111 Open Shortest Path First OSPF  OSPF Overview  OSPF Operation By Grace Deng Oct

222 OSPF Overview History  Development began 1987 by IETF  Goal—a link state protocol more efficient and scaleable than RIP  Latest revision is RFC 2328 April 1998

333 OSPF Overview OSPF versus RIP  OSPF  Link state  Efficient routing updates (sends changes only)  No hop count limit  Fast Convergence  Supports VLSM  Path selection based on bandwidth  RIP  Distance vector  Copies entire routing table  Hop count limit of 15  Hold-down timers to prevent routing loops  Does not advertise sub- net masks  Uses only hop count as metric

444 OSPF Overview Concepts  OSPF is a Link-State Routing Protocol  Uses IP as transport, IP protocol 89  Uses multicast addresses in neighbor maintenance and flooding of LSAs  – All OSPF Routers  – All DRouters  Employs Dijkstra’s Shortest Path First (SPF) algorithm to calculate the path tree

555 OSPF Overview Concepts – (cont.)  Uses Metrics—path cost  Typically faster convergence than DVRPs  Support for CIDR, VLSM, Authentication, Multi-path and IP unnumbered  Relatively low steady state bandwidth requirements

666 OSPF Overview Terminology Terminology

777 OSPF Overview Terminology  Link  Link state  Link State (LS) or topological database  Area  OSPF Metric Cost  Routing table  Adjacencies database

888 OSPF Overview Topology/Link State Database  A router has a separate Link State (LS) or topological database for each area to which it belongs  All routers belonging to the same area should have identical databases  SPF calculation is performed independently for each area  LSA flooding is bounded by area

999 OSPF Overview Areas  OSPF uses a 2 level hierarchical model  Areas labeled with a 32-bit number  Can be defined using single decimal or IP address format value  (i.e. Area or Area 0)  Area 0 reserved for the backbone area  All areas must connect to area 0

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11 OSPF Overview OSPF Metric  Cost applied on all router link paths  16-bit positive number 1–65,535  The lower the more desirable  Relevant going out an interface only  Route decisions made on total cost of path

12 OSPF Overview OSPF Packet Types OSPF Packet format

13 OSPF Packet Types (cont.)

14 OSPF Overview Router ID  Routers are identified by a unique 32-bit ID  RID: highest IP address configured on any active loopback interface  RID: if no loopback exists, highest IP address configured on any active physical interface  RID can be configured with  router-id  router-id

15 OSPF Overview OSPF Hello Packets  Multicast on all router interfaces  Hello interval 10 sec. LAN, 30 sec. NBMA  Used to form adjacencies between routers

16 OSPF Overview Database Descriptor Packets (DDP)  Contain link state database headers  Describe the current LS database  Exchange stage DD seq=x+1,S DD seq=x+n,S DD seq=x+n,M DD seq=x+1,M

17 OSPF Overview Link State Request & Update Packets  Request for specific parts of database  Send only database updates requested  Loading Stage, labeled Full when complete Link State Update Link State Request Link State Update

18 OSPF Operation  Network changes generate link-state advertisements (LSA)  Cost change to an interface  Link being added or deleted from topology  All routers exchange LSAs to build and maintain a consistent database  The protocol remains relatively quiet during steady-state conditions.

19 OSPF Operation Steps to OSPF Operation 1. Establishing router adjacencies 2. Electing DR and BDR 3. Discovering Routes 4. Choosing Routes 5. Maintaining Routing Information

20 OSPF Operation OSPF States OSPF router interfaces can be in one of seven states:  Down State  Init State  Two-way State  ExStart State  Exchange State  Loading State  Full Adjacency State

21 OSPF Operation Steps to OSPF Operation with OSPF States 1. Establishing router adjacencies  Down State  Init State  Two-way State  (ExStart State unless DR/BDR election needed) 2. Electing DR and BDR  ExStart State with DR and BDR  Two-way State with all other routers

22 3. Discovering Routes  ExStart State  Exchange State  Loading State  Full State 4. Choosing Routes 5. Maintaining Routing Information OSPF Operation Steps to OSPF Operation with OSPF States

23  Initially, an OSPF router interface is in the down state.not exchanged information with any neighbor. OSPF Operation 1. Establishing Adjacencies (1)

24 Init State  Init State - OSPF routers send Type 1 Hello packets at regular intervals (10 sec.) to establish neighbors.  When a router receives its first Hello packet, it enters the init state, meaning the router is ready to take the relationship to the next level. OSPF Operation 1. Establishing Adjacencies (2)

25 From init state to the two-way state  RTB receives Hello packets from RTA and RTC (its neighbors), and sees its own Router ID ( ) in the Neighbor ID field.  RTB declares takes the relationship to a new level, and declares a two-way state between itself and RTA, and itself and RTC. OSPF Operation 1. Establishing Adjacencies (3)

26 Two-way state to ExStart state?  RTB now decides who to establish a full adjacency with depending upon the type of network that the particular interfaces resides on.  If the interface is on a point-to-point link, the routers becomes adjacent with its sole link partner (aka “soul mates”), and take the relationship to the next level by entering the ExStart state.  If the interface is on a multi-access link (Ethernet, Frame Relay, …) RTB must enter an election process to see who it will establish a full adjacency with, and remains in the two-way state. (Next!) OSPF Operation 1. Establishing Adjacencies (4)

