FIRST KNOW ABOUT OSPFv2 :
1. OSPF Stands For Open Shortest Path First. It Also Uses The SPF (Shortest Path First) Algorithm To Determine The Best Route To Its Neighbors.
2. OSPF Is A Standard Routing Protocol, As Defined By RFC2328 And RFC1247. This Means That OSPF Can Run On Just About Any Company's' Routers And OSPF Routes From A Non-Cisco Router Can Be Exchanged With Cisco Routers.
3. The Entire Network That OSPF Routes For Is Called An "Autonomous System", Or "AS".
4. All OSPF Routing Updates Must Traverse Area 0. Because Of This, You Should Carefully Design Your Network Before You Begin Deploying OSPF.
OSPFv2 - QUICK REFERENCE :
With OSPF, Every Router Has Its Own Unique "Picture" (Topology Map) Of The Network. Routers Use "HELLO" Packets To Periodically Check With Routers To Ensure They Are Still There. Every Router In OSPF Is Identified With A "Router ID". The Router ID Can Be Manually Entered Or OSPF Will Automatically Choose The IP Address With The Highest Number.
FEATURES OF OSPF :
• A Standards-Based Routing Protocol That Works Is Very Interoperable Between Different Vendors Routers And Firewalls.
• Supports Variable Length Subnet Masks (VLSM), Making It A Classless Routing Protocols.
• Authentication Of Routing Updates Are Supported.
• Route Redistribution Is Supported Between Different Routing Protocols • OSPF Works Well In Point To Point And Point To Multipoint, Broadcast Or Non-Broadcast Configurations.
• OSPF Also Offers A Number Of OSPF-Specific Features Such As Stub Areas, Virtual Links, And OSPF On Demand Circuits.
NOTE :The Process Number Is An Arbitrary Number. It Is Recommended That The Number Match On All Routers But It Is Not Required. The Process ID Number Does Not Have To Be The AS Number.
OSPF CONFIGURATION TASK
OSPF CONFIGURATION TASK :
OSPF Typically Requires Coordination Among Many Internal Routers, Area Border Routers (Routers Connected To Multiple Areas), And Autonomous System Boundary Routers. At A Minimum, OSPF-Based Routers Or Access Servers Can Be Configured With All Default Parameter Values, No Authentication, And Interfaces Assigned To Areas.
To Configure OSPF, Complete The Tasks In The Following Sections. Enabling OSPF Is Mandatory; The Other Tasks Are Optional, But Might Be Required For Your Application.
• ENABLE OSPF
• CONFIGURE OSPF INTERFACE PARAMETERS.
• CONFIGURE OSPF OVER DIFFERENT PHYSICAL NETWORKS.
• CONFIGURE OSPF AREA PARAMETERS.
• CONFIGURE OSPF NOT SO STUBBY AREA (NSSA) .
• CONFIGURE ROUTE SUMMARIZATION BETWEEN OSPF AREAS.
• CONFIGURE ROUTE SUMMARIZATION WHEN REDISTRIBUTING ROUTES INTO OSPF.
• CREATE VIRTUAL LINKS.
• GENERATE A DEFAULT ROUTE.
• CONFIGURE LOOKUP OF DNS NAMES.
• FORCE THE ROUTER ID CHOICE WITH A LOOPBACK INTERFACE.
• CONTROL DEFAULT METRICS.
• CONFIGURE OSPF ON SIMPLEX ETHERNET INTERFACES.
• CONFIGURE ROUTE CALCULATION TIMERS.
• CONFIGURE OSPF OVER ON DEMAND CIRCUITS.
• LOG NEIGHBOR CHANGES.
• MONITOR AND MAINTAIN OSPF.
