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Open Shortest Path First

Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS).

OSPF gathers link state information from available routers and constructs a topology map of the network. The topology is presented as a routing table to the internet layer for routing packets by their destination IP address. OSPF supports Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6) networks and is widely used in large enterprise networks. IS-IS, another LSR-based protocol, is more common in large service provider networks.

Originally designed in the 1980s, OSPF version 2 is defined in RFC 2328 (1998). The updates for IPv6 are specified as OSPF version 3 in RFC 5340 (2008). OSPF supports the Classless Inter-Domain Routing (CIDR) addressing model.

Concepts
OSPF is an interior gateway protocol (IGP) for routing Internet Protocol (IP) packets within a single routing domain, such as an autonomous system. It gathers link state information from available routers and constructs a topology map of the network. The topology is presented as a routing table to the internet layer which routes packets based solely on their destination IP address.

OSPF detects changes in the topology, such as link failures, and converges on a new loop-free routing structure within seconds. It computes the shortest-path tree for each route using a method based on Dijkstra's algorithm. The OSPF routing policies for constructing a route table are governed by link metrics associated with each routing interface. Cost factors may be the distance of a router (round-trip time), data throughput of a link, or link availability and reliability, expressed as simple unitless numbers. This provides a dynamic process of traffic load balancing between routes of equal cost.

OSPF divides the network into routing areas to simplify administration and optimize traffic and resource utilization. Areas are identified by 32-bit numbers, expressed either simply in decimal, or often in the same octet-based dot-decimal notation used for IPv4 addresses. By convention, area 0 (zero), or 0.0.0.0, represents the core or backbone area of an OSPF network. While the identifications of other areas may be chosen at will, administrators often select the IP address of a main router in an area as the area identifier. Each additional area must have a connection to the OSPF backbone area. Such connections are maintained by an interconnecting router, known as an area border router (ABR). An ABR maintains separate link-state databases for each area it serves and maintains summarized routes for all areas in the network.

OSPF runs over IPv4 and IPv6, but does not use a transport protocol such as UDP or TCP. It encapsulates its data directly in IP packets with protocol number 89. This is in contrast to other routing protocols, such as the Routing Information Protocol (RIP) and the Border Gateway Protocol (BGP). OSPF implements its own transport error detection and correction functions. OSPF also uses multicast addressing for distributing route information within a broadcast domain. It reserves the multicast addresses (IPv4) and  (IPv6) for all SPF/link state routers (AllSPFRouters) and  (IPv4) and  (IPv6) for all Designated Routers (AllDRouters). For non-broadcast networks, special provisions for configuration facilitate neighbor discovery. OSPF multicast IP packets never traverse IP routers, they never travel more than one hop. The protocol may therefore be considered a link layer protocol, but is often also attributed to the application layer in the TCP/IP model. It has a virtual link feature that can be used to create an adjacency tunnel across multiple hops. OSPF over IPv4 can operate securely between routers, optionally using a variety of authentication methods to allow only trusted routers to participate in routing. OSPFv3 (IPv6) relies on standard IPv6 protocol security (IPsec), and has no internal authentication methods.

For routing IP multicast traffic, OSPF supports the Multicast Open Shortest Path First (MOSPF) protocol. Cisco does not include MOSPF in their OSPF implementations. Protocol Independent Multicast (PIM) in conjunction with OSPF or other IGPs, is widely deployed.

OSPF version 3 introduces modifications to the IPv4 implementation of the protocol. Except for virtual links, all neighbor exchanges use IPv6 link-local addressing exclusively. The IPv6 protocol runs per link, rather than based on the subnet. All IP prefix information has been removed from the link-state advertisements and from the hello discovery packet making OSPFv3 essentially protocol-independent. Despite the expanded IP addressing to 128 bits in IPv6, area and router Identifications are still based on 32-bit numbers. <!--As a link-state routing protocol it was based on the link-state algorithm developed for the ARPANET in 1980 and the IS-IS routing protocol. OSPF was first standardized in 1989 as RFC 1131, which is now known as OSPF version 1. The development work for OSPF prior to its codification as an open standard was undertaken largely by the Digital Equipment Corporation, which developed its own proprietary DECnet protocols.

Routing protocols like OSPF calculate the shortest route to a destination through the network based on an algorithm. The first routing protocol that was widely implemented, the Routing Information Protocol (RIP), calculated the shortest route based on hops, that is the number of routers that an IP packet had to traverse to reach the destination host. RIP successfully implemented dynamic routing, where routing tables change if the network topology changes. But RIP did not adapt its routing according to changing network conditions, such as data-transfer rate. Demand grew for a dynamic routing protocol that could calculate the fastest route to a destination. OSPF was developed so that the shortest path through a network was calculated based on the cost of the route, taking into account bandwidth, delay and load. Therefore, OSPF undertakes route cost calculation on the basis of link-cost parameters, which can be weighted by the administrator. OSPF was quickly adopted because it became known for reliably calculating routes through large and complex local area networks.

