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Open Shortest Path First Protocol
Open Shortest Path First (OSPF) is a dynamic routing protocol. It is a link-state routing protocol and is operating within an autonomous system (AS). OSPF is perhaps the most widely-used interior gateway protocol (IGP) in large enterprise networks.
Purpose of OSPF is to build routing tables based solely on the destination IP address found in IP packets. Originally it was designed to support variable-length subnet masking. OSPF also detects changes in the topology, such as link failures, very quickly and converges on a new loop-free routing structure within seconds. For this, each OSPF router collects link-state information to construct the entire network topology of so-called areas from which it computes the shortest path tree.
The link-state information is maintained on each router as a link-state database (LSDB) which is a tree-image of the network topology. Identical copies of the LSDB are periodically updated through flooding on all routers in each OSPF-aware area. By convention, area 0 represents the core or "backbone" region of an OSPF-enabled network, and other OSPF area numbers may be designated to serve other regions of an enterprise network - however every additional OSPF area must have a direct or virtual connection to the backbone OSPF area. The backbone area has the identifier 0.0.0.0. Inter-area routing goes via the backbone.
Routers in the same broadcast domain or at each end of a point-to-point telecommunications link form adjacencies when they have detected each other. Multicast addresses 224.0.0.5 and 224.0.0.6 are reserved for OSPF. In contrast to the Routing Information Protocol (RIP) or the Border Gateway Protocol (BGP), OSPF does not use TCP or UDP but uses IP directly, via IP protocol 89.
Area types
An OSPF network is divided into areas that are labeled with 32-bit area identifiers. The area identifiers are commonly, but not always, written in the dot-decimal notation of an IPv4 address. However, they are not IP addresses and may duplicate, without conflict, any IPv4 address. The area identifiers for IPv6 implementations of OSPF (OSPFv3) also use 32-bit identifiers written in the same notation.
Areas are logical groupings of hosts and networks, including their routers having interfaces connected to any of the included networks. Each area maintains a separate link state database whose information may be summarized towards the rest of the network by the connecting router. Thus, the topology of an area is unknown outside of the area. This reduces the amount of routing traffic between parts of an autonomous system.
Backbone area
The backbone area (also known as area 0.0.0.0) forms the core of an OSPF network. All other areas are connected to it, and inter-area routing happens via routers connected to the backbone area and to their own associated areas. It is the logical and physical structure for the 'OSPF domain' and is attached to all nonzero areas in the OSPF domain.
The backbone area (also known as area 0.0.0.0) forms the core of an OSPF network. All other areas are connected to it, and inter-area routing happens via routers connected to the backbone area and to their own associated areas. It is the logical and physical structure for the 'OSPF domain' and is attached to all nonzero areas in the OSPF domain.
All OSPF areas must connect to the backbone area. This connection, however, can be through a virtual link. For example, assume area 0.0.0.1 has a physical connection to area 0.0.0.0. Further assume that area 0.0.0.2 has no direct connection to the backbone, but this area does have a connection to area 0.0.0.1. Area 0.0.0.2 can use a virtual link through the transit area 0.0.0.1 to reach the backbone. To be a transit area, an area has to have the transit attribute, so it cannot be stubby in any way.
Stub area
A stub area is an area which does not receive external routes except the default route, but does receive inter-area routes. This kind of area is useful when, for example, all Internet access goes through autonomous system border routers (ASBRs) in Area 0.0.0.0, but there are on other non-backbone areas allows the routers in the stub area to pick the best route to another area.
A stub area is an area which does not receive external routes except the default route, but does receive inter-area routes. This kind of area is useful when, for example, all Internet access goes through autonomous system border routers (ASBRs) in Area 0.0.0.0, but there are on other non-backbone areas allows the routers in the stub area to pick the best route to another area.
Totally stubby area
A totally stubby area (TSA), which is a nonstandard but useful extension by Cisco, is similar to a stub area, however this area does not allow summary routes in addition to the external routes, that is, inter-area (IA) routes are not summarized into totally stubby areas. 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.
A totally stubby area (TSA), which is a nonstandard but useful extension by Cisco, is similar to a stub area, however this area does not allow summary routes in addition to the external routes, that is, inter-area (IA) routes are not summarized into totally stubby areas. 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
A not-so-stubby area (NSSA) is a type of stub area that can import autonomous system (AS) external routes and send them to the backbone, but cannot receive AS external routes from the backbone or other areas. The NSSA is a non-proprietary extension of the existing stub area feature that allows the injection of external routes in a limited fashion into the stub area.
A not-so-stubby area (NSSA) is a type of stub area that can import autonomous system (AS) external routes and send them to the backbone, but cannot receive AS external routes from the backbone or other areas. The NSSA is a non-proprietary extension of the existing stub area feature that allows the injection of external routes in a limited fashion into the stub area.
Cisco also implements a proprietary version of a NSSA called a NSSA totally stubby area. It takes on the attributes of a TSA, meaning that type 3 and type 4 summary routes are not flooded into this type of area. It is also possible to declare an area both totally stubby and not-so-stubby, which means that the area will receive only the default route from area 0.0.0.0, but can also contain an autonomous system border router (ASBR) that accepts external routing information and injects it into the local area, and from the local area into area 0.0.0.0.
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