For More About - > Configuring OSPF
OSPF CONFIGURATION EXAMPLE :
Interface Ethernet0/0
IP Address 1.1.1.1 255.255.255.0
Interface Serial1/2
IP Address 1.1.2.1 255.255.255.0
Interface Fddi2/0
IP Address 1.1.3.1 255.255.255.0
Router OSPF 1234
Network 1.1.1.0 0.0.0.255 Area 1
Network 1.1.2.0 0.0.0.255 Area 2
Network 1.1.3.0 0.0.0.255 Area 0
VERIFYING OSPF CONFIGURATION :
After Configured OSPF, Have To Check Its Status Displays A Summary Of The Commands Executed In This Scenario.
DEBUGGING COMMANDS :
• DEBUG IP OSPF EVENTS
• DEBUG IP PACKET
OSPF BACKBONE AREA
OSPF BACKBONE AREA :
How Each Area Connects To The Backbone Called Area 0, Or The Backbone Area. OSPF Must Have An Area 0 And All Other Areas Should Connect To This Area.
The OSPF Backbone Area Is Also Known As Area 0.The Backbone Acts As A Hub For Inter-Area Transit Traffic And The Distribution Of Routing Information Between Areas. Inter-Area Traffic Is Routed To The Backbone, Then Routed To The Destination Area, And Finally Routed To The Destination Host Within The Destination Area. Routers On The Backbone Also Advertise The Summarized Routes Within Their Areas To The Other Routers On The Backbone. These Summary Advertisements Are Flooded Into Area Routers. Therefore, Each Router In An Area Has A Routing Table That Reflects The Routes Available Within Its Area And The Routes Corresponding To The Summary Advertisements Of The ABRs Of The Other Areas In The AS.
AREA BORDER ROUTER (ABR):
Routers That Connect Other Areas To The Backbone Area Within An AS Are Called Area Border Routers (ABRs). Still At Least One Interface Of The ABR Must Be In Area 0.
A Router With Interfaces Connected To Different Areas. ABRs Have Multiple LSDBs, One For Each Attached Area.
OSPF Runs Inside An Autonomous System, But It Can Also Connect Multiple Autonomous Systems Together.
AS BOUNDARY ROUTER (ASBR):
The Router That Connects These External ASes Is Called An Autonomous System Border Router (ASBR).
A Router That Exchanges Routes With Sources Outside Of The OSPF AS. ASBRs Advertise External Routes Throughout The OSPF AS.
INTERNAL ROUTER :
A Router With All Interfaces Connected To The Same Area. Internal Routers Each Have A Single LSDB.
INTER-AREA ROUTING :
Routing Within An Area Is Performed By OSPF Routers Using The Least Cost Path To The Destination Network. Because Routes Within An Area Are Not Summarized, Each Router Has A Route To Each Network Within Its Area Or Areas.
ROUTING BETWEEN AREAS TAKES THE FOLLOWING COURSE :
1. Routers Within The Source Area Forward The Packet Along The Least Cost Path To The Nearest ABR.
2. Backbone Routers Forward The Packet Along The Least Cost Path To The Nearest ABR Connected To The Area Containing The IP Address Of The Destination Host.
3. Routers Within The Area Containing The IP Address Of The Destination Host Forward The Packet Along The Least Cost Path To The Destination Host.
OSPF - BASIC CONFIGURATION EXAMPLE
PROCESS-ID:
The Process Number Is An Arbitrary Number. It Is Recommended That The Number Match On All Routers But It Is Not Required. The Process Id Number Does Not Have To Be The As Number, Although Many People Confuse That It Is.
Router(Config)#Router OSPF Process-Id
Process-Id Is A Number Between 1 And 65535. Pick The Process-Id Which Is Not Being Used. To Determine Which Ids Are Being Used, Issue The Show Process Command.
Router#Show Ip Ospf.
Pick The Process-Id Which Is Not Being Used. To Determine What Ids Are Being Used, Issue The Show Process Command.
Router(Config)#Show Process
ROUTER ID:
Every Router In OSPF Is Identified With A "Router Id". The Router Id Can Be Manually Entered Or Ospf Will Automatically Choose The IP Address With The Highest Number.
