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OSPF Fundamental

This slide contains the basic and advanced concept of OSPF routing protocol, according to the latest version of Cisco books, and I presented it at IRAN TIC company. In the next slide, I will upload an attractive advanced feature about OSPF.

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OSPF Fundamental

  1. 1. Fundamental of IGP Protocols-OSPF by : Reza Farahani 1
  2. 2. Routing Protocol  Interior Gateway Protocol (IGP): A routing protocol that was designed and intended for use inside a single autonomous system (AS).  Exterior Gateway Protocol (EGP): A routing protocol that was designed and intended for use between different autonomous systems. 2
  3. 3. Metrics 3
  4. 4. ADMINISTRATIVE DISTANCE 4
  5. 5. IGP ALGORITHMS  Distance vector (sometimes called Bellman-Ford after its creators)  Link-state  Balanced hybrid (sometimes called enhanced distance vector) 5
  6. 6. IGP ALGORITHMS 6
  7. 7. DISTANCE VECTOR  A distance-vector routing protocol sends a full copy of its routing table to its directly attached neighbors.  This is a periodic advertisement, even if there have been no topological changes, a D.V routing protocol will, re-advertise its full routing table to its neighbors.  Layer 3 LOOP  Split Horizon  Route poisoning  Poison Reverse  Triggered update  Hold-down timer 7
  8. 8. LINK-STATE  Routers use this information, stored in RAM inside a data structure called the link- state database (LSDB), to calculate the currently best routes to each subnet.  The updates containing information called link-state advertisements (LSA). 8
  9. 9. LINK-STATE(CO) 9  This process uses something called the Dijkstra Shortest Path First (SPF) algorithm.  LSDB works like the map, and the SPF algorithm works like the person reading the map.  Each router uses itself as the starting point.
  10. 10. SUMMARY OF CATEGORIES 10
  11. 11. CLASSLESS & CLASSFUL 11  Some routing protocols must consider the Class and other routing protocols can ignore Class A,B, and C rules altogether.
  12. 12. OSPF LINK STATE CONCEPTS  OPSF uses link state (LS) logic, which can be broken into three major branches.  RFC2328  The first step, neighbor discovery  The second step, topology database exchange  Each router stores topology information in its topology database (LSDB).  The third major step, route computation  OSPF uses SPF algorithm to analyze the data, choose the best route for each reachable subnet, and add the correct information for those routes to the IP routing table 12
  13. 13. HIERARCHICAL DESIGN 13
  14. 14. OSPF TERMS 14
  15. 15. OSPF TERMS CO 15
  16. 16. Router ID 16  Step 1:  Use the router ID defined in the router-id x.x.x.x OSPF router subcommand.  Step 2:  Use the highest IP address of any up/up loopback interface.  Step 3:  Use the highest IP address of any up/up non-loopback interface.
  17. 17. OSPF CONFIGURATION 17
  18. 18. OSPF VERIFICATION  Command: show ip ospf  Show information on a variety of general OSPF and area state and configuration information.  show ip ospf interface [INTERFACE]  Show state and configuration of OSPF the specified interface, or all interfaces if no interface is given.  Command: show ip ospf route  Show the OSPF routing table, as determined by the most recent SPF calculation. 18
  19. 19. OSPF VERIFICATION 19  Command: show ip ospf neighbor  Command: show ip ospf neighbor INTERFACE  Command: show ip ospf neighbor detail  Command: show ip ospf neighbor INTERFACE detail
  20. 20. OSPF VERIFICATION  Command: show ip ospf database  Command: show ip ospf database max-age 20
  21. 21. OSPF VERIFICATION 21
  22. 22. Linux Distributions 22
  23. 23. OSPF FEATURE SUMMARY 23
  24. 24. OSPF FEATURE SUMMARY 24
  25. 25. ADJACENCIES ON LANS 25  OSPF sends multicast OSPF Hello messages on LAN interfaces, to discover OSPF neighbors, when two requirements are met:  OSPF has been enabled  The interface has not been made passive by the passive-interface router subcommand
  26. 26. Passive Interface  When a router configures an interface as passive to OSPF, OSPF quits sending OSPF Hellos, so the router will not discover neighbors.  The router will still advertise about the interface’s connected subnet if OSPF is enabled on the interface 26
  27. 27. HELLO MESSAGE FIELDS  OSPF Router ID  Hello interval  Dead interval  Subnet mask  List of neighbors reachable on the interface  Area ID  Router priority  Designated Router (DR) IP address  Backup DR (BDR) IP address  Authentication digest 27
  28. 28. NEIGHBOR REQUIREMENTS 28
  29. 29. OPTIMIZING CONVERGENCE  The Dead interval defines how long a router should wait, without hearing any Hello messages from that neighbor, before deciding that the neighbor failed.  The neighbor resets its downward-counting Hold timer to 40 upon receiving a Hello from that neighbor  To tune for faster convergence, you can configure OSPF to set a lower Hello and Dead timer.  If the interface fails, OSPF will immediately realize that all neighbors reached through that interface have also failed and not wait on the Dead timer 29
  30. 30. OPTIMIZING CONVERGENCE 30  Interface Command: ip ospf dead-interval <1-65535>  Interface Command: ip ospf dead-interval minimal hello-multiplier <2-20>
  31. 31. OPTIMIZING CONVERGENCE 31 ip ospf hello-interval <1-65535> Interface Command: no ip ospf hello-interva
