Plz Note : Some Topics That You Might Want To Pursue On Your Own That We Did Not Cover In This Article Are Listed Here. This Page Discusses OSPF DESIGNATED ROUTER (DR) AND BACKUP DESIGNATED ROUTER (BDR) ELECTION PROCESS Only, And Also We Request To The Students, Please Go Through All The Articles That Are We Posted In This Web Site And Identify All The CISCO IOS Commands In The Lab Practice Before Going To Access This Page. Thank You!
FOR MORE OSPF REFERENCES:
1. ◙ - ➤ Protocol Comparison OSPF, EIGRP, BGP And RIP:
2. ◙ - ➤ OSPFv2 (IPv4) Vs OSPFv3 (IPv6)
3. ◙ - ➤ OSPF Quick Reference:
4. ◙ - ➤ OSPF Quick IOS Commands Reference Table:
5. ◙ - ➤ Short Notes Of OSPF:
6. ◙ - ➤ OSPF LSA Operation And It’s Types:
7. ◙ - ➤ OSPF Configuration Examples:
8. ◙ - ➤ OSPF MD5:
9. ◙ - ➤ Redistribution OSPF Vs EIGRP:
10. ◙ - ➤ OSPF Review Questions And Answers:
11. ◙ - ➤ Just Recap About OSPF With Quest:
12. ◙ - ➤ IPv6 OSPF VIRTUAL-LINK:
13. ◙ - ➤ OSPF Interface Configuration Command References:
14. ◙ - ➤ OSPF Configuration Troubleshooting Command References:
SHORT RECAP INTRODUCTION OF OSPF
OPEN SHORTEST PATH FIRST (OSPF) Is A Link-State Routing Protocol Based On Open Standards (The Open In OSPF Means That It Is Open To The Public And Is Non-Proprietary). Runs The Dijkstra Algorithm To Find The Best Path.
OSPF Is A Routing Protocol That Calls For The Sending Of Link-State Advertisements (LSAs) To All Other Routers Within The Same Hierarchical Area. An Area Is A Group Of Contiguous Networks And Attached Hosts. OSPF LSAs Include Information On Attached Interfaces, Metrics Used, And Other Variables. As OSPF Routers Accumulate Information, The Routers Use The SPF Algorithm To Calculate The Shortest Path To Each Node. This Is Different From The Way Distance-Vector Protocols Work. Distance-Vector Protocols Send All Or A Portion Of Their Routing Tables In Routing-Update Messages To Their Neighbors.
OPEN SHORTEST PATH FIRST (OSPF): ◙ - ➤ SPF – Shortest Paths Are Calculated First, I.E. Before Routing Tables Are Created And Routing Of Packets Starts.
◙ - ➤ “Open” = Open Standard.
◙ - ➤ Development Began In 1987 by IETF
◙ - ➤ OSPFv2 - RFC 2328 (1998), Latest Revision Is 3.
◙ - ➤ Full Support for VLSM And Route Summarization.
◙ - ➤ Metric: Administratively Defined Cost.
◙ - ➤ (Default 100 MBPs/ Bandwidth)
◙ - ➤ Uses IP For Transport, IP Protocol 89
◙ - ➤ Uses Multicast Addresses For Neighbor Maintenance And Flooding Of LSAs.
• 224.0.0.5 – All OSPF Routers
• 224.0.0.6 – All DRouters
WHY “LINK STATE” ? ◙ - ➤ Link: Interface On A Router
◙ - ➤ Every Router Continually Checks Links To It’s Neighboring Routers (Using Hello Protocol).
◙ - ➤ Link State: Description Of An Interface And Of Its Relationship To Its Neighboring Routers, Including - IP Address/Mask Of The Interface, The Type Of Network It Is Connected To, The Routers Connected To That Network, The Metric (Cost) Of That Link
◙ - ➤ If A Link Changes State (I.E. Fails, Goes Up Or Hello Response Time Changes Considerably), Every Router In Routing Domain Is Informed.
BASIC PRINCIPLE OF LINK STATE ALGORITHM: ◙ - ➤ Hello Protocol Continually Checks Links To Neighbors
• Neighborship States Maintained In Adjacency Database.
◙ - ➤ If State Of Some Link Changes, Link State Update Is Sent To Other Routers
• Sometimes Called Link State Advertisements (LSA).
◙ - ➤ Link State Advertisements Are Flooded To Whole Routing Domain (Reliably)
◙ - ➤ All Routers Have (The Same) Topology Database, I.E. Graph Of Network Topology
• Sometimes Called Link-State Database
• Every Router Is Uniquely Identified
• Link Is A Connection Between Two Uniquely Identified Routers
• It Is Also Necessary To Be Able To Represent Multiaccess Network Segments
◙ - ➤ Every Router Computes Tree Of Shortest Paths To Other Networks With Itself As Root
• Uses Dijkstra Algorithm.
◙ - ➤ From Shortest Path Tree, Routing Table Is Created
LINK STATE CHANGE MEAN: ◙ - ➤ Neighbor Router Is Now Reachable/Unreachable
• Link Or Neighbor Router Goes Up/Down (Including Addition Of A New Link)
◙ - ➤ Metric Of A Link Changed
• Change Made By Administrator
• Average Delay Of Hellos Passed Between Neighbors Changes Considerably (ImplementedRarely)
◙ - ➤ Detected By Hardware: Interface State Changes
◙ - ➤ Detected By IGP Protocol (Hello):
• Interface Still Up, But Neighbor Lost
• New Neighbor Found
◙ - ➤ Faster Change Detection: Bidirectional Forwarding Detectiion (BFD-Cisco): CPU Offload By Distributed Processing
Note: On Multiaccess Network, Interface Can Be Up, But Some Neighbor Down
LINK STATE ADVERTISEMENT: ◙ - ➤ Sent Every Time Link State Changes
◙ - ➤ Contains
• Sending Router Identity
• List Of Neighboring Routers
• Together With Current Metric For Link To Each Of Them
• Sequence Number
• Needed To Distinguish Between Older And Newer LSAs
• Incremented Every Time New LSA Is Sent
◙ - ➤ Maxage
• To Defend Against Outdated Information; Lsas Have Limited Lifetime
• Need Of Periodic Refresh Of Lsas Generated By Every Router
• Typical Period Of 20 – 30 Mins, Randomized To Avoid Congestion
LSA FLOODING: ◙ - ➤ LSA Has To Be Processed On Each Router
◙ - ➤ Need For Reliable And Loop-Free Flooding On LSA Arrival
• If Router Already Has This Version (I.E. Seq. Number Originated By The Same Router) Of LSA, Router Ignores It
• Router Installs LSA In The Database (Or Reset Aging Timer)
• Router Resends LSA To Adjacent Neighbors Except That From Which LSA Came
• If LSA Carries Any Changes, Router Schedules SPF Tree Calculation (Full/Partial)
LINK STATE DATABASE (TOPOLOGY DATABASE): ◙ - ➤ Is A Collection Of LSAs Originated By Individual Routers (The Collection Of All The Link-States Would Form A Link-State Database).
