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Published byEdmund Hamilton Modified over 9 years ago
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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
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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
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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
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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
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LS Age OptionsLS Type Link State ID Advertising Router LS Sequence Number LS Checksum Length LSA Header 0 16
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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
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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
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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 Max Age LSAs used to delete outdated LSAs
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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 20-65535 bytes
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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
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Example of a link state database LS TypeLink State IDAdv RouterLS ChecksumLS Seq NoLS Age Router LSA10.1.1.1 0x9b470x800000060 …..…...….. ….…...
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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
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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
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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
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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
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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
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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
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A B C D G H F E IJ AA 1 2 2 1 1 33 1 31 10.2.1.0/2410.2.2.0/24 Area 0.0.0.1 10.1.2.0/24 10.1.1.0/24 Area 0.0.0.2 3 3 1 1 3 3 3 1 1 10.3.7.0/24 10.8.2.0/24 Area 0.0.0.3 Area 0.0.0.0 1
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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) 10.2.0.0 Router B’s router ID 0x80000001 28 bytes 255.255.0.0 TOS 0 (normal) Cost of 7
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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
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All the areas are connected to area 0.0.0.0 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
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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
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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
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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
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