► Used To Calculate Best Paths To Destinations
► Based On Link State Database
► Each Router In OSPF Runs SPF Itself
► Transit – Backbone – Area 0
► Regular – User Access – All Other Areas
► All Regular Areas Must Connect To Area 0
► Areas Minimize Routing Tables
► Localize Impact Of Topology Changes
► Stop Detailed LSA Floods At Area Boundaries
► Functions As Default Route Or Default Path Out Of Area
► Separates LSA Flood Zones
► Connects Multiple Areas
► Multicast Address For DR/BDR 224.0.0.6
► All Other Routers On Network Form Adjacency With DR/BDR
► LSA’s Are Only Exchanged With These Two
► All Other Routers Called Drothers
► Elected By Highest Priority (Default Priority Is 1)
► IP OSPF Priority {#} Interface Command
► Changing DR/BDR Priority To Zero Takes Effect Immediately
► Changing Priority On DROTHER To Something Else Has No Effect Until Re-Election
► If Tie In Priority, Highest Router-ID (Active IP Interface) Is Used To Break Tie
► Any New Routers Brought Onto The Network Will Form Full Adjacency With DR/BDR And Two Way State With Other Neighbors.
► Changing Priority To Zero On A DR/BDR Will Take Effect Immediately.
ASBR:
► Connects The OSPF AS To An Outside AS
► Redistribution Point
►► Hello – Discover And Building Adjacency
► Hello Packets Are Sent Every 10 Seconds On Multi Access Networks
► Dead Interval Is 4 Times Hello Interval
►► Database Description – DBD/DDP – Checks For Db Synchronization
►► Link-State Request – Requests Specific Link-State Records
►► Link-State Update – Response To LSR
►► Link-State Ack – Acknowledges Other Packet Types
► Down
► Init State – Routers Multicast Initial Hello (224.0.0.5)
► Two Way – Routers Send Unicast Hellos Listing Neighbors
► Exstart – Master/Slave Relationship Established With DR/BDR
► Exchange – Ddps Are Exchanged
► Loading – Lsrs For Specific Networks
► Full – All Lsdbs Are Synchronized With DR/BDR. Routers Are Able To Route Traffic
COMMUNICATION :
► 224.0.0.5 Multicast To Everyone
► 224.0.0.6 Multicast To DR/BDR
► Lsacks Are Sent Unicast
► LSDB Summaries Are Multicast Every 30 Minutes
► Entries Have A Max Life Of 60 Minutes
► 32 Bit Sequence Numbers Are Used For Link-State Advertisements
► The Sequence Number Can Be Seen With The “Show Ip Ospf Database” Command
► Loopback Interfaces Are Recommended For Stability And RID
If RID Was Set With Loopback, A Router Reboot Is Required For The Router-Id Command To Take Effect
If RID Was Set With Router-Id Command, It Can Be Changed With “Clear Ip Ospf Process” Command
NETWORK TYPES
►► Point To Point
► Multicast 224.0.0.5 To Discover Neighbors
► No DR/BDR
► Ip Unnumbered Is Possible Over Point To Point Links
► 10/40 Hello/Dead Interval
►► Broadcast
► DR/BDR Are The Central Point Of Contact In The Network
►► Non Broadcast Multi-Access
► By Default, OSPF Cannot Form Neighbor Adjacencies
► DR/BDR Elections Become Crucial
►► Hub/Spoke, Not All Spoke Sites Can Communicate Directly
THREE TOPOLOGIES
►► Full Mesh – Costly, Requires Separate VC’s For Connectivity Between Each Site
►► Partial Mesh
►► Star – Hub/Spoke
MODES OF OPERATION
►► IP OSPF Network {Mode} Interface Command
►► Broadcast – Cisco Proprietary
► WAN Links Are Treated Like LAN Interfaces
► Multicast Hello For Discover
►► Full/Partial Mesh
NON BROADCAST – RFC COMPLIANT
► One IP Subnet For All Spokes
► Neighbors Are Manually Configured
► DR/BDR Is Also Manually Rigged/Configured To Insure Connectivity To DR
POINT TO MULTI-POINT – RFC Compliant
► Multicast Hello For Discover
► No DR/BDR (Requires Additional Lsas)
► Mesh/Star
POINT TO MULTI-POINT NON BROADCAST – Cisco Proprietary
► Used In Place Of P2MP Where Broadcasts And Multicast Are Disabled
► Neighbors Are Manually Configured
POINT TO POINT – CISCO PROPRIETARY
► Different IP Subnet On Each Interface
► No DR/BDR Elected Or Needed
► LAN Or WAN Interface
DEFAULT MODES:
► Point To Point FR – Point To Point Mode
► Multipoint FR (Subinterfaces) – Non Broadcast
► Main FR Interface – Non Broadcast
► Neighbor {Ip} Priority {# Default 0} Poll-Interval
{#}Cost {1-65535} Database-Filter All
►►► Router LSA – Type 1
►Intraarea LSA Generated By Every Router In The Area.
