SUMMARY OF OSPF: OSPF Terms And Terminology : 1. Link: Network Communication Channel Consist Of A Circuit Or Transmission Path. 2. Link State: The Statistics Of Link Between Routers, In Other Words A Interface Of A Router And Its Relationship To Its Neighbor Router. 3. Topology Of LSDB (Link State Database): A List Of Information About All Networks. All Routers With Same Area Must Have Same LSDB. 4. Area: A Collection Of Network And Router Having Same Logical Boundary. Area Provides OSPF A 2 Layer Hierarchical Concept. a) Area 0: Known As Backbone Of Area. It Is The Area Which Acts Like A Transit To All Other Area. b) Off Backbone Area: Areas Which Must Be Connected With Area 0. It Is A Logically Or Physical Ranges Between Area 1 To Area Infinite (8). Off Backbone Cannot Communicate Each Other Without Traversing Area 0. 5. OSPF Cost: OSPF Cost Is The Value Assign To A Link As Routing Metric. OSPF Cost Is Calculate As 108/Bandwidth Of Link In Bps (Cumulative). 6. Router ID: Router ID Is A Identification Tag Used By OSPF And EIGRP. Normally, It Is The Highest Loopback Interface IP, If Loopback Is Not Configured Then The Highest Physical Interface IP Will Be The Router ID. 7. Election Criteria DR/BDR: a) Highest Router Interface Priority Will Be The DR. Priority Can Be Assign From 0 To 255. Where 0 Is Not Eligible For DR/BDR Election, 1 Is Default Priority, 255 Is The Highest Priority. b) In Case Of Tie Then Highest Router ID Will Be DR. 8. OSPF Packet Type: a) HELLO Packet - Discover Neighbor And Built Adjacency Between Them. Maintain Neighbor Relationship. b) DBD (Database Description) � Check For Database Synchronization Between Routers. c) LSR (Link State Request) � Request Specific Link State Records From Another Router. d) LSU (Link State Update) � Sends Specific Requested Link State Records From Another Router. e) LSACK (Link State Acknowledgement) � The Other Packet Types. 9.The Data Field Contains Different Information Depending In The OSPF Packet Type: a) Hello Packet Contains A List Of Known Neighbor. b) DBD Packet Contains A Summary Of The LSDB Among The Number Of The Field. c) LSR Packet Contains The Type Of LSU Needed And The Router ID Of The Router That Has The Need Of The LSU. d) LSU Packet Contains The Full LSA Entries. e) LSACK Packet Contains Null In This Field. 10. OSPF Neighbor Adjacency Parameters. Same Following Parameters Must Match To Became OSPF Neighbor: a) Hello And Dead Interval Timer. b) Area IDs. c) Authentication Type And Password (If Configured). d) Area Stub Flag � Which Indicates The Type Area Stub. 11. OSPF Area Characteristic : a) Minimizing Routing Table. b) Localized Impact Of A Topology Change. c) Detail LSA Flooding Stops At The Area Boundary. d) Required Hierarchical Network Design. 12. OSPF Route Types: a) Internal Router � Router That Have All Their Interface In The Same Area And Has Same LSDB. b) Backbone Router � Router That Are Situated On The Backbone And Have At Least One Interface Connected To Area 0. c) ABR (Area Border Router) � Router That Have Interfaces Attached To Multiple Areas, Maintain Separate LSDB For Each Area To Which They Comes. ABRs Are The Exits Point Of Area. d) ASBR (Autonomous System Boundary Router) � Router That Have At Least One Interface Attached To An Entire Router Domain (A Network Running Different Routing Protocol Other Then OSPF Known As Different AS). ASBR Can Impact As Non OSPF Network Into The OSPF Network. 13. Type Of Areas: 1. Backbone Area: Known As Area 0. It Is The Area Which Acts Like A Transit To All Other Area. 2. Off Backbone Area: Areas Which Must Be Connected With Area 0. Logically Or Physical Ranges Between Area 1 To 4294967295. Off Backbone Cannot Communicate Each Other Without Traversing Area 0. Area Types Depend Upon The Characteristic Of The Internal Router Administrative Privilege And Accommodation Of Redistributed Route. Based On The Information Area Can Be Of Three Types. a. Stub Area � Does Not Accept Information About External Routes To The OSPF Domain, Such As Route From Non OSPF Sources (Other Routing Protocol). These Mean That No Type 5 LSA Will Be Known Inside This Area And Consequently Type 4 LSA Is Unnecessary. ABRs At The Age Of The Stub Area Are Used Type 3 LSA To Advertise A Single Default Route Into The All Internal Routers Within The Stub Area, So That If The Router Need To Route To The Network Outside Of The OSPF Area Will Use