OSPF ALL THEORY (PDF) by sairanaeem


									                                 (OPEN SHORTEST PATH FIRST)
IGP (Interior Gateway Protocol)
Open Standard Protocol.
Made Up of Several Protocol Handshakes, Database Advertisements and Packet Types.
Used by Both Enterprises & Service Providers.
Is a Link-State Protocol.

        Developed to overcome Limitations of Distance Vector Routing Protocols.
        Respond Quickly to Network Changes.
        Send Triggered Updates when a Network Change Occurs.
        Send Periodic Updates/Link-State Refresh Every 30 Seconds.

LSDB is used to Calculate Best Paths through the Network. A DIJKSTRA’S ALGORITHM/SPF is applied on LSDB to Build the
SPF TREE and Best Paths are Selected from SPF TREE and Placed into the Routing Table.

        Neighbor Table/ Adjacency Table contain List of Recognized Neighbors.
        Topology Table/LSDB Contains all Routers and their Links in the Area/Network.
        In an Area Every Router has Same LSDB (Identical LSDB for all Routers within an Area).
        Routing Table/Forwarding Database Contains List of Best Paths to Destinations.
        Link-Sate Routers recognize more Information than Distance Vector about Network.
        Have Full Picture of Topology.
        Have More Accurate Decisions.
        Transmit Area (Backbone/Area 0).
        Regular Areas (Non backbone Area).
                      o Minimizes Routing Table Entries.
                      o Localizes Impact of a Topology Change with an Area.
                      o Detailed LSA Flooding Stops at the Area Boundary.
                      o Requires a Hierarchical Network Design.
        Five OSPF Packet Types enable All OSPF Information Flow between Routers.
        Hello Packets Discover Neighbors and Built Adjacency Between them
        DBD Checks for Database Synchronization between Routers.
        LSR-Requests Specific Link-State Records from Router – to –Router.
        LSU-Sends Specifically requested Link-State Records.
        LSAck-Acknowledges the Other Packet Types.
        OSPF Five Packets are encapsulated directly into an IP Payload.
        OSPF does not use TCP/UDP for Acknowledgment
        OSPF uses Reliable Transport Scheme as OSPF Packet Type 5.
        Router ID, Area ID, Checksum, Authentication.

Each Interface Participating in OSPF uses IP Multicast Address to Send Hello Packets.
Router ID, used on Multi-access Network.
Hello & Dead Intervals, Frequencies in Seconds for Sending Hello Packets.
Hello Default is 10 Seconds on Multi-access Network.
Dead is 4 Times of Hello.
Should be same on Both Sides for Neighbor Adjacency.
    • Used for Selecting DR/BDR on Multi-access Network.
    • Must Exchange Same Password for Neighbor Adjacency.
    • Used for Reducing Routing Update by Replacing them with a Default Route.
           After a DR/BDR selected, Any Router added to Network will Establish Adjacency with DR/BDR only.
                            OSPF STATES
      • DOWN                                           HELLO
      • INIT                                           HELLO
      • TWO-WAY
      • EXSTART                                        DBD
      • EXCHANGE                                       DBD, LSACK
      • LOADING                                        LSR
      • FULL                                           LSAck
           Routing Table Established after Full State.
It is very Important for the LSDBs (Topology Table) of all Routers Stay Synchronized, when a Change in Link State.
Routers use Flooding Process to Notify Other Routers in the Network of Change. LSAs Flooding used by LSUs.

