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OSPF

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  1. OSPF SAvPS 2009 Genči

  2. References • RFC2338 - OSPF Version 2. April 1998. • Moy, J.: „OSPF : anatomy of an Internet routing protocol“, Addison-Wesley Publishing Company, 1998 • Duncan Maidens: Routing in Large Networks with OSPF and beyond with BGP. Presentation at ACADEMY CONFERENCE 2005 • NetAcad&CATC OSPF presentation • OSPF Design Guide. Document ID: 7039 (Search on CISCO web page)

  3. OSPF • OSPF = Open Shortest Path First • The OSPF routing protocol is the most important link state routing protocol on the Internet • The complexity of OSPF is significant • History: • 1989: RFC 1131 OSPF Version 1 • 1991: RFC1247 OSPF Version 2 • 1994: RFC 1583 OSPF Version 2 (revised) • 1997: RFC 2178 OSPF Version 2 (revised) • 1998: RFC 2328 OSPF Version 2 (current version)

  4. [2]

  5. Representation of routers and networks [1] • The Autonomous System's link-state database describes a directed graph. • The vertices of the graph consist of routers and networks. • A graph edge connects two routers when they are attached via a physical point-to-point network. • An edge connecting a router to a network indicates that the router has an interface on the network.

  6. Representation of routers and networks [1] • The neighborhood of each network node in the graph depends on the network's type (point-to-point, broadcast, NBMA or Point-to-MultiPoint) and the number of routers having an interface to the network.

  7. Exterior Interior Distance vector Link state EGP BGP RIP v1RIP v2IGRPEIGRP OSPFIS-IS Position of OSPF among routing protocols

  8. OSPF RIP Example

  9. OSPF • Terminology

  10. OSPF Terminology Interfaces

  11. Neighbors OSPF Terminology Interfaces

  12. Neighbors OSPF Terminology Interfaces Cost = 10 Cost = 1785 Cost = 6

  13. Neighbors OSPF Terminology Interfaces Area 1 Cost = 10 Area 0 Cost = 1785 Cost = 6

  14. Neighbors OSPF Terminology Autonomous System Interfaces Area 1 Cost = 10 Area 0 Cost = 1785 Cost = 6

  15. Neighbors OSPF Terminology Autonomous System Interfaces Area 1 Cost = 10 Area 0 Cost = 1785 Cost = 6 Neighborship Database Lists Neighbors

  16. Neighbors Topology Database Lists All Routes OSPF Terminology Autonomous System Interfaces Area 1 Cost = 10 Area 0 Cost = 1785 Cost = 6 Neighborship Database Lists Neighbors

  17. Neighbors OSPF Terminology Autonomous System Interfaces Area 1 Cost = 10 Area 0 Cost = 1785 Cost = 6 Neighborship Database Lists Neighbors Topology Database Lists All Routes Routing Table Lists Best Routes

  18. 1 x x Maintaining Routing Information Link-State Change DR LSU B A Router A notifies all OSPF DRs on 224.0.0.6

  19. 2 1 x x Maintaining Routing Information Link-State Change DR LSU LSU B A • Router A notifies all OSPF DRs on 224.0.0.6 • DR notifies others on 224.0.0.5

  20. 2 1 x x 3 Maintaining Routing Information Link-State Change DR LSU LSU B A LSU • Router A notifies all OSPF DRs on 224.0.0.6 • DR notifies others on 224.0.0.5

  21. 2 1 x x 3 Maintaining Routing Information Link-State Change DR LSU I need to update my routing table. 4 LSU B A LSU • Router A notifies all OSPF DRs on 224.0.0.6 • DR notifies others on 224.0.0.5

  22. LSU LSA Maintaining Routing Information (cont.) Is entry inlink-statedatabase? No Add to database Send LSAck to DR Flood LSA Run SPF to calculate new routing table End

  23. LSU LSA Maintaining Routing Information (cont.) Is entry inlink-statedatabase? Is seq. # the same? Ignore LSA Yes Yes No Add to database Send LSAck to DR Flood LSA Run SPF to calculate new routing table End

  24. LSU LSA Maintaining Routing Information (cont.) Is entry inlink-statedatabase? Is seq. # the same? Ignore LSA Yes Yes No No Add to database Is seq. # higher? Send LSAck to DR No Send LSU with newer information to source Flood LSA Run SPF to calculate new routing table End End

  25. LSU LSA Maintaining Routing Information (cont.) Is entry inlink-statedatabase? Is seq. # the same? Ignore LSA Yes Yes No No Add to database A Go to A Is seq. # higher? Yes Send LSAck to DR No Send LSU with newer information to source Flood LSA Run SPF to calculate new routing table End End

  26. Purpose of this Lesson • Coverage of topics new to the “OSPF” module of BSCI. • What’s new in this module? • Some new terminology and acronyms • More detailed explanation of LSAs and the Link-state Database • Several new OSPF configuration and verification commands • OSPF authentication configuration and verification commands

  27. New Terminology

  28. OSPF Areas Review of OSPF area characteristics: • Minimizes routing table entries • Localizes impact of a topology change within an area • Detailed LSA flooding stops at the area boundary • Requires a hierarchical network design

  29. OSPF Areas New terminology for areas: • Transit Area • Also known as Backbone Area 0 • Regular Area • Also known as Nonbackbone areas

  30. OSPF Database • OSPF maintains three databases • Adjacency Database (show ip ospf neighbor) • Link-state Database (show ip ospf database) • Forwarding Database (show ip route)

  31. What is LSDB? • LSDB is an acronym for Link-state Database.

  32. LSAs and the Link-state Database

  33. LSA Sequence Numbering • Each LSA in the LSDB maintains a sequence number. • The sequence numbering scheme is a 4-byte number that begins with 0x80000001 and ends with 0x7FFFFFFF. • OSPF floods each LSA every 30 minutes to maintain proper database synchronization. Each time the LSA is flooded, the sequence number is incremented by one. • Ultimately, an LSA sequence number will wrap around to 0x80000001. When this occurs, the existing LSA is prematurely aged to maxage (one hour) and flushed. • When a router encounters two instances of an LSA, it must determine which is more recent. The LSA having the newer (higher) LS sequence number is more recent.