Default Route. b. Not So Stubby Area (NSSA) � An NSSA Is Another Area To The OSPF Area Terminology Which Offers The Benefit That Are Similar To The Stub Or Totally Stubby Area, But Also All External Route To Be Advertise Into The OSPF Domain. These Forms Of NSSA Allow ASBR (Redistribution Point). The ASBR Originated Type 7 LSA To Advertise The External Distribution. Type 7 LSAs Are Flooded Throughout The NSSA But Are Blocked By The ABR. The ABR Convert The Type 7 LSA Into A Type 5 LSA Which Is Propagated Through The OSPF Domain. c. Totally Stubby Area � These Type Of Area Is Even Restricted Than Stubby Area. The Internal Router Can Communicate, Within Their Networks, Within Their Area Via OSPF Routing. But, Any Communication Beyond The Area Must Be Communicated Via Default Route. Totally Stubby Cannot Contain The ASBR. Except That The ASBR May Also Be ASBR. OSPF Not-So-Stubby (NSSA): The OSPF Not-So-Stubby Area (NSSA) Feature Is Described By RFC 1587 And Is First Introduced In Cisco IOS Software Release 11.2. It Is A Non-Proprietary Extension Of The Existing Stub Area Feature That Allows The Injection Of External Routes In A Limited Fashion Into The Stub Area. Redistribution Into An NSSA Area Creates A Special Type Of Link-State Advertisement (LSA) Known As Type 7, Which Can Only Exist In An NSSA Area. An NSSA Autonomous System Boundary Router (ASBR) Generates This LSA And An NSSA Area Border Router (ABR) Translates It Into A Type 5 LSA, Which Gets Propagated Into The OSPF Domain. The Network Diagram Demonstrates This Principle. 14. OSPF Virtual Link : Virtual Link Is A Link That Allows Distinguish Area 0 To Be Connected A Disconnecting Area To Be To Be Connecting With Area 0 Via A Transit Area. [Note: Virtual Link Should Not Be Use As Primary Design; Rather It Should Be Use Only For Specific Cases, Such As Temporary Communication Or Backup For Failure. The Hello Protocol Works Over The Virtual Link As It Does Over Standard Link In 10 Second Interval, However LSA Updates Works Differently On Virtual Link And LSA Usually Refresh Every 30 Munities But, LSAs Learn Through Virtual Link Have �Do Not Age (DNA)� Option Set. So, The LSA Does Age Out. DNA Is Required To Prevent Excessive Flooding Over The Virtual Link. 15. OSPF Route Summarization: In OSPF Intra Area Route Summarization Is Not Possible But Inter Area (Within The Same Area) Route Summarization Is Possible And External Route Summarization Is Possible. Inter Area Route Summarization Is Always Occur On An ABR. We Have To Use �Area Range� Command To Instruct The ABR To Summarized Route For A Specific Area Before Injection Them. External Route Summarization Is Always Occur In The ABR With The Help Of �Summary-Address� Command Which Instruct The ASBR To Summarization To Summarized External Route Before Inject Them Into OSPF Domain As Type 5 External LSA. 16. Default Route Inclusion : A Default Route While Include In OSPF Domain Is Injected As An LSA Type 5, Default Route Distribution Is Not On By Default In OSPF We Have To Issue The �Default-Information Originated� Command Under The OSPF Routing Process. 17.OSPF IS EASILY EXPLAINED: 1. OSPF-Speaking Routers Send Hello Packets Out All OSPF-Enabled Interfaces. If Two Routers Sharing A Common Data Link Agree On Certain Parameters Specified In Their Respective Hello Packets, They Will Become Neighbors. 2. Adjacencies, Which Can Be Thought Of As Virtual Point-To-Point Links, Are Formed Between Some Neighbors. OSPF Defines Several Network Types And Several Router Types. The Establishment Of An Adjacency Is Determined By The Types Of Routers Exchanging Hellos And The Type Of Network Over Which The Hellos Are Exchanged. 3. Each Router Sends Link-State Advertisements (LSAS) Over All Adjacencies. The Lsas Describe All Of The Routers Links, Or Interfaces, The Routers Neighbors, And The State Of The Links. These Links Might Be To Stub Networks (Networks With No Other Router Attached), To Other OSPF Routers, To Networks In Other Areas, Or To External Networks (Networks Learned From Another Routing Process). Because Of The Varying Types Of Link-State Information, OSPF Defines Multiple LSA Types. 4. Each Router Receiving An LSA From A Neighbor Records The LSA In Its Link-State Database And Sends A Copy Of The LSA To All Of Its Other Neighbors. 5. By Flooding Lsas Throughout An Area, All Routers Will Build Identical Link-State Databases. 