     • To Ensure an accurate Database, OSPF Floods Refresh LSAs every 30 Minutes. The Sequence Number is
          incremented by adding One.
                                    A # show ip ospf database
          Begins 0x80000001 ends 0x7FFFFFF.
          i.e      080000008 Link ID Updated 8 Times.
                   And 48 Seconds ago Last Update.
     • A = debug ip ospf packet
Hello Protocol forms Logical Neighbor Adjacency Relationships.
Full State Means that Database are Synchronized with Adjacent Routers.
LSAs are sent on Change but are also sent every 30 Minutes to Ensure Database Integrity. Maximum that an LSA will stay in
the Database without an Update is ONE (1) Hour. The LSA Sequence Number is Incremented every Time is Advertised.
Each LSA in the LSDB has a Sequence Number, which is Incremented by one each Time the LSA is Flooded. When a Router
encounters Two Instances of an LSA, it Must determine which is more recent. The LSA having the Newer (Higher) LSA
Sequence Number is more recent.
Use the Address & Mask matches all Interfaces on the Router.
We can also give an IP Address on Area ID in ospf network Command.
A(fg-if)#ip ospf 1 area 0 [Secondary None]
Option Prevents Secondary IP Addresses on the Interface from being Advertised.
A(fg-if)# network area 0
This Technique assigns All Interfaces defined in the Network to OSPF Process.
A(fg-if)#network area 0
This Host Technique allows the Operator to define which Specific Interfaces will Run OSPF.
Network Statement and WildCard Mask are not used for Route Summarization.
The ABR is used to interconnect different OSPF Areas with BackBone Area.
Router ID is the Normally Highest IP Address on an Active Interface.
Using a LoopBack Interface/Router-ID Command is Recommended for Stability.
A Loopback Address Never goes down.
A(fg)#router ospf 1
A#clear ip ospf process
For Refresh OSPF Process.
A#sh ip ospf
To confirm Router ID.
Verify OSPF Process.
A#sh ip protocols
A#sh ip route ospf 1
A#sh ip ospf interface serial 0/1
A#sh ip ospf neighbor
* Neighbor-id
* Detail
A#sh ip route ospf
* 782 Total Cost of the Route to reach to Subnet/Destination.
A#sh ip ospf interface fa 0/0
A#sh IP ospf neighbor
A#sh ip ospf neighbor detail

   • Adjacency Type is different for each Network Types.
   • Point-to-Point Network Joins a Single pair of Routers.
   • Broadcast, A Multi-access Network as Ethernet.
   • Non-Broadcast Multi Access (NBMA), A Network that Interconnects More that Two Routers but has no Broadcast
       Capability. i.e. Frame-Relay, ATM, X.25


                 POINT-to-POINT LINKS
             •   Usually Serial Interface Running PPP/HDLC.
             •   May also use Point-to-Point Sub interface in Frame-Relay/ATM.
             •   No DR/BDR Election.
             •   OSPF Auto detects this Interface Type.
             •   OSPF Packets sent using
             •   Default Hello & Dead = 10 & 40 Sec.

