every Ospf Packet
✓ The Source Of Each Routing Update Packet Received Is Authenticated
✓ Each Participating Neighbor Must Have The Same Key (Password) Configured.
OSPF QUEST
2. WHAT IS AN OSPF ADJACENCY? An OSPF Adjacency Is A Conceptual Link To A Neighbor Over Which LSAs Can Be Sent.
3. WHAT ARE THE FIVE OSPF PACKET TYPES? WHAT IS THE PURPOSE OF EACH TYPE? OSPF Packet Types, As OSPF Link-State Information Is Shared Between Areas, An Intricate Set Of Mechanisms Is Followed, Relying On A Number Of Different OSPF Packet Types. All OSPF Traffic Is Transmitted Inside IP Packets. Receivers Recognize OSPF Traffic Because It Is Marked As IP Protocol 89.
OSPF Packet Types And Their Purposes:
✓ Hello packets — Establish Communication With Directly Attached Neighbors.
✓ Database Descriptor (DBD) — Sends A List Of Router Ids From Whom The Router Has An LSA And The Current Sequence Number. This Information Is Used To Compare Information About The Network.
✓ Link State Requests (LSR) — Follow DBDs To Ask For Any Missing LSAs.
✓ Link State Update (LSU) — Replies To A Link-State Request With The Requested Data.
✓ Link-State Acknowledgements (LSACK) — Confirm Receipt Of Link-State Information.
4. WHAT IS AN LSA? HOW DOES AN LSA DIFFER FROM AN OSPF UPDATE PACKET? 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. WHAT ARE LSA TYPES 1 TO 5 AND LSA TYPE 7? WHAT IS THE PURPOSE OF EACH TYPE? 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 \ XD5 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. WHAT IS A LINK STATE DATABASE? WHAT IS LINK STATE DATABASE SYNCHRONIZATION? 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. WHAT IS THE DEFAULT HELLO INTERVAL? The Default OSPF Hello Interval Is 10 Seconds.
8. WHAT IS THE DEFAULT ROUTER DEAD INTERVAL? The Default Router Dead Interval Is Four Times The Hello Interval.
9. WHAT IS A ROUTER ID? HOW IS A ROUTER ID DETERMINED? 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.
10. WHAT IS AN AREA? An Area Is An OSPF Sub-Domain, Within Which All Routers Have An Identical Link State Database.
11. WHAT IS THE SIGNIFICANCE OF AREA 0? Area 0 Is The Backbone Area. All Other Areas Must Send Their Inter-Area Traffic Through The Backbone.
12. WHAT IS MAXAGE? Maxage, 1 Hour, Is The Age At Which An LSA Is Considered To Be Obsolete.
13. WHAT ARE THE FOUR OSPF ROUTER TYPES? ◙ - ➤ 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. WHAT ARE THE FOUR OSPF PATH TYPES? ✓ Intra-Area Paths
✓ Inter-Area Paths
✓ Type 1 External Paths
✓ Type 2 External Paths
15. WHAT ARE THE FIVE OSPF NETWORK TYPES? ✓ Point-To-Point Networks
✓ Broadcast Networks
✓ Non-Broadcast Multi-Access (NBMA) Networks
✓ Point-To-Multipoint Networks
✓ Virtual Links
16. WHAT IS A DESIGNATED ROUTER? A Designated Router Is A Router That Represents A Multiaccess Network, And The Routers Connected To The Network, To The Rest Of The OSFP Domain.
17. HOW DOES A CISCO ROUTER CALCULATE THE OUTGOING COST OF AN INTERFACE? Cisco IOS Calculates The Outgoing Cost Of An Interface As 108/BW, Where BW Is The Configured Bandwidth Of The Interface.
18. WHAT IS A PARTITIONED AREA? 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. WHAT IS A VIRTUAL LINK? A Virtual Link Is A Tunnel That Extends An OSPF Backbone Connection Through A Non-Backbone Area.
20. WHAT IS THE DIFFERENCE BETWEEN A STUB AREA, A TOTALLY STUBBY AREA, AND A NOT-SO-STUBBY AREA? ✓ 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. WHAT IS THE DIFFERENCE BETWEEN OSPF NETWORK ENTRIES AND OSPF ROUTER ENTRIES? 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. WHY IS TYPE 2 AUTHENTICATION PREFERABLE OVER TYPE 1 AUTHENTICATION? Type 2 Authentication Uses MD5 Encryption, Whereas Type 1 Authentication Uses Clear-Text Passwords.
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? 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.
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The “OSPF Quick References" And Also We Hope This Guide Will Help Every Beginner 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!Hands - On Experience Is An Invaluable Part Of Preparing For The Lab Exam And Never Pass Up An Opportunity To Configure Or Troubleshoot A Router ( If You Have Access To Lab Facilities, Take Full Advantage Of Them) There Is No Replacement For The Experience You Can Gain From Working In A Lab, Where You Can Configure Whatever You Want To Configure And Introduce Whatever Problems You Want To Introduce, Without Risk Of Disrupting A Production Network. Thank You And Best Of Luck
This Article Written Author By: Premakumar Thevathasan - CCNA, CCNP, MCSE, MCSA, MCSA - MSG, CIW Security Analyst, CompTIA Certified A+ And Etc.
WARNING AND DISCLAIMER:
Routers Direct And Control Much Of The Data Flowing Across Computer Networks. This Guide Provides Technical Guidance Intended To Help All Network Students, Network Administrators And Security Officers Improve Of Their Demonstrated Ability To Achieve Specific objectives Within Set Timeframes.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, But No Warranty Or Fitness Is Implied.
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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