ROUTER ID Is The Tie-Breaker For OSPF Path Selection. The Path Selection Process Uses A Variety Of Metrics To Select A Route. If All Other Metrics (Accessibility, Administrative Weight, Local Preference, Etc.) Are Equal, OSPF Determines The Router Id Using The Following Priority:
1. Use The Address Configured By The OSPF ROUTER-ID Command
2. Use The Address Of The Loopback 0 Interface
3. Use The Highest Ip Address Of Any Interface
4. If No Interface Exists, Set The ROUTER-ID To 0.0.0.0
If No OSPF ROUTER ID Is Explicitly Configured, OSPF Computes The Router-Id Based On The Items 2, 3, And 4 And Restarts OSPF (If The Process Is Enabled And Router-Id Has Changed). The OSPF ROUTER-ID Command Causes The Ospf Process To Restart Using The New Router-Id (If The Processes Are Enabled And Router-Id Has Changed).
Use In Router Configuration Mode OSPF ROUTER-ID IP-ADDRESS Command To Set The OSPF Router Id For The System. Use The NO OSPF ROUTER-ID To Configure The OSPF ROUTER ID As The Default Value (Address Of The Loopback 0 Interface). Factory Default Is Loopback 0 Interface Address.
Router(Config-Router)#Router Id 1.1.1.1
It Is Very Recommend Checking Router’s ROUTER ID By Giving The Next Command.
Router#Show IP OSPF
ADD THE NETWORK NUMBER, MASK AND AREA-ID :
Router(Config-Router)#Network Network-Number Mask Area AREA-ID
The Network-Number Identifies The Network Using OSPF. The Mask Tells Which Bits To Use From The Network-Number, And The AREA-ID Is Used For Determining Areas In An OSPF Configuration.
EXAMPLE:
Router(Config-Router)#Network 192.168.10.0 255.255.255.0 Area 0.0.0.0
Repeat This Step For All The Network Numbers. To Turn Off Ospf, Use The Following Command.
Router(Config)#No Router Ospf Process-Id
VIRTUAL LINK:
A Virtual link can be built up for network shown in Above Figure.
1. Defining Router IDs for the both ends.
2. Giving in router configuration mode the area x virtual-link A.B.C.D, where x is
AREA ID In The Both Routers. For Example In Router 1.1.1.1
Router(Config-Router)#Area 1 Virtual-Link 2.2.2.2
To Ensure A Virtual Link Is Up, Give The Next Command.
Router#Show Ip OSPF
CONFIGURE OSPF INTERFACE PARAMETERS :
You Are Not Required To Alter Any Of These Parameters, But Some Interface Parameters Must Be Consistent Across All Routers In An Attached Network.
CONFIGURE OSPF AUTHENTICATION :
Cisco Router Supports Message-Digest Hashing Algorithm To Guarantee The Integrity Of OSPF Packets And Authenticate Between The Routers. The Key Is Set On Interface By The Following Command.
Router(Config-If)#IP OSPF Message-Digest-Key Key-Id Md5
Encryption-Type Key
The KEY-ID Is An Identifier And Takes The Value In The Range Of 1 Through 255.The Encryption Type Field Refers To The Type Of Encryption, Where 0 Means None And 7 Means Proprietary. The Key Is An Alphanumeric Password Up To Sixteen Characters. Neighbor Routers Must Use The Same Key Identifier With The Same Key Value.
After The Key Is Configured, Authentication Must Be Enabled In Global Configuration Mode.