  34. The OSPF Link-State Database • The show ip ospf database command displays the current LSDB for the local router. • The next slide discusses the purpose of the Age and Seq# fields highlighted below. RTC#show ip ospf database OSPF Router with ID (192.168.1.253) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 192.168.1.249 192.168.1.249 1705 0x80000005 0x00D5B0 5 192.168.1.253 192.168.1.253 1578 0x80000006 0x009F91 5

  35. Link-State Data Structures: LSA Operation

  36. Verifying LSA Age and Sequence Number • In the first output below, notice the age timer will expire sometime after 1800 seconds or 30 minutes. RTC#show ip ospf database OSPF Router with ID (192.168.1.253) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 192.168.1.249 192.168.1.249 1705 0x80000005 0x00D5B0 5 192.168.1.253 192.168.1.253 1578 0x80000006 0x009F91 5 • A few minutes later, the router has received an LSU for both links. Note the refreshed age timer and incremented sequence number. RTC#show ip ospf database OSPF Router with ID (192.168.1.253) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 192.168.1.249 192.168.1.249 106 0x80000006 0x00D3B1 5 192.168.1.253 192.168.1.253 58 0x80000007 0x009D92 5

  37. Basic OSPF Configuration

  38. Configuring OSPF • An excellent resource for information on the many different OSPF configurations is the Cisco white paper, “Configuring OSPF”, which can be downloaded from the Cisco website: • http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.html

  39. Configuring Basic OSPF Router(config)# router ospf process-id[vrfvpn-name] • Enable one or more OSPF routing processes. Router(config-router)# network ip-address wildcard-maskarea area-id • Define the interfaces that OSPF will run on. Router(config-if)# ip ospf process-id area area-id [secondaries none] • Optional method to enable OSPF explicitly on an interface.

  40. Configuring OSPF for Multiple Areas

  41. OSPF Router ID • The router is known to OSPF by the OSPF router ID number. • LSDBs use the OSPF router ID to differentiate one router from the next. • By default, the router ID is the highest IP address on an active interface at the moment of OSPF process startup. • If no interface is up when the OSPF process starts, you will get the following error message: • p5r2(config)#router ospf 1 • 2w1d: %OSPF-4-NORTRID: OSPF process 1 cannot start. • A loopback interface can override the OSPF router ID. If a loopback interface exists, the router ID is the highest IP address on any active loopback interface. • The OSPF router-id command can be used to override the OSPF router ID. • Using a loopback interface or a router-id command is recommended for stability.

  42. OSPF router-id Command Router(config-router)# router-id ip-address • This command is configured under the router ospf [process-id] command. • Any unique arbitrary 32-bit value in an IP address format (dotted decimal) can be used. • If this command is used on an OSPF process that is already active, then the new router ID takes effect after the next reload or after a manual restarting of the OSPF process using: Router#clear ip ospf process Router(config)#router ospf 1 Router(config-router)#router-id 172.16.1.1 Router#clear ip ospf process

  43. NBMA OSPF Configuration

  44. OSPF over NBMA Topology Modes of Operation • RFC 2328-compliant modes are as follows: • Nonbroadcast (NBMA) • Point-to-multipoint • Additional modes from Cisco are as follows: • Point-to-multipoint nonbroadcast • Broadcast • Point-to-point Router(config-if)# ip ospf network [{broadcast | non-broadcast | point-to-multipoint [non-broadcast] | point-to-point}] • This interface command defines OSPF network type.

  45. RFC-compliant Non-broadcast Mode • One IP subnet. • Neighbors must be manually configured. • DR and BDR elected. • DR and BDR need to have full connectivity with all other routers. • Typically used in a full mesh topology. RTB(config-if)#ip ospf network non-broadcast -------- RTB(config-router)#network 3.1.1.0 0.0.0.255 area 0 RTB(config-router)#neighbor 3.1.1.1 RTB(config-router)#neighbor 3.1.1.3

  46. RFC-compliant Point-to-Multipoint Mode • One IP subnet. • Uses multicast OSPF hello packet to automatically discover neighbors. • DR and BDR not required • Typically used in a partial-mesh or hub-and-spoke topology. RTB(config-if)#ip ospf network point-to-multipoint -------- RTB(config-router)#network 3.1.1.0 0.0.0.255 area 0

  47. Cisco’s Point-to-Multipoint Non-broadcast mode • Cisco extension to RFC-compliant point-to-multipoint mode • Must statically define neighbors, like nonbroadcast mode • Like point-to-multipoint mode, DR/BDR not elected • Used in special cases where neighbors cannot be automatically discovered RTB(config-if)#ip ospf network point-to-multipoint non-broadcast -------- RTB(config-router)#network 3.1.1.0 0.0.0.255 area 0 RTB(config-router)#neighbor 3.1.1.1 cost 10 RTB(config-router)#neighbor 3.1.1.3 cost 20

  48. Cisco’s Broadcast Mode • Makes a WAN interface appear to be a LAN • One IP subnet • Uses multicast hellos to discover neighbors • DR and BDR elected • Requires a full mesh. RTB(config-if)#ip ospf network broadcast -------- RTB(config-router)#network 3.1.1.0 0.0.0.255 area 0