LINK STATE ALGORITHMS SCALABILITY: ◙ - ➤ Hierarchical Routing Principle (2-Level Hierarchy)
◙ - ➤ Autonomous System Can Be Divided Into Areas
• Area – A Set Of Routers Sharing The Complete Topology Information For That Area
◙ - ➤ Routers In The Same Area Have The Same Topology Database
◙ - ➤ Routers At Area Boundary Maintain Topology Databases For All Adjacent Areas
◙ - ➤ Topology Of The Area Is Not Known Outside That Area
◙ - ➤ Only Summary Information About Networks In The Area Is Exported
◙ - ➤ Router Has Separate Topological Database For Every Connected Area
◙ - ➤ LSA Flooding Bounded By Area Boundary
◙ - ➤ Only Summary Information About Networks In The Area Are Propagated By The Area Border Router To The Backbone
◙ - ➤ SPF Calculation Is Performed Independently For Each Area
◙ - ➤ Distance Vector Routing Principle Between Areas
◙ - ➤ Hierarchical Addressing Allows Summarization
ROUTE FILTERING IN LS ROUTING: ◙ - ➤ In Filtering
◙ - ➤ Filtering Of Routes Accepted To Routing Tables
◙ - ➤ Does Not Affect Flooding Of LSAs
◙ - ➤ Done On Every Router Independently
◙ - ➤ Out Filtering
◙ - ➤ Only On Area Or As Boundary
◙ - ➤ Filtering Of (Summary) LSAs Generated By ABR or ASBR – Inter-Area Filtering (Out Of Area, To Area)
ADVANTAGES OF LS ROUTING ALGORITHMS: ◙ - ➤ Fast Convergence
◙ - ➤ Minimal Overhead After Reaching Convergence
◙ - ➤ Event-Driven - Updates Are Sent Only On Link State Change
• Incremental Updates
◙ - ➤ All Routers Have The Same And Complete Information About Network Topology
• Less Chance Of Calculating Wrong Routing Tables
◙ - ➤ Better Metric Than Hop Count
• Cost Defined By Administrator For Each Link
• No Hop-Count Limit On Network Topology
DISADVANTAGES OF LS ROUTING ALGORITHMS: ◙ - ➤ Higher Processor Utilization
◙ - ➤ Higher Memory Requirements
◙ - ➤ More Planning Before Implementation
◙ - ➤ Route Filtering Not So Transparent As With DV Algorithms
◙ - ➤ Route Summarization Only On ABRS & ASBRs
THE OSPF PROCESS BUILDS AND MAINTAINS THREE SEPARATE TABLES: ◙ A Neighbor Table – > Contains A List Of All Neighboring Routers.
◙ A Topology Table – > Contains A List Of All Possible Routes To All Known Networks Within An Area.
◙ A Routing Table – > Contains The Best Route For Each Known Network.
OSPF Is Built Upon A Hierarchy Of Network Components. The Highest Level Of The Hierarchy Is The Autonomous System (AS). An Autonomous System Is Defined As A Number Of Networks, All Of Which Share The Same Routing And Administration Characteristics.
An AS Can Be Divided Into Multiple Areas. Each Area Represents A Collection Of Contiguous Networks And Hosts. Areas Limit The Area To Which Link-State Advertisements Are Broadcast, Thereby Limiting The Amount Of Flooding That Occurs Within The Network. An Area Is Represented In OSPF By Either An IP Address Or A Number.
An OSPF Router Can Be A Member Of Multiple Areas. Routers With Membership In Multiple Areas Are Known As Area Border Routers (ABRs). Each ABR Maintains A Separate Topological Database For Each Area The Router Is In. Each Topological Database Contains All Of The LSA Databases For Each Router Within A Given Area. The Routers Within The Same Area Have Identical Topological Databases. The ABR Is Responsible For Forwarding Routing Information Or Changes Between Its Border Areas.
You Can Further Limit The Broadcast Area Of Flooding By Defining An Area Range. The Area Range Allows You To Assign An Aggregate Value To A Range Of IP Addresses. This Aggregate Value Becomes The Address That Is Advertised Instead All Of The Individual Addresses It Represents Being Advertised.
OSPF AREAS: ◙ OSPF AREAS: The Primary Purpose Of An OSPF Area Is Scalability Of The Protocol. Boundaries Are Defined In The Network To Limit The Flooding Of Specific LSA Types. Each Newly Created Area Is Assigned A Unique 32-Bit Area ID Value. This Is Represented In A Quad-Octet Format Of 0.0.0.0, Much Like An IP Address. Although The Router Works With Area Numbers In This Fashion, Most Humans Prefer To Use Whole Numbers, Such As Area 0.
One Of The Newly Defined Areas, The Backbone Area, Forms The Core Of The Network. All Other OSPF Areas Must Connect To The Backbone Area. The Backbone Connects All Areas And Redistributes All Non-Backbone Routing Information Between The Areas.
OSPF ROUTER TYPES: ◙ INTERNAL ROUTER: All Router Interfaces Belong To Only One Area (A Router That Has All Its Interfaces In The Same Area). A Router That Maintains All Operational Interfaces Within A Single Area Is Known As An Internal Router. An Internal Router May Belong To Any OSPF Area.
◙ BACKBONE ROUTER (Area 0): Contain At Least One Interface In Area 0. A Router That Has At Least One Interface In Area 0 Is Known As A Backbone Router.
◙ AREA BORDER ROUTER (ABR): Contains Interfaces In At Least Two Separate Areas. The Area Border Router (ABR) Connects One Or More OSPF Areas To The Backbone. This Means That At Least One Interface Is Within Area 0 While Another Interface Is In Another Area. The ABR Plays A Very Important Role In An OSPF Network.
◙ AUTONOMOUS SYSTEM BOUNDARY ROUTER (ASBR): Contain A Connection To A Separate Autonomous System. Is A Router That Is Running Multiple Protocols And Serves As A Gateway To Routers Outside An Area And Those Operating With Different Protocols. The ASBR Is Able To Import And Translate Different Protocol Routes Into OSPF Through A Process Known As Redistribution.
ASBRs PROVIDE ACCESS TO EXTERNAL NETWORKS. OSPF DEFINES TWO “TYPES” OF EXTERNAL ROUTES: ◙ Type 2 (E2) – > Includes Only The External Cost To The Destination Network. External Cost Is The Metric Being Advertised From Outside The OSPF Domain. This Is The Default Type Assigned To External Routes.