27 OSPF Operation Designated Router  Reduce OSPF traffic on multiaccess links  Routers form FULL adjacencies with DR/BDR  Store and distribute neighbors LSDBs  Backup DR for redundancy  OSPF priority used in DR selection  Range 1–255 default 1, 0 for non-candidate. Priority carried in Hello packet  ip ospf priority  ip ospf priority

28 OSPF Operation Function of DR/BDR DR BDR Link change Flood Link change AllOSPFRouters AllDRrouters

29 OSPF Operation 2.Electing a DR and BDR (1)  On point-to-point links adjacencies are established with all neighbors, because there is only one neighbor.  On multi-access networks,OSPF elects a DR and BDR to limit the number of adjacencies.  Reduce routing update traffic

30  DR - Designated Router  BDR – Backup Designated Router  DR’s serve as collection points for Link State Advertisements (LSAs)  A BDR back ups the DR.  If the IP network is multi-access, the OSPF routers will elect 1 DR and 1 BDR (unless there is only 1 router on the network). OSPF Operation 2.Electing a DR and BDR (2)

31  The formation of an adjacency between every attached router would create many unncessary LSA (Link State Advertisements), n(n-1)/2 adjacencies.  Flooding on the network itself would be chaotic.  To prevent this problem, a Designated Router is elected on multi-access networks. OSPF Operation 2.Electing a DR and BDR (3)

32  All other routers, “DRother”, establish adjacencies with only the DR and BDR.  DRother routers multicast LSAs to only the DR and BDR  ( all DR routers)  DR sends LSA to all adjacent neighbors  ( all OSPF routers) OSPF Operation 2.Electing a DR and BDR (4)

33  Once a DR is established, a new router that enters the network with a higher priority or router id will NOT become the DR or BDR. (Bug in early IOS 12.0)  If DR fails, BDR takes over as DR and selection process for new BDR begins.  State of the relationship  DRothers enter ExStart state with DR and BDR and two-way state with all other routers OSPF Operation 2.Electing a DR and BDR (5)

34 DR - Summary DR Election  Router with the highest interface priority (priority = 0 cannot become DR or BDR)  Router with the highest router ID.  Loopback address used first  IP Address on active interface used second  BDR is the second highest OSPF Operation 2.Electing a DR and BDR (6)

35 DR - Summary Adjacencies and multicasting  All other routers, DRother, establish adjacencies with only the DR and BDR.  All routers continue to multicast Hello packets to AllSPFRouters ( ) so they can track neighbors.  But updates (LSAs) are multicast to DR and BDR only ( AllDRrouters) and in turn  DR floods updates (LSAs) to all adjacent neighbors ( AllSPFRrouters) OSPF Operation 2.Electing a DR and BDR (7)

36 BDR-summary  Listens, but doesn’t act.  If LSA is sent, BDR sets a timer.  If timer expires before it sees the reply from the DR, it becomes the DR and takes over the update process.  The process for a new BDR begins. OSPF Operation 2.Electing a DR and BDR (8)

37 OSPF Operation 3. Discovering Routes and reaching Full State

38  Link state database  Created with Link State Packets (LSPs) from each router  TENT database  Tentative triples (ID, path cost, direction)  PATH database  Best path triples (ID, path cost, direction)  Forwarding database  The Routing Table Dijkstra - Shortest Path First (SPF) Algorithm OSPF Operation 4. Choosing routes (1)

39  All routers exchange Link State Packets (LSPs)  Each router starts with itself as root  Tent is built from LSPs  Path is created by examining and comparing TENT triples  Once path is final the forwarding table is populated Dijkstra (SPF) Overview (Cont.) OSPF Operation 4. Choosing routes (2)

40 BCDEF B/4 G/2 G A/4 C/1 A B/1 D/4 E/2 C/4 E/1 C/2 D/1 F/2 E/2 G/2 A/2 F/2 B A C D E F G Lowest cost best Link State Packet (LSP) Data OSPF Operation 4. Choosing routes (3)

41 .. OSPF Operation 5. Maintaining routes Router 2, Area 1 Old Routing TableNew Routing Table Link State Table LSA Dijkstra Algorithm ACK  Every router in area receives the new LSA via flooding  Each router computes shortest path routing table when a link changes State. Router 1, Area 1

42 Issues with large OSPF nets  Large routing table  Large link-state table  Frequent SPF calculations

43 reference  RFC 1403, "BGP OSPF Interaction", K. Varadhan, RFC 1403 RFC 1403  RFC 1584, "Multicast Extensions to OSPF", J. Moy, March RFC 1584 RFC 1584  RFC 1850, "OSPF Version 2 Management Information Base", F. Baker and R. Coltun, Nov RFC 1850 RFC 1850  RFC 2328, "OSPF Version 2", J. Moy, April 1998, also STD 54. RFC 2328 RFC 2328  RFC 2370, "The OSPF Opaque LSA Option", R. Coltun, July RFC 2370 RFC 2370  “OSPF”, B. Daniel, B. Omer, R. Carmel.  Internetworking with TCP/IP (Vol I) - Comer  Technology Innovation Centre Brimingham”.