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Open Shortest Path First

Open Shortest Path First. Pedro Tsao E-mail:Pedro_Tsao@dlink.com.tw. Link state OSPF Areas OSPF behavior OSPF Packets Type OSPF Network Type OSPF LSA Type OSPF Route Summarization Configuring OSPF Special Area types Configuring OSPF Authentication. Agenda. Link State. Link State.

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Open Shortest Path First

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  1. Open Shortest Path First Pedro TsaoE-mail:Pedro_Tsao@dlink.com.tw

  2. Link state OSPF Areas OSPF behavior OSPF Packets Type OSPF Network Type OSPF LSA Type OSPF Route Summarization Configuring OSPF Special Area types Configuring OSPF Authentication Agenda

  3. Link State

  4. Link State • The information available to a distance vector router has been compared to the information available from a road sign. Link state routing protocol are like a road map. • Link State routing include the following: • Open Shortest Path First (OSPF) for IP • The ISO’s Intermediate System-to-Intermediate System (IS-IS) for CLNS and IP • DEC’s DNA Phase V • Novell’s NetWare Link Services Protocol (NLSP)

  5. OSPF as a Link-State Protocol • OSPF propagates link-state advertisements rather than routing table updates • LSAs are flooded to all OSPF routers in the area • The OSPF link-state database(LSDB) is pieced together from the LSAs generated by the OSPF routers • OSPF use the SPF algorithm to calculate the shortest path to a destination • Link = router interface • State = description of an interface and its relationship to neighboring routers

  6. Link State Data Structures • Neighbor table: • Also known as the adjacency database • Contains list of recognized neighbors • Topology table: • Typically referred to as LSDB • Contains all routers and their attached links in the area or network • Identical LSDB for all routers within an area • Routing table: • Commonly named a forwarding database • Contains list of best paths to destinations

  7. Link State Routing Protocol • Link-state routers recognize more information about the network • than their distance vector counterparts. • Each router has a full picture of the topology. • Consequently, link-state routers tend to make more accurate decisions.

  8. OSPF Areas

  9. Link State Data Structure: Network Hierarchy • Link-state routing requires a hierachical networkstructure • that is enforced by OSPF. • This two-level hierarchy consists of the following: • Transit area (backbone or area 0) • Regular areas (non-backbone areas)

  10. OSPF Areas Backbone Area • 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 Area3 Area1 Area2

  11. OSPF Terminology Backbone Area • Router A and B are backbone routers • Backbone routers make up area 0 • Router C, D and E are known as Area Border Routers (ABRs) • ABRs attach all other areas to area 0 Area3 Area1 Area2

  12. OSPF Behavior

  13. OSPF Adjacencies Hello • Routers discover neighbors by exchange hello packets • Routers declare neighbors to be up after checking certain parameters or options in the hello packet

  14. Forming OSPF Adjacencies • Point-to-point WAN links: • Both neighbors become full adjacent • LAN Links • Neighbors form a full adjacency with the DR and BDR • Routers maintain two-way state with the other routers (DROTHERs) • Routing updates and topology information are passed only between adjacent routers • Once an adjacency is formed, LSDBs are synchronized by exchanging LSAs • LSAs are flooded reliably through the area (or network)

  15. 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 • In descending other of specificity, the Router-id may be one of following: • Router-id command • Highest loopback address • Highest Active IP address

  16. OSPF DR/BDR Election • DR/BDR will be electing by the following rules: • The router With Highest priority value is the DR • The router with the second highest priority value is BDR • In case of a tie. The highest Router ID is DR, the second is BDR • A router with priority of 0 cannot be the DR or BDR • A router that’s not DR or BDR is a DROther • If a router with higher priority comes into the network, it does not preempt the DR or BDR

  17. OSPF Calculation • Routers find the best paths to destinations by applying Dijkstra’s SPF algorithm to link-state database as follows: • Every router in an area has the identical link-state DB • Each router in the area places itself into the root of the tree that is built • The best path is calculated with respect to the lowest total cost of links to a specific destination • Best routes are put into the forwarding database(routing table)

  18. OSPF Calculation(cont.) Link-state DB Shortest Path x x B A B A Dijkstra’s algorithm C C D D E E F F G H G H Assume all links are Ethernet, with an OSPF cost of 10

  19. OSPF Packets Type

  20. OSPF Packet Types 1.Hello 2.Destination Description 3.Link-State Request 4.Link-State Update 5.Link-State Acknowledgement

  21. Neighborship: The Hello Packet Hello • Router ID • Hello and dead intervals • Neighbors • Area ID • Router priority • DR IP address • BDR IP address • Authentication password • Stub area flag hello • Entry must match on neighboring routers

  22. Establishing Bidirectional Communication Port2 172.16.5.1/24 B A Port1 172.16.5.2/24 Down state hello I am router id 172.16.5.1, and I see no one To 224.0.0.5 Initial State Router B neighbor List 172.16.5.1/24,in Port2 Unicast to A hello I am router id 172.16.5.2, and I see 172.16.5.1 Router A neighbor List 172.16.5.2/24,in Port1 Two-way State

  23. Discovering the Network Routes Port2 172.16.5.1/24 B A Port1 172.16.5.2/24 Exstart state DBD I will start exchange because I have router id 172.16.5.1 DBD No, I’ll start exchange because I have a higher RID exchange State DBD Here is a summary of my LSDB DBD Here is a summary of my LSDB

  24. Adding the Link-State Entries Port2 172.16.5.1/24 B A Port1 172.16.5.2/24 LSAck LSAck Thanks for the information! Loading state LSR I need complete entry for network 172.16.6.0/24 LSU Here is the entry for network 172.16.6.0/24 LSAck Thanks for the information! Full State