  32. 32. Regular Routing Method bottle-necks 32  Interface Command: ip ospf retransmit-interval <1-65535>  Interface Command: no ip ospf retransmit interval
  33. 33. OSPF AUTHENTICATION  Enabling per interface  Enabling on all interfaces in an area  Clear text per area  OSPF Command: area <0-4294967295> authentication  Enable on interface by : ip ospf authentication-key AUTH_KEY 33
  34. 34. OSPF AUTHENTICATION  MD5 per area :  area #n authentication message-digest  Enable on interface by : ip ospf message-digest-key KEYID md5 KE  Per Interface  ip ospf authentication-key AUTH_KEY  ip ospf authentication message-digest  Note: The maximum length of the key is 16  Show ip ospf interface 34
  35. 35. DR AND BDR  The end result of the DR election is that topology information is exchanged only between special neighbors not all. 35
  36. 36. DR AND BDR  The DR concept prevents overloading a subnet with too much OSPF traffic when many routers are on a subnet.  Because the DR is so important to the exchange of routing information, the loss of the elected DR could cause delays in convergence  OSPF includes the concept of a Backup All routers except the DR and BDR are typically called “DROther” in IOS show command output.  on each subnet, so when the DR fails or loses connectivity to the subnet, the BDR can take over as the DR. 36
  37. 37. DR AND BDR Election  The router sending the Hello with the highest OSPF priority setting becomes the DR.  If two or more routers tie with the highest priority setting, the router sending the Hello with the highest RID wins.  The router with the second-highest priority becomes the BDR.  After the DR and BDR have been elected, the new better candidate does not preempt the existing DR/BDR 37
  38. 38. DR AND BDR Command 38  Interface Command: ip ospf priority <0-255>  The range of priority values that allow a router to be a candidate are 1 ~ 255
  39. 39. OSPF MESSAGES 39
  40. 40. OSPF MESSAGES 40
  41. 41. OSPF NEIGHBOR STATES 41
  42. 42. STATIC DISCOVERY  For example in NBMA Networks : Frame Relay …  Interface Command: ip ospf network (broadcast|non-broadcast|point-to- multipoint|point-to-point)  In our device like cisco LoopBack advertise by /32 and we can use above command for change real define subnet loopback interface 42
  43. 43. OSPF NETWORK TYPES 43
  44. 44. LSA TYPES 44
  45. 45. LSA TYPES 45
  46. 46. LSA TYPE 1  An LSA type 1, called a router LSA, identifies an OSPF router based on its OSPF router ID (RID).  Each router creates a Type 1 LSA for itself and floods the LSA throughout the same area.  The Type 1 LSA in one area will list only interfaces in that area and only neighbors in that area. 46
  47. 47. LSA TYPE 2 47  OSPF defines the Type 2 network LSA, used as a pseudonode
  48. 48. LSA TYPE 3  ABRs do not forward Type 1 and Type 2 LSAs from one area into another area, and vice versa  This convention results in smaller per-area LSDBs, saving memory and reducing complexity for each run of the SPF algorithm, which saves CPU and improves convergence time.  OSPF advertises inter-area routes using the Type 3 summary LSA.  Type 3 summary LSAs do not contain all the detailed topology information, so in comparison to Types 1 2, these LSAs summarize the information–hence the name summary LSA. 48
  49. 49. PERIODIC FLOODING  OSPF does reflood each LSA every 30 minutes based on each LSA’s age variable.  If 30 minutes pass with no changes to an LSA resets the timer to 0, and refloods the LSA.  Note also that when a router realizes it needs to flush an LSA from the LSDB for an area, it actually sets the age of the LSA to the MaxAge setting (3600) and refloods the LSA. 49
  50. 50. BEST OSPF ROUTES  Analyze the LSDB to find all possible routes to reach the subnet.  Note that OSPF supports equal-cost load balancing.  Lower cost but SPECIAL RULES :  Step 1.When choosing the best route, an intra-area route is always better than a competing interarea route, regardless of metric  If an ABR learns a Type 3 LSA inside a nonbackbone area, the ABR ignores that LSA when calculating its own routes 50
  51. 51. METRIC TUNING 51 OSPF Command: auto-cost reference-bandwidth <1-4294967> Interface Command: ip ospf cost <1-65535>
  52. 52. OSPF Virtual Links 52  In some cases two backbone areas exist; in other cases, a non-backbone area may not have a point of connection to the backbone area.
  53. 53. OSPF Virtual Links 53  Extend Backbone Area but  Change use unicast packet  It’s temporary
  54. 54. TYPE 3 LSA FILTERING  On ABR1, filter subnet 3 from being advertised  On ABR2, filter both subnet 2 and 3 54
  55. 55.  When “in” is configured, IOS filters prefixes being created and flooded into the configured area  When “out” is configured, IOS filters prefixes coming out of the configured area. 55
  56. 56. ROUTE SUMMARIZATION  OSPF allows summarization at both ABRs and ASBRs but not on other OSPF routers  The main reason is again that the LSDB must be the same for all routers in a single area. 56
  57. 57. END OF PART 1 Next session : Redistribution Other LSA Types OSPF V3 Thanks for your Attention 57

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  • RezaFarahani3

    Mar. 8, 2018

This slide contains the basic and advanced concept of OSPF routing protocol, according to the latest version of Cisco books, and I presented it at IRAN TIC company. In the next slide, I will upload an attractive advanced feature about OSPF.

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