                    • LAN Technologies like Ethernet & Token Ring.
                    • DR & BDR Election Required.
                    • All Neighboring Routers form Null Adjacencies with the DR & BDR only.
                    • Packets to DR & BDR use
                    • Packets to all other Routers use sent by DR.
                           o ELECTING DR & BDR
                               • Hello Packets are exchanged via IP Multicast.
                               • With Highest OSPF Priority =DR and with Second-Highest OSPF Priority =BDR
                               • Use the Router ID as Tie Breaker.
                               • DR Election is Non-Preemptive.
                               • In Multi-Access Broadcast Environment, Each Network Segment has its own DR &
                               • Router Connected to Multiple Multi-access Broadcast Networks can be a DR on One
                                          Segment and a Regular on Other Segment.
                           o PRIORITY FOR DR ELECTION
                                    • A(fg)#int fa 0/1
                                    • A(fg-if)#ip ospf priority 10
                                    • Different Priority Values could be Assign to Different Interfaces.
                                    • Default Priority is 1.
                                    • 0 Means that Router can not be DR/BDR.
                                    • Not DR/BDR = DR Other.
                                    • The Priority of an Interface takes effect only when the Existing DR goes Down.
                           o NBMA TOPOLOGY NETWORK (X.25, Frame-Relay, ATM)
                                    • A Single Interface interconnects Multiple Sites.
                                    • NBMA Topologies Support Multiple Routers, But without Broadcasting
                                    • Due to Non-Broadcast Capability, if the NBMA is not fully Meshed, then a
                                        Broadcast/Multi-access sent by one Router will not Reach at All other Routers.
                                    • To Implement Broadcast/Multicasting on NBMA Network, The Router
                                        Replicates the Packets to be Broadcast/Multicast and sends them Individually
                                        on each Permanent Virtual Circuit (PVC) to all Destinations. This Process is CPU
                                        and Bandwidth Intensive.
                                    • Default Hell0 = 30 Sec, Dead = 120 Sec.
                           o DR ELECTION IN NBMA TECHNOLOGY
                                    • OSPF Considers NBMA Like other Broadcast Media.
                                                   •  The DR/BDR needs to have Full Meshed Connectivity with Other Routers, but
                                                      NBMA Networks are not always Fully Meshed.
                                                  • NBMA Clouds are usually built in HUB-and-Spoke Topology, using PVCs or
                                                      Switched Virtual Circuits (SVCs).
                                                  • A Hub-and-Spoke Topology means that the NBMA Networks is only a Partial
                                                  • The DR/BDR needs a List of Neighbors.
                                                  • OSPF Neighbors are not Automatically Discovered over NBMA Network by the
                                                      (OSPF can not automatically build Adjacencies with Neighboring Routers over
                                                      NBMA Interfaces.)
                                       o       OSPF OVER FRAME-RELAY CONFIGURATIONS, FRAME-RELAY TOPOLOGIES
                                                  • There are several OSPF Configurations Choices for a Frame-Relay Network
                                                      depending on the Frame-Relay Network Topology.
                                                  • With Frame-Relay, Remote Sites interconnect in a Variety of ways.
                                                      By default, Interfaces that Frame-Relay Supports are Multipoint Connection.
                                                              STAR TOPOLOGY/HUB-and-SPOKE TOOPLOGY (Most Common)
                                                              *Remote Sites Connect to a Central Site that generally provides
                                                              *The Least Expensive Topology, because it requires Smallest Number
                                                              of PVCs.
                                                              *The Central Router Provides a Multipoint Connection, it use a Single
                                                              Interface to Interconnect Multiple PVCs.
                                                              FULL-MESH TOPOLOGY
                                                              *All Routers have Virtual Circuits to all other Destinations. Provides
                                                              Link Redundancy.
                                                              *Costly due to Number of Nodes Increasing.
                                                              *Formula used for How Many Virtual Circuits are required for Full
                                                              Meshed. i.e. N (N-1)/2 N = Number of Nodes.
                                                              *Not all Sites have Direct Access to a Central Site.
                                                              *Method reduces cost for Full-Mesh Topology Implement.
                                        OSPF OVER NBMA TOPOLOGY SUMMARY
          OSPF MODE             NBMA TOPOLOGY             SUBNET ADDRESS        HELLO     ADJACENCY          RFC/CISCO
          Broadcast             Full/Partial              Same                  10        Auto               CISCO
                                                                                          DR/BDR Elected
          Non-Broadcast         Full/Partial              Same                  30        Manual             RFC
          (NBMA)                                                                          DR/BDR
          Point-to-Multipoint   Partial/Star              Same                  30        Auto               RFC
                                                                                          No DR/BDR
          Point-to-Multipoint   Partial/Star              Same                  30        Manual             CISCO
          Non-Broadcast                                                                   No DR/BDR
          Point-to-Point        Partial/Star using        Different for Each    10        Auto               CISCO
                                Sub Interfaces            Sub Interface                   No DR/BDR