◙ Type 1 (E1) – > Includes Both The External Cost, And The Internal Cost To Reach The ASBR, To Determine The Total Metric To Reach The Destination Network. Type 1 Routes Are Always Preferred Over Type 2 Routes To The Same Destination.
PROCESS ID: The Process ID Is The ID Of The OSPF Process To Which The Interface Belongs. The Process ID Is Local To The Router, And Two OSPF Neighboring Routers Can Have Different OSPF Process IDs. (This Is Not True Of Enhanced Interior Gateway Routing Protocol [EIGRP], In Which The Routers Need To Be In The Same Autonomous System). Cisco IOS® Software Can Run Multiple OSPF Processes On The Same Router, And The Process ID Merely Distinguishes One Process From The Another. The Process ID Should Be A Positive Integer. In This Example, The Process ID Is 1.
ROUTER ID: The OSPF Router ID Is Used To Uniquely Identify Each Router In The OSPF Routing Domain. It's Simply An IP Address. It's Derived Based On Three Criteria With The Following Precedence: The OSPF Router ID Is A 32−Bit IP Address Selected At The Start Of The OSPF Process. The Highest IP Address Configured On The Router Is The Router ID. If A Loopback Address Is Configured, It Is The Router ID. In The Case Of Multiple Loopback Addresses, The Highest Loopback Address Is The Router ID. Once The Router ID Is Elected, It Does Not Change Unless OSPF Restarts Or Is Manually Changed With The Router−ID 32−Bit−IP−Address Command Under Router OSPF Process−ID.
◙ The Router ID Can Be Manually Specified. Use The IP Address Configured With ODPF Router-ID Command.
◙ If Router-ID Is Not Configured, The Router Chooses The Higest IP Address Of Any Of Its Loopback Interfaces. It Is More Common To Find Routers Configured With A Loopback Interface Because Older Routers Do Not Have The Router-Id Command Available.
◙ If No Loopback Interfaces Are Configured, The Router Chooses The Highest IP Address Of Any Of Its Physical Interfaces. This Does Not Have To Be An OSPF-Enabled Interface, But It Must Be In The Up State.
The Router ID Is Showed Under The “Show IP Protocols” Command. The Router ID Must Be Unique, Otherwise, Chaos Will Reign. It's Kinda Like Dividing By 0.
OSPF NEIGHBOURS AND ADJACENCY: ◙ ADJACENCY: When Two Routers Have Exchanged Routing Information And Have The Same Topology Table.
◙ NEIGHBOURS: Are Routers That Are Directly Connected (Share A Subnet). OSPF Forms Neighbor Relationships, Called Adjacencies, With Other Routers In The Same Area By Exchanging Hello Packets To Multicast Address 224.0.0.5. Only After An Adjacency Is Formed Can Routers Share Routing Information.
After Two-Way Communication Between Neighbors Is Established, OSPF Routers Move On To The Next Step, Which Is Building Adjacencies. Adjacent Routers Are Routers That Go Beyond The Hello Protocol Exchange And Proceed Into The Database Exchange Process. Not All Neighboring Routers Become Adjacent. Whether Or Not An Adjacency Is Formed Depends On The Type Of Network To Which The Neighboring Routers Are Connected.
OSPF Neighbors Are Identified By Their Router IDs. A Router ID Is An IP Address By Which The Router Is Uniquely Identified Within The OSPF Domain. A Cisco Router Selects Its Router ID As The Highest IP Address On Any Loopback Interfaces Configured On The Router. If No Loopback Interfaces Are Configured On The Router, The Router Chooses The Highest IP Address Of Any Of Its Physical Interfaces.
TWO ROUTERS WILL BECOME NEIGHBORS IF THE FOLLOWING PARAMETERS ARE AGREED: ◙ AREA ID: The Two Routers Sharing A Common Network Segment Must Have Their Interfaces Configured To Be In The Same Area.
◙ AUTHENTICATION: OSPF Allows For Configuration Of A Password For A Specified Area. Routers That Want To Become Neighbors Must Exchange The Same Password Over The Common Segment.
◙ HELLO AND DEAD INTERVALS: The Hello Interval Is The Amount Of Time Between Hello Packets That A Router Sends Out On An OSPF-Enabled Interface. The Dead Interval Is The Amount Of Time, In Seconds, That A Router Will Wait For A Hello Packet From A Neighbor Before Declaring The Neighbor Down. These Interval Times Are Included In The Hello Packet And Must Be Agreed Upon By Neighbors.
◙ STUB AREA FLAG: Two Neighboring Routers Must Also Agree On The Stub Area Flag In The Hello Packets In Order To Become Neighbors. (Stub Areas Will Also Be Discussed Later).
Routers That Share A Common Segment May Become Neighbors On That Segment. Neighbors Are Discovered Via The OSPF Hello Protocol And Are Recorded In A Neighbor Table.
◙ HELLO PACKETS: Are Sent Every 10 Seconds On Multicast Address 224.0.0.5
◙ HELLO PACKETS: Are Sent Every 30 Seconds On NBMA Networks Via Unicast.
FORMING OSPF HELLO
◙ Acts As A Keepalive Between Neighbors.
◙ Ensures Bi-Directional Communication Between Neighbors.
◙ Is Used For Designated Router (DR) And Backup Designated Router (BDR) Election On Certain Types Of Networks.
Hello Packets Are Sent Out All OSPF-Enabled Interfaces. They Are Sent Out Periodically With A Special Multicast Address As The Destination. Routers Will Become Neighbors When They See Themselves (Their Own Router ID) In Their Neighbor's Hello Packets And They Agree Upon Certain Parameters Included In The Hello Packets. Neighbor Negotiation Will Take Place On The Primary IP Address Only, Not Over Secondary Addresses. If Secondary Addresses Are Configured On The Interface, They Are Restricted To Be In The Same OSPF Area As The Primary Address.
INCLUDED IN HELLO PACKETS ARE: ◙ Router ID - > A 32-Bit Number That Is Unique To This Router.
◙ Hello And Dead Interval - > Period Of Time Between Hellos And The Timeout.
◙ Neighbor List - > List Of Neighbor Router Ids. Area ID - > Area Number.
◙ Priority - > Highest Priority Is Elected The Designated Router.
◙ DR And BDR - > IP Address Of Designated Routers.
◙ Authentication - > Password, If Enabled.
◙ Stub Area Flag - > TRUE If This Is A Stub Area.
Router IDs Of The Originating Router Neighbors. By Default, Hello Packets Are Sent Out OSPF-Enabled Interfaces Every 10 Seconds For Broadcast And Point-To-Point Interfaces, And 30 Seconds For Nonbroadcast And Point-To-Multipoint Interfaces.