6. When The Databases Are Complete, Each Router Uses The SPF Algorithm To Calculate A Loop-Free Graph Describing The Shortest (Lowest Cost) Path To Every Known Destination, With Itself As The Root. This Graph Is The SPF Tree. 7. Each Router Builds Its Route Table From Its SPF Tree. This Fundamental Procedure Of Calculating Routes From The Link-State Database, Rather Than By Exchanging Routes With Neighbors, Has Repercussions For Route Filtering. When All Link-State Information Has Been Flooded To All Routers In An Area And Neighbors Have Verified That Their Databases Are Identicalthat Is, The Link-State Databases Have Been Synchronizedand The Route Tables Have Been Built, OSPF Is A Quiet Protocol. Hello Packets Are Exchanged Between Neighbors As Keepalives, And Lsas Are Retransmitted Every 30 Minutes. If The Network Topology Is Stable, No Other Activity Should Occur. OSPF Neighbors And Adjacencies Before Any Lsas Can Be Sent, OSPF Routers Must Discover Their Neighbors And Establish Adjacencies. The Neighbors Will Be Recorded In A Neighbor Table, Along With The Link (Interface) On Which Each Neighbor Is Located And Which Contains Other Information Necessary For The Maintenance Of The Neighbor The Following Is A Summary Of The States An Interface Passes Through Before Becoming Adjacent To Another Router: Down: No Information Has Been Received From Anybody On The Segment. Attempt: On Non-Broadcast Multi-Access Clouds Such As Frame Relay And X.25, This State Indicates That No Recent Information Has Been Received From The Neighbor. An Effort Should Be Made To Contact The Neighbor By Sending Hello Packets At The Reduced Rate Pollinterval. Init: The Interface Has Detected A Hello Packet Coming From A Neighbor But Bi-Directional Communication Has Not Yet Been Established. Two-Way: There Is Bi-Directional Communication With A Neighbor. The Router Has Seen Itself In The Hello Packets Coming From A Neighbor. At The End Of This Stage The DR And BDR Election Would Have Been Done. At The End Of The 2way Stage, Routers Will Decide Whether To Proceed In Building An Adjacency Or Not. The Decision Is Based On Whether One Of The Routers Is A DR Or BDR Or The Link Is A Point-To-Point Or A Virtual Link. Exstart: Routers Are Trying To Establish The Initial Sequence Number That Is Going To Be Used In The Information Exchange Packets. The Sequence Number Insures That Routers Always Get The Most Recent Information. One Router Will Become The Primary And The Other Will Become Secondary. The Primary Router Will Poll The Secondary For Information. Exchange: Routers Will Describe Their Entire Link-State Database By Sending Database Description Packets. At This State, Packets Could Be Flooded To Other Interfaces On The Router. Loading: At This State, Routers Are Finalizing The Information Exchange. Routers Have Built A Link-State Request List And A Link-State Retransmission List. Any Information That Looks Incomplete Or Outdated Will Be Put On The Request List. Any Update That Is Sent Will Be Put On The Retransmission List Until It Gets Acknowledged. Full: At This State, The Adjacency Is Complete. The Neighboring Routers Are Fully Adjacent. Adjacent Routers Will Have A Similar Link-State Database. 18.OSPF NETWORK TYPES : OSPF Type Of Network To Which The OSPF Interface Is Connected. One Of The Following OSPF Network Types Must Be Selected When Configuring An Interface On An OSPF Router. Broadcast A Network That Can Connect More Than Two Routers With A Hardware Broadcast Facility Where A Single Packet Sent By A Router Is Received By All Routers Attached To That Network. Ethernet, Token Ring, And FDDI Are Broadcast Networks. OSPF Messages Sent On Broadcast Networks Use IP Multicast Addresses. Point-To-Point A Network That Can Connect Only Two Routers. Leased-Line WAN Links Such As Dataphone Digital Service (DDS) And T-Carrier Are Point-To-Point Networks. OSPF Messages Sent On Point-To-Point Networks Use IP Multicast Addresses. Non-Broadcast Multiple Access A Network That Can Connect More Than Two Routers But Has No Hardware Broadcast Facility. X.25, Frame Relay, And ATM Are Non-Broadcast Multiple Access (NBMA) Networks. Because Multicasted OSPF Messages Do Not Reach All The OSPF Routers On The Network, OSPF Must Be Configured To Unicast To The IP Addresses Of The Routers On The NBMA Network.
Posted on: Tue, 12 Nov 2013 13:36:32 +0000
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