Router(Config)#Router OSPF Process-Id
Router(Config-Router)#Area Area-Id Authentication Messagedigest
To Turn Off Any Feature Of OSPF Routing, For Example To Turn Off Network 10.12.0.4 255.255.255.252 Area 0, Give The Next Command :
Router(Config-Router)#No Network 10.12.0.4 255.255.255.252 Area 0
To Turn Off RIP, Use The No Router Rip Command:
Router(config)#No Router OSPF Process-Id
RIP AND OSPF REDISTRIBUTION :
RIP:
If You Like To Redistribute Routing Information To OSPF, You Have To Tell RIP Running Router Process-Id Of OSPF And Default-Metric Value. For Example Router(Config-Router)#Redistribute OSPF Process-Id
And Default-Metric Value:
Router(Config-Router)#Default-Metric Value
Value Is A Positive Integer:
OSPF:
In OSPF You Have To Mention Any Default-Metric Value, Just Give The Next Command, If You Like To Transform Routing Information To RIP.
Router(Config-Router)#Redistribute Rip
In Addition In Classless Addressing, You Have To Write Subnets, Like
Router(Config-Router)#Redistribute Rip Subnets
GATEWAY :
When You Have A Network With Running A Routing Protocol And You Like To Connect It To Another Network Running B Routing Protocol Throughout One Or More Routers, You Have To Tell About Border Area Router Or Gateway Router With The Following Command. B Network Is Generally Much Bigger And For Example The Internet.
Router(Config-Router)#Default-Information Originate
LAN :
If A LAN Is Connected To A Router As Shown In Fig, You Have To Tell About It To The Router’s Routing Protocol. Just Write
Router(Config-Router)#Redistribute Connected
STEP BY STEP LAB GUIDANCE FOR OSPF POINT TO POINT EXAMPLE
OSPF POINT TO POINT EXAMPLE:
Router> - User Exec Mode
Router# - Privileged Exec Mode
Router(Config)# - Configuration Mode (Notice The # Sign Indicates This Is Only Accessible At Privileged Exec Mode.)
Router(Config-If)# - Interface Level Within Configuration Mode.
Router(Config-Router)# - Routing Engine Level Within Configuration Mode.
SAVING CONFIGURATIONS - ALWAYS ISSUE THIS COMMAND AFTER EACH CONFIGURATION :
Router# Copy Running-Config Startup-Config - Saves Configuration Into NVRAM
A Cisco IOS Router Stores Configurations In Two Locations - RAM And NVRAM. The Running Configuration Is Stored In RAM And Is Used By The Router During Operation. Any Configuration Changes To The Router Are Made To The Running-Configuration And Take Effect Immediately After The Command Is Entered.
The Startup-Configuration Is Saved In NVRAM And Is Loaded Into The Router's Running-Configuration When The Router Boots Up. If A Router Loses Power Or Is Reloaded, Changes To The Running Configuration Will Be Lost Unless They Are Saved To The Startup-Configuration.
To Save The Running-Configuration To The Startup Configuration, Type The Following From Privileged EXEC Mode (I.E. At The "Router#" Prompt.)
Router# Copy Running-Config Startup-Config
TO CHECK YOUR ROUTING TABLE :
Router#Show Ip Int Brief
Router#Clear Ip Route*
Router#Show Ip Route
Router#Show Run
VERIFYING OSPF CONFIGURATION :
After Configured OSPF, Have To Check Its Status.
Show IP OSPF - > Displays The OSPF Output
Show IP OSPF Border - >Display Border Routers Details
Show Ip OSPF Database – > Display Contents Of The Topology Database
Show IP OSPF Interface – > Display Interfaces Participating In OSPF
Show IP OSPF Neighbor – > Display Neighbor Information
Show IP OSPF Virtual - > Links
Show IP Protocols - > Displays All Active Routing Protocols
Show IP Route – > Display Routing Table Details
DEBUGGING COMMANDS :
• Debug IP OSPF Events
• Debug IP Packet
OSPF CONFIGURATION ON ROUTER A
Router>enable (Switches To Privileged EXEC Level)
Router#Configure Terminal (Switches To Global Configuration Level)
Router(Config)#Hostname Router A (Assign Host Name To Router A)
Router A(Config)#
Router(config)#Hostname Router A
Router A(config)#
CONFIGURATION ON ROUTER A - INTFACE E0
Router A(Config)#int e0 (Switches To Configure The E0 Interface)
Router A(Config-if)#IP address 10.0.0.1 255.0.0.0 (Configures An IP Address On Ethernet0 (Interface))
Router A(Config-if)#No shutdown (Activates Serial0 (Interface))
Router A(Config-if)#No Keepalive (Keepalive Packets Are Disabled On The Interface To Pass Through Other Interface)
Router A(Config-if)#Exit (Exits Back To Global Configuration Level)
KEEPALIVE :
A Keepalive Is A Message Sent By One Device To Another To Check That The Link Between The Two Is Operating, Or To Prevent This Link From Being Broken. No Keepalive Will Be Bringing Down The Interface Or Before Bringing The Tunnel Protocol Down For A Specific Interface.
