 Development began in 1987  OSPF Working Group (part of IETF)  OSPFv2 first established in 1991  Many new features added since then  Updated OSPFv2 specification in RFC 2178

 Faster Convergence and less consumption of network resources  A more descriptive routing metric ◦ configurable ◦ value ranges between 1 and 65,535 ◦ no restriction on network diameters  Equal-cost multipath ◦ a way to do load balancing

 Routing Hierarchy ◦ support large routing domains  Separate internal and external routes  Support of flexible subnetting schemes ◦ route to arbitrary [address,mask] combinations using VLSMs  Security  Type of Service Routing

 Distributed, replicated database model ◦ describes complete routing topology  Link state advertisements ◦ carry local piece of routing topology  Distribution of LSAs using reliable flooding  Link state database ◦ identical for all the routers

LS Age OptionsLS Type Link State ID Advertising Router LS Sequence Number LS Checksum Length LSA Header 0 16

 Identifying LSAs ◦ LS type field ◦ Link State ID field  mostly carries addressing information  e.g. IP address of externally reachable network ◦ Advertising Router field  originating router’s OSPF router ID

 Identifying LSA instances ◦ needed to update self-originated LSAs ◦ LS Sequence Number field  32 bit values  monotonically increasing until some max value  600 years to roll over!  LSA checksum and LS Age guard against potential problems

 Verifying LSA contents ◦ LS Checksum field  computed by the originating router and left unchanged thereafter  LS age field not included in checksum  Removing LSAs from databases ◦ LS Age field  ranges from 0 to 30 min.  Max Age LSAs used to delete outdated LSAs

 Other LSA Header fields ◦ Options field  sometimes used to give special treatment during flooding or routing calculations ◦ Length field  includes LSA header and contents  ranges from bytes

 Collection of all OSPF LSAs  databases exchanged between neighbors  synchronization thru reliable flooding  gives the complete routing topology  each OSPF router has identical link-state database

 Example of a link state database LS TypeLink State IDAdv RouterLS ChecksumLS Seq NoLS Age Router LSA x9b470x …..…...….. ….…...

 OSPF packets encapsulated in IP packets ◦ standard 24 byte header ◦ OSPF packet type field ◦ OSPF router ID of sender ◦ Packet checksum ◦ Authentication fields ◦ OSPF Area ID

 OSPF Hello Protocol  Hello packets sent out every 10 seconds  helps to detect failed neighbors  RouterDeadInterval (default 40 seconds)  also ensures that link is bidirectional  neighboring routers agree on intervals ◦ hello interval set so that a link is not accidentally brought down

 Crucial to ensure correct and loop free routing  must be done before 2 neighbors start communication  also whenever new LSAs are introduced ◦ uses reliable flooding  each router sends LSA headers to its neighbor when connection comes up  requests only those LSAs which are recent

 Neighboring routers first exchange hellos  a database description packet packet establishes the sequence number  the other router sends LSA headers  sequence number incremented for every pair od database description packets ◦ implicit acknowledgement for the previous pair  after examining LSA headers explicit request sent for complete LSAs

 Starts when a router wants to update self- originated LSAs  Link State Update packets  Neighbor installs more recent LSAs into its database  floods out on all interfaces except the one on which it arrived  reliability-retransmissions until acks received

 Two-level hierarchical routing scheme through the use of areas  areas identified by 32-bit id  each area has its own link state database which is a collection of network-LSAs and router-LSAs  area’s topology hidden from all other areas  interconnection of areas through area border routers (ABRs)  ABR leaks IP addressing information to other areas through summary LSAs

A B C D G H F E IJ AA / /24 Area / /24 Area / /24 Area Area

 Example of Summary LSA(router B) LS Age OptionsLS Type Link State ID Advertising Router LS Sequence Number LS Checksum Length Network Mask TOS Metric 0 0x2, Type 3(summary-LSA) Router B’s router ID 0x bytes TOS 0 (normal) Cost of 7

 Reduction in link state databases of an area  reduction in amount of flooding traffic needed for synchronization  reduction in the cost of the shortest path calculations  increased robustness  routing protection  Hidden prefixes

 All the areas are connected to area also called the backbone area  need not have a direct physical connection though ◦ virtual links provide logical link to backbone ◦ summary LSAs tunneled across non backbone areas  exchange of routing information between areas using Distance Vector Protocol ◦ absence of redundant paths between areas ◦ not subject to convergence problems

 Special routers called AS boundary routers at the edge of OSPF domain  ASBRs originate AS-External LSAs  only routes for which the choice of an ASBR makes sense are imported  otherwise default routes are used  AS external LSAs similar to Summary LSAs with 2 additional fields ◦ Forwarding address ◦ external route tag

 AS-External LSAs flooded across borders  ASBR summary LSAs used to know the location of the originator of AS-External LSA  Link State ID of ASBR Summary LSA set to the OSPF router ID of the ASBR whose location is advertised  similar to summary LSA in all other respects

 Restrict the amount of external routing information within an area  used when resources especially router memory is very limited  two types of restricted areas ◦ Stub Areas ◦ NSSAs or Not-So-Stubby-Areas