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Open Shortest Path First ('''OSPF''') is a Link-state , hierarchical Interior Gateway Protocol (IGP) Routing protocol. Dijkstra's Algorithm is used to calculate the Shortest Path Tree . It uses ''cost'' as its routing metric. A link state database is constructed of the Network Topology which is identical on all Router s in the area. OSPF is perhaps the most widely used IGP in large networks. It can operate securely, using MD5 to authenticate peers before forming adjacencies, and before accepting Link-state Advertisement s. A natural successor to RIP , it was VLSM capable or ''classless'' from its inception. A newer version of OSPF (OSPFv3) now supports IPv6 as well. Multicast extensions to OSPF ( MOSPF ) have been defined, however these are not widely used. OSPF can "tag" routes, and propagate these tags along with the routes. An OSPF network can be broken up into smaller networks. A special area called the ''backbone area'' forms the core of the network, and other areas are connected to it. Inter-area routing goes via the backbone. All areas must connect to the backbone; if no direct connection is possible, a ''virtual link'' may be established. Routers in the same Broadcast Domain or at each end of a Point To Point link form ''adjacencies'' when they have discovered each other. The routers elect a ''designated router'' (DR) and ''backup designated router'' (BDR) which act as hub to reduce traffic between routers. OSPF uses both Unicast and Multicast to send 'hello packets' and link state updates. Multicast Address es 224.0.0.5 and 224.0.0.6 are reserved for OSPF. In contrast to RIP or BGP , OSPF does not use TCP or UDP but uses IP directly, using IP protocol 89. AREA TYPES An OSPF network is divided into ''areas''. These are logical groupings of routers whose information may be summarized towards the rest of the network. Several "special" area types are defined: Backbone area The backbone area (also known as ''area zero'') forms the core of an OSPF network. All other areas are connected to it, and inter-area routing happens via a router connected to the backbone area. Stub area A stub area is an area which doesn't receive ''external'' routes. External routes are defined as routes which were distributed in OSPF via another routing protocol. Therefore, stub areas typically need to rely on a Default Route to send traffic to routes outside the present domain. Totally stubby area A totally stubby area is similar to a stub area, however this area does not allow ''summary'' routes in addition to the ''external'' routes, i.e., inter-area (IA) routes are not summarized into totally stubby areas. The only way for traffic to get routed outside of the area is a default route which is the only Type-3 LSA advertised into the area. When there is only one route out of the area, fewer routing decisions have to be made by the route processor, which lowers system resource utilization. Not-so-stubby area Also referred to as NSSA, a not-so-stubby area is a type of stub area that can import AS external routes and send them to the backbone, but cannot receive AS external routes from the backbone or other areas. Cisco also implements a proprietary version of NSSAs called a NSSA Totally Stubby area. It takes on the attributes of a Totally Stubby area, meaning that type 3 and 4 summary routes are not flooded into this type of area. OSPF ROUTER TYPES OSPF defines various router types. These are logical definitions, and a router that uses OSPF may be classified as more than one of the following types. For example, a router that is connected to more than one area, and which receives routes from a BGP process connected to another AS, is both an ABR and an ASBR. Area Border Router An Area Border Router (ABR) is a router that connects one or more OSPF areas to the main backbone network. It is considered a member of all areas it is connected to. An ABR keeps multiple copies of the link-state database in memory, one for each area. Autonomous System Boundary Router An ASBR is a router connected to more than one Autonomous System (AS), and which exchanges routing information with routers in other ASs. ASBRs typically also run a non-IGP routing protocol, such as BGP. An ASBR is used to distribute routes received from other ASs throughout its own AS. Internal router A router is called an internal router (IR) if it only has OSPF adjacencies with routers in the same area. Backbone router A backbone router (BR) is a router with an interface in to the backbone area. An ABR would be a BR, although the reverse need not be true. Designated Router A designated router (DR) is the router elected by the network by elections. The DR is elected based on the following default criteria:
DR's exist for the purpose of reducing network traffic by providing a source for routing updates, the DR maintains a complete topology table of the network and sends the updates to the other routers via multicast. This way all the routers do not have to constantly update each other, and can rather get all their updates from a single source. The use of multicasting further reduces the network load. DRs and BDRs are always setup/elected on Broadcast networks (Ethernet). DR's can also be elected on NBMA (Non-broadcast Multi-Access Networks) such as Frame Relay. DRs or BDRs do not configure on point-to-point links (such as a point-to-point WAN connection) because the bandwidth between two hosts cannot be further optimized. Backup Designated Router A backup designated router (BDR) is a router that becomes the designated router if the current designated router has a problem or fails. RFC HISTORY
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