NOTE :If You Enter The No Keepalive Command, Keepalive Packets Are Disabled On The Interface.
Router A(Config)#Exit
Router A# Copy Running-Config Startup-Config (Saves Configuration Into NVRAM)
CONFIGURATION ON ROUTER A - INTFACE S1
Router A#Conf T
Router A(Config)#Int s1 (Switches To Configure The Serial1 Interface)
Router A(Config-if)#Ip address 20.0.0.1 255.0.0.0
Router A(Config-if)#No shut (Enable An Interface)
Router A(Config-if)# Clock Rate 56000 (Set The Clock Rate 56000 For DCE Interface)
Router A(Config-if)# Bandwidth 64 (Set A Logical Bandwidth Assignment Of 64k To The Serial Interface)
To Enable The Back-To-Back Serial Connection Between Two Routers, You Need To Configure One Router As DCE Using The Following Command In Interface Configuration Mode For The Serial Connection On Two different Routers.
Router A(Config-if)#Exit
ROUTER - DTE / DCE :
In A Real-Life Network, Your Serial Interfaces Will Almost Certainly Be Configured As DTE Interfaces. Recall That A CSU/DSU Usually Handles The Clocking For A Synchronous Serial Interface.
The DCE-To-DTE Crossover Cable Will Have Two Different DB-60 Interfaces – One Marked DTE, And The Other Marked DCE. The Router Connected To The DCE End Of The Cable Will Need Its Serial Interface Configured As A DCE Device.
NOTE : If Your Device Is The DCE, You Must Provide Clocking Using The Clock Rate Command.
Example - > Router A(Config-if)# Clock Rate 56000
Router A(Config-if)#^Z
Router A#
Router A# Copy Running-Config Startup-Config
Router A#Show Run
Router A#Show IP Route
OSPF CONFIGURATION ON ROUTER B
Router>Enable (Switches To Privileged Exec Level)
Router#Configure Terminal (Switches To Global Configuration Level)
Router(Config)#Hostname Router B (Assign Host Name To Router B)
Router B(Config)#
CONFIGURATION ON ROUTER B - INTFACE S1
Router B(Config)#Int S0
Router B(Config-if)#IP Address 20.0.0.2 255.0.0.0
Router B(Config-if)#No shutdown
Router B(Config-if)# Bandwidth 64
Router B(Config-if)#Exit
Router B(Config)#Exit
Router B# Copy Running-Config Startup-Config
CONFIGURATION ON ROUTER B - INTFACE E0
Router B(Config)#Int e0
Router B(Config-if)#IP Address 30.0.0.1 255.0.0.0
Router B(Config-if)#No shutdown
Router B(Config-if)#No Keepalive
Router B(Config-if)#Exit
Router B(Config)#Exit
Router B# Copy Running-Config Startup-Config
Router B#Show Run
Router B#Show IP Route
OSPF CONFIGURATION ON ROUTER A
Router A(Config)#Router OSPF 17 - > (Indicate The Process Identification Number 17).
Router A(Config-Router)#Network 10.0.0.0 0.255.255.255 area 0 - > (10.0.0.0 Indicate The Network ID And 0.225.255.255 Wildcard Mask With Area 0).