        A(fg-if)#ip ospf network ?
        -Broadcast, Non-Broadcast, Point-Multipoint, Point-Multipoint Non-Broadcast, Point-to- Point.
        i.e. A(fg)#int serial 0/0/0
                (fg-if)#encapsulation frame-relay
                        #ip ospf network broadcast
        A(fg-router)#neighbor ip address
        i.e. #neighbor
        For Point-to-Point Mode.
        A#debug ip ospf adj
        For Broadcast Mode
        A#debug ip ospf adj
        OSPF default behavior on a Fast Ethernet Link is Broadcast Mode.
        First DR/BDR are elected, and then Exchange Process occurs in Broadcast Mode.
        More Important thing in OSPF is, How the Topology Database is Built ?
        The Core Concept of OSPF ARE THE LINK-State Database (LSDB) and LSAs.
        OSPF usually operate within a Single Area.
        Following Problem Occurs with Single Area.
             • SPF (Shortest Path First) Algorithm, Recalculation whenever a Network Change occurs, Processor Intensive.
             • Large Routing Table, As OSPF does not perform Route Summarization by Default. Therefore, Big Routing Table
                  becomes in Single Area.
             • Large LSDB, LSDB Covers the Topology of the Entire Network, Each Router must maintain an entry in the LSDB for
                  every Network in Area, even not every Route is being used for Routing Table.
             • OSPF Areas are used for,
                  -    Consists of Areas and Autonomous System.
                  -    Minimizes Routing Update Traffic.
             • Link State Update (LSU) contains different LSAs and Summary Information.
             • Rather than send an LSU about Each Network within Areas only Summarized Route/Small Number is sent.
        INTERNAL Routers, which have all their Interfaces in the Same Area and have their Identical LSDBs.
        BACKBONE Routers, which sit in the Backbone Area and Maintain Routing Information as Internal Routers.
        AREA BORED Routers, which have Interfaces attached to Multiple Areas, Maintain Separate LSDBs, and Exit Point for an
        Area. ABR can be configured to summarize the Routing Information from the LSDBS of their Attached Areas. ABRs distribute
        the Information to Backbone & Backbone Routers then Forward Information to Other ABRs. In a Multi-Area Network, An
        Area can be/have More than One ABR.
        ASBR Routers, which have at Least one Interface attached to an External/Non-OSPF Network, ASBRs can Import Non-OSPF
        Network Information to OSPF Network and can also send OSPF Network Information to Non-OSPF Network (Called Route
        A Router can exist more than One Router Type, i.e. A Router Connects Area 0, Area 1 & also Non-OSPF Network then it will
        be ABR & ASBR.
        A Router has Separate LSDB for each Area to which it connects; therefore an ASBR has one LSDB for Area 0 and one for
        Two Routers in Same Area maintain Identical LSDBs for that Area.
        An LSDB is synchronized between Pairs of Adjacent Routers.
        On Broadcast Network like Ethernet LSDB is synchronized between DR Other and its DR & BDR.
        Virtual Links are used to connect a Discontigeous Area to Area 0.
        A Logical Connection is built between Routers.
        Virtual Links are recommended for Back up or Temporary Connections.
         Hello Protocol works over Virtual Links as it works on Standard Links, in 10 Seconds Interval.
        LSA Updates work differently on Virtual Links. Normally, LSA refreshes after every 30 Minutes. But LSA Learned through
        Virtual Links has the DoNot Age option. So, The LSA through Virtual Link does not Age Out.
        For Getting Router ID knowledge,
        A#sh ip ospf
        A#sh ip protocol
        A#sh ip ospf interface
        A#sh ip ospf virtual-links
        Virtual Link to Router Up
        DoNot Age LSA allowed.
        Virtual Link TROUBLESHOOTING
        A#sh ip ospf neighbor
        A#sh ip ospf database router
        A#debug ip ospf adj
        LSAs are Building Blocks of OSPF LSDB.
        Act as Database Records.
        Describe Entire Topology of Entire Network/Area of OSPF.
        For all LSA Types, 20 Byte Header is used; One of the Field LSA Header is Link-State ID.
        Generally, if A Router has different Networks and by chance if any Network goes Down, Then its Entry from the Routing
        Table will be Out.
         LSA 1 = Router LSA
                   Link-State ID = Originating Router ID
                   Any thing which is generated by New Added Router for DR is called Router LSA in Broadcast Network.
                   One Router LSA (Type 1) for every Router in an Area. Includes Directly Connected Interfaces/Links.
                   Identified by Router ID of Originating Router.
                   Floods within its Area only, do not Cross ABR.
                 -    LSA 1 (Router LSA) Link Types,
                      CONNECTION                                 LINK-STATE ID
                      Point-to-Point                             Neighbor Router ID
                      Connection to a Transit Network            DR IP Address
                      Connection to a Stub Network IP Network/Subnet Number
                      Virtual Link                               Neighbor Router ID
                 A Stub Network is a Dead-End-Link has only One Router Attached.
                 LSA 1 describes whether Router is ABR/ASBR.
          LSA 2 = Network LSA
                   Link-Stat ID = IP Interface Address of DR that advertises it.
                   Any thing which is generated by DR is called Network LSA.
                   LSA 2 is generated for every Transit/Broadcast/Non-Broadcast Network within an Area.
                   Transit Network has at least Two Directly Attached OSPF Routers. i.e. Multi-access Network like Ethernet is an
                   example for Transit Network.
             LSA ¾ = Summary LSA ¾
                 Link-State ID = Router ID of Described ASBR.
                 Any thing which is generated by ABR.
                 -    All information is not sent about Networks from an Area to other, but only Required/Important Information in
                      form of Summary is sent by ABR.
                      Summary = LSA 3
                 -    Also an ASBR is attached to An Area; only its Information sent to ABR is, like Summary of External Networks
                      sent to ABR.
                      LSA 4 = External Network Summary
                      O               ----- ----       LSA 1 , LSA 2
                      OIA             ----------       LSA ¾
                      OEx             ----------        LSA 5
                 -    Type LSA 4 is needed to find the ASBR.
                 -    By default, Router are not Summarized.
             LSA 5 = External LSAs
                 Link-State ID = External Network Number
                 Any thing which is generated by ASBR.
             LSA 6 = Special LSAs, used for Multicast OSPF Applications.
             LSA 7 = used for NSSAs.
             LSA 8 = used for Internetworking between OSPF & BGP.
             LSA 9, 10, 11 = Opaque LSAs
             Used for Future OSPF Application Specific-Purposes, i.e. Opaque LSAs used for MPLS with OSPF.