OSPF Also Has A Dead Interval, Which Indicates How Long A Router Will Wait Without Hearing Any Hellos Before Announcing A Neighbor As “Down.” Default For The Dead Interval Is 40 Seconds For Broadcast And Point-To-Point Interfaces, And 120 Seconds For Non-Broadcast And Point-To-Multipoint Interfaces. By Default, The Dead Interval Timer Is Four Times The Hello Interval.
The Hello Packets Also Serve As Keepalives To Allow Routers To Quickly Discover If A Neighbor Is Down. Hello Packets Also Contain A Neighbor Field That Lists The Router Ids Of All Neighbors The Router Is Connected To.
◙ DEAD INTERVAL: 40 Seconds ( 4 Hello’s) On Multicast And 120 ( 4 Hello’s) On NBMA
◙ NEIGHBOUR TABLE: A List Of Neighbours That Have Been Found Via Hello Multicasts, The Hello Packet Also Has A List Of All Known Neighbours From The Router That Sent It.
◙ COST: Metric Used By OSPF, Higher The Speed, Lower The Cost.
◙ DATABASE DESCRIPTOR PACKET(DDP): Used During The Exchange Of Topology Information During The Forming Of An Adjacency, They Are A Cut Down LSA (Link State Advertisement)
◙ INIT STATE: A State When Hello Packets Are Being Sent By The Router And It Is Waiting For A Reply So It Can Start To Forum A Relationship.
◙ LINK-STATE DATABASE: Also Known As Topology Map, Contains The Information About All Know Links And All Known Networks.
◙ LINK STATE REQUEST (LSR): When A Router Get A Database Descriptor That Doesn’t Match Any Of Its Known Links It Will Send A Link State Request, To Ask For A Full LSA Of That Link To Be Sent.
◙ LINK STATE UPDATE (LSU): Sent In Response To The LSR.
◙ LOADING STATE: The State Where LSR’s And LSU’s Are Sent And Received Is Known As The Loading State.
LINK STATE ADVERTISEMENT ( LSA): The Protocol Uses LINK-STATE ADVERTISEMENTS (LSA) To Update Neighboring Routers Regarding Its Interfaces And Information On Those Interfaces. The Routing Switch Floods These LSAs To All Neighboring Routers To Update Them Regarding The Interfaces.
OSPF Routers Keep Track Of The Status Of Links Within Their Respective AREAs. A Link Is Simply A Router Interface. From These Lists Of Links And Their Respective Statuses, The Topology Database Is Created. OSPF Routers Forward Link-State Advertisements (LSAs) To Ensure The Topology Database Is Consistent On Each Router Within An Area.
Sent When Something In The Topology Changes ( Link Up/Link Down) There Are Many Types Of LSA And They Must Be Acknowledged By The Receiving Router (Hello Is Acknowledged By Been Seen As A Neighbour In That Neighbours Hello Packets: 1 Hello
2 Database Description
3 Link State Request
4 Link State Update
5 Link State Acknowledgment
LSAs MAY INCLUDE TWO TYPES OF GENERIC INFORMATION: ◙ Router Link Information Advertises A Router's Adjacent Neighbors With A Triple Of (Router ID, Neighbor ID, Cost), Where Cost Is The Cost Of The Link To The Neighbor.
◙ Stub Network Information Advertises A Router's Directly Connected Stub Networks (Networks With No Neighbors) With A Triple Of (Router ID, Network ID, Cost).
The Important Information For The Shortest Path Determination Process Is The Advertising Router's ID, Its Attached Networks And Neighboring Routers, And The Cost Associated With Those Networks Or Neighbors.
◙ - ➤ For More About - > OSPF LSA Operation And It’s Types:
◙ RXMTINTERVAL: How Long A Router Will Wait To Retransmit A Packet That Hasn’t Been Acknowledged, By Default 5 Seconds.◙ EQUAL COST LOAD BALANCING: Up To 6 Links.
◙ TIMED UPDATES: OSPF Will Send DDPS Of All Its Routes To Its Neighbours Once Every 30mins If There Have Been No Changes.
◙ MAXAGE TIMER: How Long An LSA Can Stay In The Topology Table Before Being Aged Out Unless Refreshed Default 3600 Seconds.
◙ LS-REFRESH TIMER: How Long Is Waited Until An LSA Refreshed, Makes The Maxage Timer Reset, Default 1800.
◙ LSA-GROUP-PACING: 240 Seconds , Use To Hold Back LSA’s From Refreshing So More Can Be Sent As Once. So Refreshing LSU’s Are Sent Every 240 SECS Containing Many LSA.
◙ POLL INTERNAL: On NBMA(Neighbour Statement)How Long The Interval Between Hellos When A Neighbour Down.
◙ FLOOD PACING: Min Interval Between Flooding An LSA , Default 33ms.
◙ RETRANSMISSION PACING: Min Interval Between Sending A Retransmission.
◙ BACK UP DESIGNATED ROUTER(BDR): A Backup Dr (Designated Router) It Does None Of The DR Roles While The DR Is Active.
◙ DESIGNATED ROUTER (DR): A Router That Is Responsible For Forming Adjancies With All Routers On A LAN Segment, If More Then One Router Is Connected To A Segment The There Must Be A Dr Router Election, The Election Can Be Controlled By Used A Priority Command On A Per Interface Basis. Default Priority Is 1. If The Priority Command Isn’t Used And The Election Isn’t Controlled Then The Default Is To Use High Router ID ( IP Address).
224.0.0.6, Is The Destination IP Address Used By Non DR/BDR Routers When Sending LSA’s ( LSU,LSR,ACK ) Known As ALLDROUTERS (Note: Hello’s Still Via .5). 224.0.0.5 All Packets Originating From The DR/BDR Routers Have This Destination Known As ALLSPFROUTERS.
◙ DROTHER: A Normal Router, Neither DR Or BDR
◙ PRIORITY: Thing, That Allows For Control Of The DR/BDR Election From 0 To 255 (1 Is Default, 0 Cant Be Elected DR Or BDR). The Neighbour With The Highest Priority Becomes The BDR, If There Is No DR Then The BDR Is Promoted To DR And The First Step Repeats For The BDR.
OSPF ROUTING TABLES ARE BUILT IN TWO DIFFERENT WAYS: Either Established Databases Have To Adjust To A Change In The Network, Or A New Router Has To Create The Topology And Forwarding Databases When It Enters The Network.
Different Techniques Are Used For These Different Routing Table Requirements. Essentially, THE DIFFERENCE BETWEEN THE TWO TECHNIQUES IS SIMPLE:
◙ If A New Router Connects To A Network, It Will Find A Neighbour Using The Hello Protocol And Will Exchange Routing Information.
◙ If A Change Occurs In An Existing Network, The Router That Sees The Change Will Flood The Area With The New Routing Information.