Router A(Config-Router)#Network 20.0.0.0 0.255.255.255 area 0 - > (20.0.0.0 Indicate The Network ID And 0.225.255.255 Wildcard Mask With Area 0).
Router A(Config-Router)#Exit
Router A(Config)#Exit
Router A#
PING :
Now On Both Routers Each Network Will Be Added Automatically, You Can Ping To Router A And Router B To Check Communication. You Can Verify This On Router-A Or Router-B By Using Command Router A# Ping 30.0.0.1 (Ping For Router B).
OSPF CONFIGURATION ON ROUTER B
Router B(Config)#Router OSPF 4 - > (Indicate The Process Identification Number 4).
Router B(Config-Router)#Network 20.0.0.0 0.255.255.255 area 0 - > (20.0.0.0 Indicate The Network ID And 0.225.255.255 Wildcard Mask With Area 0).
Router B(Config-Router)#Network 30.0.0.0 0.255.255.255 area 0 - > (30.0.0.0 Indicate The Network ID And 0.225.255.255 Wildcard Mask With Area 0).
Router B(Config-Router)#^z
Router#
PING :
Now On Both Routers Each Network Will Be Added Automatically, You Can Ping To Router A And Router B To Check Communication. You Can Verify This On Router-A Or Router-B By Using Command Router B# Ping 10.0.0.1 (Ping For Router A).
TO CHECH OSPF CONFIGURATION ON ROUTER A & Router B
Router A# Clear IP Route*
Router A# Clear IP OSPF Processor - > (It will Ask Yes/No)
To Clear Redistribution Based On The OSPF Routing Process ID, Use The Clear Ip Ospf Command In Privileged EXEC Mode.
The Following Example Clears All OSPF Processes:
Clear Ip Ospf Process
s Router A# Show IP OSPF Neighbor
Displays OSPF Neighbors In Detail, Providing Parameters, Such As Neighbor Address, Hello Interval, And Dead Interval.
These Hello Packets Are Exchanged Between Directly-Connected Neighbors To Learn More About Each Other. You Can Use The SHOW IP OSPF NEIGHBOR Command To Observe The Neighbor Data Structure. This Command Displays OSPF-Related Neighbor Information.
The Following Sections Describe The SHOW IP OSPF NEIGHBOR Command Output From The EXAMPLE Above.
Neighbor Id,
Priority,
State,
Dead Time,
Address,
Interface
Roure A# Show IP OSPF
To Display General Information About Open Shortest Path First (OSPF) Routing Processes, Use The Show Ip Ospf Command In EXEC Mode.
Router A# Show IP OSPF Database
To Display Lists Of Information Related To The OSPF Database For A Specific Router, Use The SHOW IP OSPF DATABASE Command In EXEC Mode. The Various Forms Of This Command Deliver Information About Different OSPF Link-State Advertisements (LSAs).
Router A# Show IP OSPF Interface
Router A# Show IP OSOF Interface Serial 1
To Display OSPF-Related Interface Information, Use The Show Ip Ospf Interface Command In EXEC Mode.
SHOW IP OSPF INTERFACE [Interface-Type Interface-Number] [Brief]
The Following Is Sample Output Of The Show Ip Ospf Interface Command When Ethernet Interface 0 Is Specified: Router# SHOW IP OSPF INTERFACE ETHERNET 0
Router A# Show IP Route
Router A# Show IP Int Brief
Router A# Show Run
For More About - > OSPF IP Routing Protocols Commands :
TO CHECH OSPF HELLO PACKET ON ROUTER A & Router B
DEBUGGING COMMANDS :
• Debug IP OSPF Events
• Debug IP Packet
Router A# Debug IP OSPF HELLO
TO CHENGE THE HELLO INTERAVAL
Router A(Config)#Int S1
Router A(Config-if)# IP OSPF Hello-Interval 20
NOTE : Hello Interval Should be same in Both Router.
The Hello Interval Is The Number Of Seconds This Router Waits Before Sending Out The Next Hello Packet.