  25. int2172.17.2.1/24 int1172.17.1.1/24 Area 0 Case Study: OSPF Packets int110.1.1.3/24 Router ID: 2.2.2.2 System10.1.1.2/24 Router ID: 1.1.1.1 int1192.168.1.1/24 int2192.168.2.1/24 Area 2

  26. Link-State Data Structures: LSA Operation Is seq# the same? IS entry in LSDB? YES LSA Ignore LSA YES NO NO Is seq# higher? YES Add to DB Send LSAck NO Flood LSA Send LSU with newer information to source Run SPF to calculate new routing table END END

  27. Maintaining Routing Information 3 DR 1 2 B A • Router A notifies all OSPF DRs on 224.0.0.6 • DR notifies others on 224.0.0.5

  28. Command (OSPF)

  29. Case Study: OSPF Configuration(D-Link) int2172.17.2.1/24 int1172.17.1.1/24 Area 0 DES-3852 Router ID: 2.2.2.2 System10.1.1.3/24 System10.1.1.2/24 DES-3828P Router ID: 1.1.1.1 int1192.168.1.1/24 int2192.168.1.1/24 Area 2

  30. int2172.17.2.1/24 int1172.17.1.1/24 DES-3852 # OSPF config ospf ipif System area 0.0.0.0 state enable config ospf ipif int1 area 0.0.0.0 state enable config ospf ipif int2 area 0.0.0.0 state enable config ospf router_id 2.2.2.2 enable ospf System10.1.1.3/24 System10.1.1.2/24 DES-3828P # OSPF create ospf area 0.0.0.2 type normal config ospf ipif System area 0.0.0.0 state enable config ospf ipif int1 area 0.0.0.2 state enable config ospf ipif int2 area 0.0.0.2 state enable config ospf router_id 1.1.1.1 enable ospf int1192.168.1.1/24 int2192.168.1.1/24

  31. int2172.17.2.1/24 int1172.17.1.1/24 DES-3852 # OSPF config ospf ipif System area 0.0.0.0 state enable config ospf ipif int1 area 0.0.0.0 state enable config ospf ipif int2 area 0.0.0.0 state enable config ospf router_id 2.2.2.2 enable ospf System10.1.1.3/24 System10.1.1.2/24 DES-3828P # OSPF create ospf area 0.0.0.2 type normal config ospf ipif System area 0.0.0.0 state enable config ospf ipif int1 area 0.0.0.2 state enable config ospf ipif int2 area 0.0.0.2 state enable config ospf router_id 1.1.1.1 enable ospf int1192.168.1.1/24 int2192.168.1.1/24

  32. int2172.17.2.1/24 int1172.17.1.1/24 DES-3852 # OSPF config ospf ipif System area 0.0.0.0 state enable config ospf ipif int1 area 0.0.0.0 state enable config ospf ipif int2 area 0.0.0.0 state enable config ospf router_id 2.2.2.2 enable ospf System10.1.1.3/24 System10.1.1.2/24 DES-3828P # OSPF create ospf area 0.0.0.2 type normal config ospf ipif System area 0.0.0.0 state enable config ospf ipif int1 area 0.0.0.2 state enable config ospf ipif int2 area 0.0.0.2 state enable config ospf router_id 1.1.1.1 enable ospf int1192.168.1.1/24 int2192.168.1.1/24

  33. Case Study: OSPF Configuration(Cisco) Fa1/188.88.88.88/24 Fa1/310.1.3.2/24 Area 0 Fa1/210.1.2.1/24 Fa1/310.1.3.1/24 Area 1 Fa1/210.1.2.1/24 Fa1/1172.31.1.2/24 Fa1/1172.31.1.1/24 SW1 Fa1/299.99.99.99/24 SW3 SW2 Area 3 • X is the Switch Number • Each Switch has a loopback: X.X.X.X/32 except SW3 SW4

  34. OSPF Network Type

  35. OSPF Network Type • The three types of networks defined by OSPF are: • Point-to-point: A network that joins a single pair of routers • Broadcast: A multiaccess broadcast network, such as Ethernet • Non-Broadcast multiaccess(also called NBMA): A network that interconnects more than two routers but that has no broadcast capability. Frame Relay, X.25 and ATM are examples of NBMA networks

  36. Point-to-point Links • Usually a serial interface running either PPP or HDLC • May also be a point-to-point interface running Frame Relay or ATM • No DR or BDR election required • OSPF autodetects this interface type • OSPF packets are send using multicast 224.0.0.5

  37. Broadcast Network • Generally these are LAN technologies like Ethernet and Token Ring • DR and BDR election are required • All neighbor routers form full adjacencies with the DR and BDR only • Packets to the DR and BDR use 224.0.0.6 • Packets from DR to all other routers use 224.0.0.5

  38. Electing the DR and BDR Hello • Hello packets are exchange via IP multicast • The router with the highest priority is selected as the BR. The second-highest one is the BDR • Use the OSPF RID as the tie breaker • The DR election is nonpreemptive

  39. OSPF Network Type summary

  40. OSPF Router Type ABR and Backbone Router Area0 Backbone/Internal routers Internal routers Internal routers ABR and Backbone Router Area1 ExternalAS Area2 ASBR and Backbone Router

  41. OSPF Virtual Link Area1 172.16.0.0 Area0 10.0.0.0 Area0 10.0.0.0 Virtual Link A B • Virtual links are used to connect a discontinuous area to area 0 • A logical connection is built between router A and router B • Virtual links are recommended for backup or temporary connections

  42. Case Study: OSPF Virtual Link (D-Link)

  43. Case Study: OSPF Virtual Link (Cisco)

  44. Q&A

  45. END

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