Both Of These Events Must Occur As Stated Because, Although The New Router Must Learn The Network Topology, Its Addition Is A Change To The Rest Of The Network.
OSPF NETWORK TYPES
OSPF IDENTIFIES FIVE DISTINCT NETWORK TYPES OR TECHNOLOGIES: ◙ BROADCAST MULTI-ACCESS - > Uses Multicast Hello’s.
◙ POINT-TO-POINT - > Uses Multicast Hello But There Is No DR/BDR. (For Example, The Interfaces Of Two Routers Connected Through E1 Or T1 Links).
◙ POINT-TO-MULTIPOINT - > Treats Each Connection To A Router As A Point To Point Circuit And Forwards Hello For Each Circuit Individually. (Such As Frame Relay).
◙ NONBROADCAST MULTIACCESS (NBMA) - > Requires Manual Configuration , Will Use Unicast For Hello. (Such As X.25 And Frame Relay).
◙ VIRTUAL LINKS - > Are Links To An Area That Doesn’t Connect To The Backbone( Area 0) And Are Sent Via Unicast.
NETWORK TYPES: POINT-TO-POINT NETWORKS: Such As Serial Lines, Connect A Single Pair Of Routers.
◙ OSPF Will Always Form An Adjacency With The Neighbor On The Other Side Of A Point-To-Point Interface.
◙ There Is No Concept Of DR Or BDR On Point-To-Point Networks.
◙ The Destination Address Of OSPF Packets On These Networks Will Always Be Sent To 224.0.0.5, Otherwise Known As The All SPF Routers Multicast Address.
BROADCAST NETWORKS: Such As Ethernet, Token Ring, And Fiber Distributed Data Interface (FDDI), Are Multi-Access, Meaning They Are Able To Connect More Than Two Devices; A Packet Sent By One Router Will Be Received By All Connected Routers.
◙ On Broadcast Networks, OSPF Will Elect A DR And A BDR.
◙ Hello Packets On Broadcast Networks Are Sent To The Destination Address Of 224.0.0.5.
All Packet Originated By The DR And BDR Are Also Sent To The This Address. All Other Non-DR And Non-BDR Routers Will Send Link-State Updates To The Address 224.0.0.6, Also Known As All DROUTERS.
NON-BROADCAST MULTI-ACCESS NETWORKS (NBMA): Such As Frame Relay, ATM, And X.25, Can Connect Multiple Devices, But They Have No Broadcast Capability.
◙ A Packet Sent By A Router Will Not Be Received By All The Other Routers Attached To The Network.
◙ Special Care Should Be Taken When Configuring OSPF Over NBMA Networks. OSPF Considers These Media To Be Just Like Any Other Broadcast Media Such As Ethernet Or Token Ring. As A Result, Extra Configuration May Be Required For NBMA Networks.
◙ OSPF Routers Elect A DR And BDR, And All OSPF Packets Are Unicast.
POINT-TO-MULTIPOINT NETWORKS: Are NBMA Networks In Which The Networks Are Treated As A Collection Of Point-To-Point Links.
◙ Routers On These Networks Do Not Elect A DR And BDR Because The Network Is Seen As Point-To-Point Links. OSPF Packets Are Multicast On These Networks.
FORMING OSPF NEIGHBOUR RELATIONSHIPS
◙ 2. THE INIT STATE: The New Router Waits For A Reply. Typically This Is Four Times The Length Of The Hello Timer. The Router Is In The Init State. Within The Wait Time, The New Router Hears A Hello From Another Router And Learns The Dr And The BDR. If There Is No DR Or BDR Stated In The Incoming Hello, An Election Takes Place. However, In Accordance With The Description Of The Hello Protocol, The DR Has Been Elected: It Is The 7200 Router, Which Connects The Campus To The Campus Backbone. Upon Hearing The Hello Protocol From The 2500 Router, A Router On The Segment Adds The Router Id Of The 2500 And Replies As A Multicast (224.0.0.5) With Its Own Id And A List Of Any Other Neighbors.
◙ 3. THE TWO-WAY STATE: The New Router Sees Its Own Router Id In The List Of Neighbors, And A Neighbor Relationship Is Established. The New Router Changes Its Status To The Two-Way State.
OSPF EXCHANGING ROUTING INFORMATION WITH A NEIGHBOUR
The Exchange State Both Routers Will Send Out Database Description Packets, Changing The State To The Exchange State. Each Link Will Have An Interface ID For The Outgoing Interface, A Link ID, And A Metric To State The Value Of The Path. The Database Description Packet Will Not Contain All The Necessary Information, But Just A Summary (Enough For The Receiving Router To Determine Whether More Information Is Required Or Whether It Already Contains That Entry In Its Database).
◙ 5. THE LOADING STATE: The LSR Will Prompt The Master Router To Send The LSU Packet. For Example, If There Is A Discrepancy Between The Information In The Received DDPs And The Router’s Topology Database, The Router Requests More Detailed Information From Its Neighbor About Those Routes Of Which It Was Unaware.
◙ 6.THE FULL STATE: When These LSRs Are Received And The Databases Are Updated And Synchronized, The Neighbors Are FULLY ADJACENT.
ACKNOWLEDGEMENTS OF LSA’s
IMPLICIT ACKNOWLEDGMENT
A Neighbor Can Implicitly Acknowledge The Receipt Of An LSA By Including A Duplicate Of The LSA In An Update Back To The Originator. Implicit Acknowledgments Are More Efficient Than Explicit Acknowledgments In Some Situations, For Instance, When The Neighbor Was Intending To Send An Update To The Originator Anyway.
◙ EXPLICIT ACKNOWLEDGMENT: A Neighbor Explicitly Acknowledges The Receipt Of An LSA By Sending A Link State Acknowledgment Packet. A Single Link State Acknowledgment Packet Is Capable Of Acknowledging Multiple LSAs. The Packet Carries Only LSA Headers—Enough To Completely Identify The LSA—Not The Complete LSA. When A Router First Sends An LSA, A Copy Of The LSA Is Entered Into The Link State Retransmission List Of Every Neighbor To Which It Was Sent. The LSA Is Retransmitted Every RXMTINTERVAL Until It Is Acknowledged Or Until The Adjacency Is Broken. The LINK STATE UPDATE PACKETS CONTAINING RETRANSMISSIONS ARE ALWAYS UNICAST, Regardless Of The Network Type.
IF OSPF ROUTE IS LOST
If An LSA Is Received From Either An Adjacent Or DR Router It Is Flooded Out All OSPF Interfaces.
IF NEW OSPF ROUTE IS FOUND
◙ 2. The Router Verifies That The Type Of LSA Is One That Can Be Accepted By This Router.
◙ 3. Having Ascertained That It Is A Valid LSA Which It Can Receive, The Router Issues A Lookup To Its Topological Database.