       A#sh ip ospf database
       Four Types of LSAs are in View, after this command given.
       Following information which also exists in Output of above Command.
       Link ID = Identify each LSA.
       ADV Router = Router, that advertises LSA.
       AGE = Maximum age 1 Hour.
       Sequence # = Number Begins from 0x80000001.
       Checksum = Total Number of Directly attached Links used only on Router LSA (Type 1).
       Link-Count includes all Point-to-Point, Transit and Stub Links.
        Point-to-Point Includes as 2 (written under Link-Count), and All other Count as 1 including Ethernet Link.
       O        =         Intra Area
         OIA      =        Inter Area
         OE1      =        Type 1 External, Networks of Routers which are outside of OSPF Autonomous System.
         OE2      =        Type 2 External, Advertised by way of External LSA.
        Cost of an External Route depends on External Type configured on ASBR.
        E1 (OE1) External Routes Calculate cost by adding External Cost to Internal Cost of each Link that Packet Crosses. Use this
        Type when there is Multiple ASBRs advertising an External Route to Same/One OSPF Autonomous System to avoid
        Suboptimal Routing.
        E2 (OE2): By Default enabled.
        External Cost only is used by External Routes; Use this Type if there is only one ASBR is Advertising External Routes to OSPF
        Autonomous System.
        A#sh ip route
        [110/50] Here, 110 = Admin Distance & 50 = Total cost of Route to a Specific Destination Network.
        A(fg-router)#max-lsa ?
         Other Options Too.
        Excessive LSAs generated by other Routers can Drain Local Router Resources.
        This Feature can limit the Processing of Non-Self-Generated LSAs on the Router for a defined OSPF Process.
        Dijkstra’s/SPF Algorithm determine best Path by adding all Link Costs along a Path.
        The Cost/Metric is Indication to send Packets over an Interface. Default is 100 Mbps/Bandwidth in Mbps.
        A(fg-if)#ip ospf cost 110
        A(fg-router)#auto-cost reference-bandwidth 10 (in Mbps)
        Sets Reference Bandwidth to Values than 100 Mbps.
        When you use Bandwidth to determine Cost Remember for Exact cost in Kbps.
        A(fg-if)#bandwidth 64
        If Interfaces Faster than 100 Mbps, you should use “Auto-Cost Reference-Bandwidth” Command, on All Routers in Network
        to ensure accurate Route Calculation in Mbps.
        Route Summarization is used to Summarize Routes at Area & Autonomous System Boundaries.
        It reduces OSPF Link-State Flooding Advertisements and Link-State Database (LSDB) & Routing Table Sizes.
        OSPF Network can scale to very Large Sizes in Parts because of Route Summarization.