Use The IP OSPF Hello-Interval Seconds Command To Set The Number Of Seconds This Router Waits Before Sending The Next Hello Packet Out This Interface.
Use The No OSPF Hello-Interval To Set The Hello-Interval To The Default Value Of 20 Seconds.
Factory Default: 10 Seconds.
Command Mode: Interface Configuration.
In The Following Example, The IP OSPF Hello-Interval Seconds Command Sets The Number Of Seconds Between Sending Hello Packets To 20 Seconds, And The Show IP OSPF Interface Pos Command Displays The Setting:
IP OSPF DEAD-INTERVAL:
Sets The Number Of Seconds Without Hello Packets Before An Adjacency Is Declared Down.
Syntax: IP OSPF Dead-Interval Seconds
No Ip Ospf Dead-Interval
The Number Of Seconds Not Hearing A Router's Hello Packets Before The Router's Neighbors Will Declare It Down. Valid Values Are 1 To 65535
Description: The Dead-Interval Is A Timer Used To Time Out Inactive Adjacencies.
Use The IP OSPF Dead-Interval Seconds Command To Configure The Number Of Seconds This Router Should Wait Between Receiving Hello Packets From A Neighbor Before Declaring The Adjacency To That Neighbor Down.
Use The No IP OSPF Dead-Interval Command To Configure The Dead Interval At The Default Value Of 40 Seconds.
Factory Default: 40 Seconds.
Command Mode: Interface Configuration.
Example: In The Following Example, The IP OSPF Dead-Interval Seconds Command Sets The Number Of Seconds Without A Hello Packet Before The Router Declares An Adjacency Down To 60 Seconds, And The Show IP OSPF Interface Pos Command Displays The Setting.
OSPF Hello Packets Are Packets That An OSPF Process Sends To Its OSPF Neighbors To Maintain Connectivity With Those Neighbors. The Hello Packets Are Sent At A Configurable Interval (In Seconds). The Defaults Are 10 Seconds For An Ethernet Link And 30 Seconds For A Non Broadcast Link. Hello Packets Include A List Of All Neighbors For Which A Hello Packet Has Been Received Within The Dead Interval. The Dead Interval Is Also A Configurable Interval (In Seconds), And Defaults To Four Times The Value Of The Hello Interval. The Value Of All Hello Intervals Must Be The Same Within A Network. Likewise, The Value Of All Dead Intervals Must Be The Same Within A Network.
These Two Intervals Work Together To Maintain Connectivity By Indicating That The Link Is Operational. If A Router Does Not Receive A Hello Packet From A Neighbor Within The Dead Interval, It Will Declare That Neighbor To Be Down.
The Hello Interval And Router Dead Interval Are The Two Timers That You Can Adjust To Speed Up Network Convergence In An OSPF Network. The Hello Interval Determines The Interval Between Sending OSPF Hello Messages On An Interface, While The Router dead interval Is The Interval In Which A Router Must Receive An OSPF Hello Message From A Neighbor Before It Considers That Neighbor To Be Down.
You Can Adjust The Hello interval And Router dead interval With The IP OSPF Hello-Interval And IP OSPF Dead-Interval Interface Configuration Commands. Doing So Can Reduce OSPF Convergence, But You Should Be Careful To Take Into Consideration The Quality (And Possibly Speed) Of Your Links Before Doing so.
OSPF NEIGHBOR ADJACENCY
OSPF NEIGHBOR ADJACENCY (DEBUG IP OSPF ADJACENCY):
Router A# Debug IP OSPF Adjacency
OSPF adjacency events debugging is on
Getting Open Shortest Path First (OSPF) Neighbors To Become Fully Adjacent. In A Successful Formation Of OSPF Adjacency, OSPF Neighbors Will Attain The FULL Neighbor State.
In Order To Examine The Formation Of Adjacency, Issue The DEBUG IP OSPF ADJ Command.