◙ 4. If The LSA Entry Is Not In The Topological Database, It Is Flooded Immediately Out All The OSPF Interfaces, Except For The Receiving Interface.
◙ 5. If The LSA Entry Is In The Topological Database, Further Questions Are Required.
◙ 6. The Router Determines Whether The New LSA Has A More Recent (Higher) Sequence Number.
◙ 7. If The Sequence Numbers Are The Same, The Router Calculates The Checksum For The LSAs And Uses The LSA With The Higher Checksum.
◙ 8. If The Checksum Numbers Are The Same, The Router Checks The Maxage Field To Ascertain Which Is The Most Recent Update.
◙ 9. Having Found That The Latest LSU Is The One That Was Received, The Router Determines Whether It Has Arrived Outside The Wait Period, Before Another Computation Is Allowed (Minslsarrival).
◙ 10. If The New LSA Entry Passes These Tests, It Is Flooded Out All The OSPF Interfaces, Except For The Receiving Interface.
◙ 11. The Current Copy Replaces The Old LSA Entry. If There Was No Entry, The Current Copy Is Just Placed In The Database.
◙ 12. The Received LSA Is Acknowledged.
◙ 13. If The LSA Entry Was In The Database, But The LSA That Has Just Been Received Has An Older Sequence Number, The Router Asks Whether The Information In The Database Is The Same.
◙ 14. If The Information Is The Same And The New LSA Has An Older Sequence Number, The Process Discards The Packet. It Might Be Old News, But There Is No Inconsistency In The Database.
◙ 15. If The Information Is Different And The Newly Received LSA Has An Older Sequence Number, However, The Receiving Router Discards The LSA Update. It Issues A Copy Of The LSA It Has In Its Database, Sending It Out Of The Receiving Interface To The Source Address Of The Out-Of-Date LSA.
The Logic Is That The Sending Router Has Bad Or Old Information And Must Be Updated Because Its Topological Database Is Obviously Not Synchronized With The Rest Of The Area. OSPF Operation In A Single Area; This Ensures That Any Packets That Get Out Of Sequence Will Be Verified Before Action Is Taken. It Also Attempts To Rectify A Problem That It Sees—That Of Multiple Routers Offering Different Paths Because Their Topological Databases Are Completely Confused.
◙ 16. After The Initial Flood, Things Calm Down, And Updates Are Sent Only When There Are Changes In The Area Or When The 30-Minute Timer Goes Off. This Timer Ensures That The Databases Stay Synchronized.
OSPF DESIGNATED ROUTER (DR) AND BACKUP ROUTER (BDR)
The Election Process Works Pretty Much The Same For Both OSPFv2 (For IPv4) And OSPFv3 (For IPv6). At Least One Routing Device On Each Logical IP Network Or Subnet Must Be Eligible To Be The Designated Router For OSPFV2. At Least One Routing Device On Each Logical Link Must Be Eligible To Be The Designated Router For OSPFv3.
DESIGNATED ROUTER (DR) And BACKUP DESIGNATED ROUTER (BDR) Elections On A Multi-Access Segment Of OSPF Routers. On Point-To-Point OSPF Segment Links A DR And BDR Are Not Elected Since Only Two Routers Are Directly Connected. Remember That The DR And BDR Concepts Are Per OSPF Multiaccess Segment.
Every Router On A Network Segment Establish A Full Neighbor Relationship With The DR And BDR. Non-DR And Non-BDR Routers Establish A Two Way Neighbor Relationship Between Themselves.
◙ On Point-To-Point Links,Adjacencies Are Established With All Neighbors, Because There Is Only One Neighbor (In A Point-To-Point Network , DR/BDR Election Doesn’t Happen).
◙ On Multi-Access Networks, OSPF Elects A DR And BDR To Limit The Number Of Adjacencies, Using OSPF Hello Packets.
In Multi-Access Networks Such As Ethernet, There Is The Possibility Of Many Neighbor Relationships On The Same Physical Segment. In OSPF A Router Can Have A Number Of Different Roles To Play.
If Two Routers On The Same Network Segment Can Learn Routing Information Through The Establishment Of The Adjacency Relation. If Too Many Routers Are On The Same Network Segment, And Adjacency Relations Are Established Between Every Two Routers, Adjacency Relations Are Accumulated Exponentially. As A Result, Protocol Packets On The Network Occupy Mass Bandwidths.
REDUCE ROUTING UPDATE TRAFFIC: ◙ The Formation Of An Adjacency Between Every Attached Router Would Create Many Unnecessary LSA (Link State Advertisements). Using The Following Formula n(n-1)/2 For Adjacencies. “n” Is The Number Of Routers
◙ A Router Would Flood An LSA To All Its Adjacent Neighbors, Which In Turn Would Flood It To All Their Adjacent Neighbors, And So On, Creating Many Copies Of The Same LSA On The Same Network.
TO PREVENT THIS PROBLEM, A Designated Router (DR) Is Elected On Multi-Access Networks, The DR And BDR Are Established For OSPF. On The Networks Of The Broadcast Or NBMA Type, All Routers Establish Adjacency Relations With Only The DR And BDR, And Routing Information Is Exchanged. Then The DR Complies The Link Status Information Of The Network Segment And Spreads The Information To All Routers On The Network Segment. The BDR Serves As The Backup Of The DR. When The Previous DR Cannot Work Normally, The BDR Becomes The New DR On The Network Segment.
DESIGNATED ROUTER (DR): The Designated Broadcasting Router Interface For An AS. It Looks After All The Initial Contact And Other Routing Administration Traffic. Having Only One Router Do All This Greatly Reduces The Network Traffic And Collisions.
If Something Happens And The Designated Router Goes Offline, The Backup Designated Router (BDR) Takes Over. An OSPF Fortigate Unit Interface Can Become Either A DR Or BDR. Both The DR And The BDR Cover The Same Area, And Are Elected At The Same Time.
BACKUP DESIGNATED ROUTER (BDR): ◙ Listens, But Does Not Act.
◙ If An LSA Is Sent, BDR Sets A Wait Timer.
◙ If Timer Expires Before It Sees The Reply From The DR, It Becomes The DR And Takes Over The Update Process.
◙ The Process For A New BDR Begins.
◙ Once A DR Is Established, A New Router That Enters The Network With A Higher Priority Or Router ID Will Not Become The DR Or BDR.
◙ If The DR Fails, The BDR Takes Over As The DR And Selection Process For New BDR Begins.
An Election Chooses The DR And BDR From All The Available Routers. The Election Is Primarily Based On The Priority Setting Of The Routers—The Highest Priority Becomes The DR, And The Second Highest Becomes BDR. To Resolve Any Ties, The Router With The Highest Router ID Wins.