       Minimizes Number of Routing table Entries.
       Localizes Impact of a Topology.
       Reduces LSA Type ¾ & LSA Type 5 Flooding and Saves CPU Resources.
       With Route Summarization, only Summarized Routes can propagate into Backbone (Area 0).
       By Default, LSA (Type 3/4) & External LSA (Type 5) do not Contain Summarized routes.
       Inter Area Route Summarization occurs on ABRs and Applies on Routers from within each Area. It does not apply on
       External Routes Injected into OSPF via Redistribution.
       External Router Summarization is a Specific to External Routes Injected into OSPF via Redistribution.
       It is Important to ensure the Contiguity of External Addresses Ranges that are being Summarized.
       Only ASBRs generally Summarize External Routes.
       OSPF is a Classless Routing Protocol; it carries Subnet Mask Information with Route Information.
       Therefore, OSPF Supports Multiple Subnet Masks for the Same Major Network, as VLSM.
       Discontigeous Subnets are also supported by OSPF, because Subnet Masks are part of the LSDB.
       RIPv1 & IGRP do not support VLSM or Discontigeous Subnets.
       If a Major Network Crosses the Boundaries of OSPF and RIPv1/IGRP domain, then VLSM Information redistributed into
       RIPv1/IGRP is Lost. And Static Routes may have to be configured in the RIP/IGRP domain.
       Inter-Area Summary Link Carries Mask.
       One/More entries can Represent Several Subnets.
       OSPF does not perform Automatic Summary on Major Network Boundaries.
       Use “Area Range” Command to instruct the ABR to Summarize Routes for a Specific Area before injecting them into a
       different Area via the Backbone as Type 3 LSAs.
       Cisco IOS Software creates a Summary Route to Interface Null 0, when Manual Summarization is configured, to prevent
       routing Loops.
       If Summarizing Router receives a Packet to an Unknown Subnet that is part of Summarized Route, Packet Matches
       Summary route based on the Longest Match.
       Packet is Forwarded to Null 0 Interface/Dropped, which prevents the Router from forwarding the Packet to a default Route
       and possibly creating a Routing Loop.
       Area Range = For ABR, Summary-address = For ABR/ASBR.
       You may not want to Summarize Area 0 Networks into other Areas. If, you have More than one ABR between an Area &
       Backbone Area. Type 3 LSA ensures Shortest Path into an Area ensures Shortest Path to a Destination.
       A Default Route is injected into OSPF as an External LSA Type 5.
       Default Route Distribution is not on By Default, use the “default-information originate” Command under OSPF.
       Normally, this Command advertises default into OSPF Network only if default Routes already exist in Routing Table.
       The “Always” keyword allows the default to be advertised even when the default route does not exist in Routing
       DEFAULT ROUTE CONFIGURATION (Running-Configuration)
                 Router ospf 100
                 Network area 0
                 Default-information originate metric 10
                 ip route
       The Purpose behind Special Area types is to Inject Default Routes into an Area so, that External Routes and Summary Link-
       State advertisements (LSAs) are not flooded in.
       Stub Areas are designed to reduce the Amount of Flooding LSDB Size, Routing Table Size in Routers within the Area.
       Standard Area, Accepts Link Updates, Route Summaries, External Routes.
       Backbone Area/Transit Area, The Central Entity to which all other areas connect. The Backbone Area Labeled O. All Other
       Connect to this Area to exchange and Route Information. OSPF Backbone includes all Properties of Standard Area.
       Stub Area, does not Accept External Routes (LSA 5), if Routers need to Route Networks Outside the OSPF Autonomous
       System, they use default Route
       Totally Stub Area, does not accept External Routes (LSA 5) & Summary Routes (LSA ¾); Default Route is used for
       External to OSPF Autonomous System.
       ***STUB/Totally Stub Areas can not contain ASBRs Except that ABRs may also be ASBRs.
       NSSA (OSPF RFC.)
       Defines Special LSA Type 7; Benefits Similar to Stub/Totally Stub Area. However, NSSAs allow ASBRs which is against the
       Rule in a Stub Area.
       There is a Single ABR, if there is more than one ABR Suboptimal Routing Paths to other Areas or External Autonomous
       Systems are accepted.
       All Routers in an Area are configured as Stub Routers
       There is not ASBR in the Area.
       The Area is not Area 0.
       No Virtual-Links go through the Area.
       A(fg-router)#area 1 stub/no-summary
                 -    This Command Turns on Stub Area Networking.
                 -    All Routers in a Stub Area must use the Stub Command.
       A(fg-router)#area 1 default cost 10