TO CLEAR DEBUGGING, THE FOLLOWING COMMAND IS ISSUED. ROUTER A(config-router)#do un all
All possible debugging has been turned off
ADJACENCIES :
The Fact That Routers Are Neighbors Is Not Sufficient To Guarantee An Exchange Of Link-State Updates; They Must Form Adjacencies To Exchange Link-State Updates. Adjacency Is An Advanced Form Of Neighborship Formed By Routers That Are Willing To Exchange Routing Information After Negotiating Parameters Of Such An Exchange. Routers Reach A FULL State Of Adjacency When They Have Synchronized Views On A Link-State Database.
Interface Type Plays A Major Role In How The Adjacencies Are Formed. For Example, Neighbors On Point-To-Point Links Always Try To Become Adjacent, While Routers Attached To Broadcast Media Such As Ethernet Can Choose To Become Adjacent Only With A Subset Of Neighboring Routers On The Interface.
Once A Router Decides To Form An Adjacency With A Neighbor, It Starts By Exchanging A Full Copy Of Its Link-State Database. The Neighbor, In Turn, Exchanges A Full Copy Of Its Link-State Database With The Router. After Passing Through Several Neighbor States, The Routers Become Fully Adjacent.
NEIGHBOR STATES :
You Can Use The SHOW IP OSPF NEIGHBOR Command To Determine The State Of The OSPF Neighbor Or Neighbors. The Output Of This Command Will Most Likely Reveal One Of These:
• Nothing At All
• State = Down
• State = Init
• State = Exstart
• State = Exchange
• State = 2-Way
• State = Loading
There Are Other OSPF States, But Those Shown Here Are The Most Common Ones Seen In SHOW IP OSPF NEIGHBOR Command Output.
GLOSSARY
GLOSSARY :
INTERNAL ROUTER : In OSPF Routing, If All The Interfaces Of A Router Are All Within The Same Area Where The Router Resides, It Is Called An Internal Router (I). Internal Routers Are Used For Routing Traffic Within The Area, And Their Routing Tables Contain Information Only About All Other Internal Routers In That Area.
BACKBONE ROUTER: A Backbone Router Is A Router Which Has At Least One Interface Connected To The Backbone Area Which Is Used For Routing Traffic Within The Backbone Area.
AREA BORDER ROUTER (ABR) In Area Border Router (ABR), One Of The Interfaces Of A Router That Has Multiple Interfaces Must Be Connected To The Backbone Area. Abrs Are Used For Routing Traffic Between The Backbone Area And Other Areas.
AUTONOMOUS SYSTEM BOUNDARY ROUTER (ASBR) : ASBR Router Is Used To Connect An Autonomous System Running OSPF Routing To An Autonomous System Running A Different Interior Gateway Protocol.
DESIGNATED ROUTER : OSPF Separates The Network Into Regions Called Areas. Within Each Area, There Is A Router Identified As The Designated Router. This Router Is Responsible For The Distribution Of All Intra- And Inter-Area Routing Information.
LINK STATE ADVERTISEMENT (LSA): The LSA Is Similar To A RIP Update. The LSA Is Used By Routers To Exchange Information About The Status Of Their Connections To Neighbors.
HELLO MESSAGES : These Are Messages Generated By Each Router To Identify Its Neighboring Routers And To Verify Their Status. If A Router Does Not Reply To A Hello Message, That Indicates The Loss Of A Connection And Results In The Generation Of An Updated LSA.
BACKBONE : OSPF Separates The Network Into OSPF Areas. All OSPF Areas Must Connect Directly To A Special Area Called The Backbone Area. This Area Is Used To Exchange Routing Information Between All Other Areas.
ROUTER ID: A 32-Bit Number Assigned To Each Router Running The OSPF Protocol. This Number Uniquely Identifies The Router Within An Autonomous System.
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The “Cisco - OSPF Configuration Examples". Hope This Article Will Help Every Beginners Who Are Going To Start Cisco Lab Practice Without Any Doubts. 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:
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