DR And BDR Election Is Done Via The Hello Protocol. Hello Packets Are Exchanged Via IP Multicast Packets On Each Segment. The Router With The Highest OSPF Priority On A Segment Will Become The DR For That Segment. The Same Process Is Repeated For The BDR. OSPF Router Can Elect One Router To Be A Designated Router (DR) And One Router To Be A Backup Designated Router (BDR).
OSPF Routers Will Form Adjacencies With The DR And BDR. If A Change Occurs To A Link, The Update Is Forwarded Only To The DR, Which Then Forwards It To All Other Routers. This Greatly Reduces The Flooding Of LSAs.
DR And BDR Elections Are Determined By A Router’s OSPF Priority, Which Is Configured On A Per-Interface Basis (A Router Can Have Interfaces In Multiple Multi-Access Networks). The Router With The Highest Priority Becomes The DR; Second Highest Becomes The BDR. If There Is A Tie In Priority, Whichever Router Has The Highest Router ID Will Become The DR.
The Router Priority Can Vary From 0 To 255, But At 0 A Router Will Never Become A DR Or BDR. If A Router With A Higher Priority Comes On Line After The Election, It Must Wait Until After The DR And BDR Go Offline Before It Would Become The DR.
The Default For The Interface OSPF Priority Is One. Remember That The DR And BDR Concepts Are Per Multiaccess Segment. Setting The OSPF Priority On An Interface Is Done Using The IP OSPF Priority
A Priority Value Of Zero Indicates An Interface Which Is Not To Be Elected As DR Or BDR. The State Of The Interface With Priority Zero Will Be DROTHER.
Router(Config-If)# IP OSPF PRIORITY <0-255> Note: The RID Is Determined By The Highest IP Address On Any Interface At The Moment Of OSPF Startup. This Can Be Overridden With A Loopback (Logical) Interface. If You Set A Routers Interface To A Priority Value Of Zero, That Router Won’t Participate In The DR Or BDR Election On That Interface. The State Of The Interface With Priority Zero Will Then Be DROTHER.
If The Original DR Goes Offline, But Then Is Available When The BDR Goes Offline Later On, The Original DR Will Be Promoted Back To DR Without An Election Leaving The New BDR As It Is.
If Two Neighbors Share The Same Priority, The Router With The Highest Router ID Is Designated As The DR. The Router With The Next Highest Router ID Is Designated As The BDR.
OSPF USES THE ROUTER IDENTIFIER FOR TWO MAIN PURPOSES: ◙ 1. To Elect A Designated Router, Unless You Manually Specify A Priority Value, And To Identify The Routing Device From Which A Packet Is Originated. At Designated Router Election, The Router Priorities Are Evaluated First, And The Routing Device With The Highest Priority Is Elected Designated Router.
◙ 2. If Router Priorities Tie, The Routing Device With The Highest Router Identifier, Which Is Typically The Routing Device�s IP Address, Is Chosen As The Designated Router. If You Do Not Configure A Router Identifier, The IP Address Of The First Interface To Come Online Is Used. This Is Usually The Loopback Interface. Otherwise, The First Hardware Interface With An IP Address Is Used.
EXAMPLE - 1: On OSPF Multiaccess Segment, We Wanted R1 To Become The DR And R2 To Become The BDR.
R1(config)# Interface f0/0
R1(config-if)# IPv6 OSPF Priority 100
R2(config)# Interface f0/0
R2(config-if)# IPv6 OSPF Priority 90
DRs Cannot Be Preempted By Routers With A Higher Priority Once They Have Been Elected, So We'll Need To Disconnect All Routers From The Network In Order To Force A New Election. When The Interfaces Come Back Online, We See That R1 And R2 Have Been Elected As The DR And BDR, Respectively.
YOU WANT TO CHANG DR OR BDR AFTER YOU CHANGE THE PRIORITY, YOU CAN DO LIKE THIS: Router1#Clear IP OSPF Process
Reset ALL OSPF Processes? [No]: Yes
Router2#Clear IP OSPF Process
Reset ALL OSPF Processes? [No]: Yes
TO VERIFY: Which Neighbors Are The OSPD DR/BDR/DROTHER By Using The “Show IP OSPF Neighbor” Command In Privileged Mode.
Router#Show IP OSPF Neighbor
The “SHOW IP OSPF INTERFACE” Command. This Command Shows Important Information Such As The DR, The BDR, A List Of Neighbors, And The Network Type.
Once You Are Certain The Interfaces Are Properly Assigned And Operational, You Should Check The Status Of The Neighbor’s Adjacency By Using The Show OSPF Neighbor Command, You Can Use “SHOW IP OSPF NEIGHBOR” To Verify And Troubleshoot Neighbor Relationships.
THE FIELDS IN THIS OUTPUT REPRESENT: Address - > The Physical Interface Ip Address Of The Neighbor Is Displayed In This Column.
Interface - > This Column Shows The Ospf Interface That The Neighbor Is Reachable Across.
State - > The Current OSPF Adjacency State Is Displayed Here. The Possible State Values Are Discussed.
ID - > This Field Shows The Router ID Of The Neighbor. This Is Used With The PRI Field To Elect A DR Or BDR On A Broadcast Segment.
PRI - > The Router Priority Is Displayed In This Field. This Value Is Used With The ID Field To Elect A DR Or BDR On A Broadcast Or NBMA Segment.
Dead - > The Time Remaining Until The Ospf Neighbor Is Declared Unreachable Appears In This Column. Each Received Hello Packet Resets This Timer To The Router Dead Interval Value.
◙ - ➤ “SHOW IP OSPF INTERFACE”COMMAND: This Command Shows Important Information Such As The DR, The BDR, A List Of Neighbors, And The Network Type.
The First Troubleshooting Step Is Often To Determine The State Of The Local Router’s Interfaces. Each Configured OSPF Interface Must Be Operational Before Any Packets Are Sent. A Non-Operational Interface Means That No Neighbors Will Be Located, No Adjacencies Will Form, And The Link-State Database Won’t Be Populated.
The “SHOW IP OSPF INTERFACE” Command Provides Insight Into This Information: THE VARIOUS FIELDS IN THE COMMAND OUTPUT ARE: INTERFACE - > Configured Ospf Interfaces That Are Physically Present In The Router Are Displayed In This Column. Failure To Properly Enter A Logical Unit Value Results In The Interface Not Appearing In This Output.
STATE - > The Current State Of The Interface Is Displayed In This Column. Possible Values Include:
_ BDR— > The Local Router Is The Backup Designated Router.
_ DOWN—> The Interface Is Not Currently Operational.
_ DR— > The Local Router Is The Designated Router.
_ DROTHER—> The Local Router Is Neither The DR Nor The BDR.
_ PTTOPT—> This Is A Point-To-Point Interface.