                 -    This Command defines the Cost of Default Route sent into the Stub Area.
                 -    The Default Cost is 1.
       Default Route is Denoted with an O*IA.
       Note that “No-Summary” Extension with Stub Command is used only on ABRs.
       Type 7 LSA is described in Routing Table as ON2/ON1 like OE2/OE1.
       A(fg-router)#area 1 nssa
                 -    Use this Command instead of the “Area Stub” Command to define the Area as NSSA.
                 -    The No-Summary key word creates an NSSA Totally Stubby Area; This is a Cisco Proprietary Feature.
                      o A(fg)#router ospf 1
                      o A(fg-router)#area 1 nssa
                      o #area 1 nssa default-information-generate
                           Option “default-information-generate” is used on NSSA ABR.
                                    i.e. NSSA Totally Stubby Configuration
                                        o    A(fg-router)#area 1 nssa no-summary
                                             Works exactly like the Totally Stubby Technique. A Single Default Route is used for both
                                             Inbound –to-External (Type 7 LSAs) and (Type ¾ Summary LSAs) in the Area.
                                             Option “default-information-generate” is not required here.
       All Other Routers in NSSA need to Configure “area 1 nssa”
       Cisco Proprietary like Totally Stubby Area.
       SHOW Commands for STUB & TOTALLY STUBBY AREA.
                          -   A#sh ip ospf            (Areas Normal, STUB, NSSA)
                          -   A#sh ip ospf database (LSAs Detail)
                          -   A# sh ip ospf database nssa-external (Specific Detail for Each LSA Type 7.)
                          -   A#sh ip route           (All Routes used for Checking Routing Table)
       OSPF Neighbor Authentication /Neighbor Router Authentication/Route Authentication can be configured such that Routers
       can participate in Routing based on Predefined Passwords.
            • MD5 AUTHENTICATION
                      -   Router Generates and Checks every OSPF Packet. Router Authenticates the Source of each Routing
                          Update Packet that it receives.
                      -   Configure a KEY/PASSWORD for each Participating Neighbor Must has same key Configured.
                          CONFIGURING SIMPLE PASSWORD
                          *A(fg-if)#ip ospf authentication-key HUSSAIN
                          *A(fg-if)#ip ospf authentication [Message-digest] ! [Null]     (For IOS 12.0)
                          *A(fg-router)#area 1 authentication [message-digest]           (For IOS 12.0)
                          *Password will contain only on First 8 Characters.
       If the “service password-encryption” not used Key will Store as Plain Text. And if “service password-encryption” is used Key
       will be Store and Display in Encryptions Form. And it will be Encryption –Type 7.
       If the Authentication for Interface is not being use, Authentication for Area could be use; Default for Area is Null
       A#sh ip ospf neighbor
       With this Command, in State Checking “FULL/” We can guess that Routers have Made Fully Adjacency with Same Passwords
       enabled Routers.
       “ping & show ip route” could also be used. For Neighbor Adjacency Checking.
       The Password created by this Command is used as a KEY; which is generated directly into the OSPF Header, when Routing
       Protocol Originates a Packet. A Separate Password assigned to Each Network on a per-interface bases. All Neighbor Routers
       on Same Network must have the Same Password to Exchange Information.

                          CONFIGURING MD5 PASSWORD
                          *Warning Message, if Configured Large than 16 Character.
                          *A(fg-if)#ip ospf message-digest-key key-id md5 key
                          *A(fg-if)#ip ospf authentication [Message-digest] ! [Null]    (For IOS 12.0)
                          *A(fg-router)#area 1 authentication [Message-digest]

        The “key and key-id” in this Command is used to Generate a Message digest Hash Value of Each Packet.
        A Separate Password is assigned to each Network on a Per Interfaces bases.
        “Key-id” is used for Un-interrupted Transition between Keys.
        Roll Over detects New-Key and Stops duplicate Packets Receiving.
        The Old Key Must to Remove.
        - A(fg-if)#ip ospf message-digest-key 100 md5 OLD
        - #if ospf message-digest-key 101 md5 NEW
        #no ip ospf message-digest-key 100
        I.e.       Configuring MD5 Authentication
                   - A(fg-if)#ip ospf authentication message-digest
                   -#ip ospf message-digest-key 1 md5 HUSSAIN
        -A#debug ip ospf adj

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