►Advertises Link States. The LSID = RID Of Originator
►►► Network LSA – Type 2
► Network LSA Generated For Multiaccess Networks
► Generated By DR – LSID = RID Of DR
►►► Summary LSA – Type 3
► Summary Advertisements Generated By ABR
► Summarizes Type 1 Lsas From One Area To Another
► Describes Routes To Area’s Networks (Aggregate Routes)
►►► LSID = Destination Network #
► Not Flooded To Stubby, Totally Stubby, Or Not So Stubby Areas
►Routes Are NOT Automatically Summarized
►►► Summary LSA – Type 4 ► Generated By ABR To Advertise The Presence Of An ASBR. ASBR Sends Type 1 With E-Bit Set To ID Itself
► Routes To ASBR
► LSID = RID Of Describe ASBR
► Not Flooded To Stubby, Totally Stubby, Or Not So Stubby Areas
► Routes Are NOT Automatically Summarized
►►► AS External LSA – Type 5
► Generated By Asbrs To Advertise External Networks And Autonomous Systems. LSID = External Network #
► Contains All Routes Separately, Unless Manually Summarized
► Abrs Pass Type 5 Lsas On To The Rest Of The AS
►►► Multicast OSPF LSA – Type 6
►►► NSSA External LSA – Type 7
► ASBR In A Stubby Area That Needs To Pass External Routes Back Into The AS. Stubby Area Needs To Be Reclassified As NSSA To Allow Those Routes.
► ABR Receives Type 7 Lsas And Forwards Them As Type 5 To The Rest Of The AS
►►► External LSA For BGP – Type 8
COST/METRIC :
►►► 100mbps/Link Speed = Cost
► Example 100mbps Interface Has Cost 1
► Unfortunately, 1000mbps Interface Also Has Cost 1
► Auto-Cost Reference-Bandwidth {Ref Bw} Interface Command To Change This Behavior
► Bandwidth {Value} Interface Command To Define Actual Bandwidth
► IP OSPF Cost {Value} Interface Command
ROUTE SUMMARIZATION :
► Occurs At ABRS, Relies On Contiguous IP Design
► Area # Range {Addr | Mask} Advertise/Not-Advertise Cost {#}
► Router Will Create A Summarized Route To Null 0
►►► ASBR Summarization
► Summary-Address {Addr | Mask } Tag Router Config Command
DEFAULT ROUTE :
► Default-Information Originate Always Metric {Value}
► Per Cisco, Default Metric Of 10.
► Standard Area
► Accepts Link Updates, Route Summaries, And External Routes
►►► STUB AREA:
► Blocks Type 5 LSAS. No Routes External To The AS. If Stubs Need To Connect To External AS, They Use Default Routes.
► Totally Stubby Area
►►► Blocks Type 3, 4, And 5 Lsas. No External AS, No Summary Routes. Uses Default Route For Everything Outside Of The Local Area.
►►► NOT SO STUBBY AREA
► Acts Like A Stub/Tsa But Allows ASBR Connected To It
► ASBR In An NSSA Generates Type 7 LSA
► ABR To NSSA Translates Type 7 To Type 5 Before Passing Along To The Rest Of The AS
► Routes From Type 7 LSA Show In Routing Table As O N1 Or O N2 (Type 2 Is Default)
►►► STUB/TSA/NSSA CONFIGURATION:
► Area # Stub Router Configuration Command On All Routers In Area
► Area # Stub No-Summary Router Configuration Command On ABR For TSA
► Area # Nssa No-Summary
►►► VIRTUAL LINKS:
►► Allow Extension Of Area 0 Through Another Area
►► Area # Virtual-Link RID Where RID Is The RID Of The Neighbor On The Other Side Of The VL
►► SHOW IP OSPF Virtual-Links
►►► HELPFUL COMMANDS:
►► SHOW IP OSPF NEIGHBOR
►► SHOW IP OSPF DATABASE
►► SHOW IP OSPF ADJ This Is ADJ, Not Adjacency!
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The “OSPF QUICK REFERENCE ". 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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