Area - > This Field Displays The Current Area Id Assigned To The Interface.
DR ID - > The Router ID Of The Current Designated Router Is Displayed In This Column. Point-To Point Interfaces Use A Value Of 0.0.0.0.
BDR ID - > The Router Id Of The Current Backup Designated Router Is Displayed In This Column. Point-To-Point Interfaces Use A Value Of 0.0.0.0.
NBRS - > The Value In This Column Represents The Total Number Of OSPF Neighbors Discovered Across This Interface.
“SHOW IP OSPF INTERFACE BRIEF” Command - > (Optional) Displays Brief Overview Information For OSPF Interfaces, States, Addresses And Masks, And Areas On The Router.
This “Show IP OSPF Interface Brief” Command Gives A Brief Summary Of What Interface Is Currently Configured With OSPF On The Router, As Well As The IP Address And Subnet Mask Of That Interface.
◙ - ➤ “CLEAR IP OSPF NEIGHBOR” COMMAND: It May Be Necessary To Reset The Peer Session To A Neighbor. This May Occur If The Remote Router Is Malfunctioning Or If You Want To Refresh The Link-State Database With New Information.
This Is Accomplished With The “Clear OSPF Neighbor Neighbor-Address” Command. The Optional Neighbor-Address Switch Clears That Specific Neighbor.
Use The Clear IP OSPF Neighbor Command To Clear Neighbor Information From The Show IP OSPF Neighbor Command. Use The Instance-Tag Argument To Clear The Neighbor Details From One OSPF Instance. If You Do Not Use The Instance -Tag Argument, Cisco NX-OS Clears The Neighbor Details From All OSPF Instances. Use The Show IP OSPF Neighbor Command To Find The Neighbor ID. This Command Requires The LAN Base Services License.
This Example Shows How To Clear All OSPF Neighbor Details For Neighbor 192.0.2.1 For Instance Tag 201:
Switch# Clear IP OSPF 201 Neighbor 192.0.2.1
This Example Shows How To Clear All OSPF Neighbor Details For All OSPF Instances:
Switch# Clear IP OSPF Neighbor *
This Example Shows How To Clear All OSPF Neighbor Details For All Neighbors On Ethernet Interface 1/2 For OSPF Instance 202:
Switch# Clear IP OSPF 202 Neighbor Ethernet 1/2
EXAMPLE - 2: Router R1: IP Address Fa2/0 – 10.1.1.1 ,Router-ID 1.1.1.1
Router R2: IP Address Fa2/0 – 10.1.1.2,Router-ID 2.2.2.2
Router R3: IP Address Fa1/0 – 10.1.1.3 ,Router-ID 3.3.3.3
Router R1 Config:
Router R1(Config-If)# Interface Fastethernet2/0
Router R1(Config-If)# IP Address 10.1.1.1 255.255.255.0
Router R1(Config-If)# No Shut
Router R1(Config)# Router OSPF 1
Router R1(Config-Router)#Router-ID 1.1.1.1
Router R1(Config-Router)#Network 10.0.0 0.255.255.255 Area 0
Router R2 Config:
Router R2(Config)# Interface Fastethernet2/0
Router R2(Config-If)# IP Address 10.1.1.2 255.255.255.0
Router R2(Config)# IP OSPF 1 Area 0
Router R2(Config-If)# No Shut
Router R2(Config)# Router OSPF 1
Router R2(Config-Router)# Router-ID 2.2.2.2
R3 Config:
Router R3(Config)# Interface Fastethernet1/0
Router R3(Config-If)# IP Address 10.1.1.3 255.255.255.0
Router R3(Config-If)# IP OSPF 1 Area 0
Router R3(Config-If)# No Shut
Router R3(Config)# Router OSPF 1
Router R3(Config-Router)# Router-ID 3.3.3.3
If The Router R1 As A DR, Therefore Configure A Loopback Interface On Router R1. And Restart The OSPF Process By Using Command “Clear IP OSPF Process” On All Routers.
Router R1(Config)# Interface Loopback 1
Router R1(Config-If)# IP Address 10.1.1.5 255.255.255.0
Then Check Again The R1 Status By “Show IP OSPF Neighbor” Command. Now On RouterR2 Is BDR And R3 Is DR-Other.
The Purpose Of A DR Is To Be The One Router To Which All Other Routers Are Adjacent. Using A DR Reduces The Number Of Adjacencies That Consume Bandwidth And Processing To n - 1. Yes, Larger Networks Still Involve More Processing, But With A DR, The Adjacencies Scale With The Network.
Note: DRs Are Elected On Point-To-Point Ethernet Links, Which Are Very Common In Modern Networks. Because The DR And BDR Are Not Necessary On A Point-To-Point Ethernet Link, Some Current Design Guides Recommend Changing The Ethernet Interfaces To Point-To-Point Mode.
If A DR Fails, The BDR Is Promoted. The BDR Is Elected On The Basis Of Highest OSPF Priority And Ties Are Broken In Favor Of The Highest IP Address. The Default Priority Is 1 And A Priority Of 0 Prevents A Router From Being Elected. Priority Can Be Set From 0–255;
To Change The Priority From Its Default For A Particular Interface, Use The Following Command:
Router(Config-If)#IP OSPF Priority Number
To Rebooting, Clearing The OSPF Process Using The Privilege EXEC Mode Command ”Clear IP OSPF Process *” On The DR Will Force A DR/BDR Election.
SUMMARY OF OSPF - DR/BDR PROCESS
THE FOLLOWING CONDITIONS TO DETERMINE WHICH ROUTER WOULD BE THE DR: ◙ - ➤ Router With The Highest OSPF Priority Value Is Selected As The DR.
◙ - ➤ Router With The Second-Highest OSPF Priority Value Is The BDR.
◙ - ➤ The Default For The Interface OSPF Priority Is 1. In The Case Of A Tie, The Router ID Is Used. The Router With The Highest Router ID Becomes The DR. The Router ID Is Determined By The Highest IP Address On A Loopback Interface Or Any Physical Interface (The Loopback Interface Is Preferred).
◙ - ➤ A Router With A Priority Set To 0 Cannot Be Elected The DR Or BDR. It Also Does Not Participate In The DR Or BDR Election On That Interface.
◙ - ➤ A Router That Is Not Elected As The DR Or BDR Is Called A DROTHER.
◙ - ➤ Suppose A New Router With A Higher Priority Value Is Added To The Network, It Will Not Be Elected As The DR And BDR. If The Current DR Is Down, The BDR Becomes The DR And A New BDR Is Elected. If The BDR Is Out Of Service, A New BDR Is Elected.
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The “OSPF Designated Router (DR) And Backup Designated Router (BDR) Election Process 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: Mr. Premakumar Thevathasan - CCNA And CCNP (Routing & Switching), 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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