As a link-state routing protocol, OSPF maintains link-state databases, which are really network topology maps, on every router on which it is implemented. The state of a given route in the network is the cost, and OSPF algorithm allows every router to calculate the cost of the routes to any given reachable destination. Unless the administrator has made a configuration, the link cost of a path connected to a router is determined by the bit rate (1 Gbit/s, 10 Gbit/s, etc.) of the interface. A router interface with OSPF will then advertise its link cost to neighboring routers through multicast, known as the hello procedure. All routers with OSPF implementation keep sending hello packets, and thus changes in the cost of their links become known to neighboring routers. The information about the cost of a link, that is the speed of a point-to-point connection between two routers, is then cascaded through the network because OSPF routers advertise the information they receive from one neighboring router to all other neighboring routers. This process of flooding link-state information through the network is known as synchronization. Based on this information, all routers with OSPF implementation continuously update their link-state databases with information about the network topology and adjust their routing tables.

OSPF has become a popular dynamic routing protocol. Other commonly used dynamic routing protocols are the RIPv2 and the Border Gateway Protocol (BGP). Today routers support at least one interior gateway protocol to advertise their routing tables within a local area network. Frequently implemented interior gateway protocols besides OSPF are RIPv2, IS-IS, and EIGRP (Enhanced Interior Gateway Routing Protocol). -->

Router relationships
OSPF supports complex networks with multiple routers, including backup routers, to balance traffic load on multiple links to other subnets. Neighboring routers in the same broadcast domain or at each end of a point-to-point link communicate with each other via the OSPF protocol. Routers form adjacencies when they have detected each other. This detection is initiated when a router identifies itself in a hello protocol packet. Upon acknowledgment, this establishes a two-way state and the most basic relationship. The routers in an Ethernet or Frame Relay network select a designated router (DR) and a backup designated router (BDR) which act as a hub to reduce traffic between routers. OSPF uses both unicast and multicast transmission modes to send "hello" packets and link-state updates.

As a link-state routing protocol, OSPF establishes and maintains neighbor relationships for exchanging routing updates with other routers. The neighbor relationship table is called an adjacency database. Two OSPF routers are neighbors if they are members of the same subnet and share the same area ID, subnet mask, timers and authentication. In essence, OSPF neighborship is a relationship between two routers that allow them to see and understand each other but nothing more. OSPF neighbors do not exchange any routing information – the only packets they exchange are hello packets. OSPF adjacencies are formed between selected neighbors and allow them to exchange routing information. Two routers must first be neighbors and only then, can they become adjacent. Two routers become adjacent if at least one of them is designated router or backup designated router (on multiaccess-type networks), or they are interconnected by a point-to-point or point-to-multipoint network type. For forming a neighbor relationship between, the interfaces used to form the relationship must be in the same OSPF area. While an interface may be configured to belong to multiple areas, this is generally not practiced. When configured in a second area, an interface must be configured as a secondary interface.

Operation modes
The OSPF can have different operation modes on the following setups on an interface or network:
 * Point-to-point. Each router advertises itself by periodically multicasting hello packets. No designated router is elected. The interface can be IP unnumbered (without a unique IP address assigned to it).
 * Broadcast (default), each router advertises itself by periodically multicasting hello packets.
 * Non-broadcast multi-access, with the use of designated routers. May need static configuration. Packets are sent as unicast.
 * Point-to-multipoint, where OSPF treats neighbours as a collection of point-to-point links. No designated router is elected. Separate hello packets are sent to each neighbor.
 * Point to Multipoint Non Broadcast (P2MP-NB), No designated router is elected. Separate hello packets are sent to each neighbor, Packets are sent as unicast.
 * Passive, Only advertised to other neighbours. No adjacency is advertised on network.

Indirect connections
Virtual link over Virtual links, tunneling and sham links, are a form of connections that goes over the routing engine, and is not a direct connection to the remote host.

 * Virtual links: The packets are sent as unicast. Can only be configured on a non-backbone area (but not stub-area). Endpoints need to be ABR, the virtual links behave as unnumbered point-to-point connections. The cost of an intra-area path between the two routers is added to the link.
 * Virtual link over tunneling (like GRE and WireGuard): Since OSPF does not support virtual links for areas other than the backbone, a workaround is to use of tunneling. If the same IP or router ID is used, the link creates two equal-cost routes to the destination.
 * Sham link: An intra-area link that connects two sites via the MPLS VPN backbone that is preferred to an internal intra-area "OSPF backdoor link" between the same two sites. A sham link is only needed if the MPLS VPN backbone is preferred over the OSPF backdoor link.

Adjacency state machine
Each OSPF router within a network communicates with other neighboring routers on each connecting interface to establish the states of all adjacencies. Every such communication sequence is a separate conversation identified by the pair of router IDs of the communicating neighbors. RFC 2328 specifies the protocol for initiating these conversations (Hello Protocol) and for establishing full adjacencies (database description packets, link-state request packets). During its course, each router conversation transitions through a maximum of eight conditions defined by a state machine:

Neighbor state changes

 * 1) Down: The state down represents the initial state of a conversation when no information has been exchanged and retained between routers with the Hello Protocol.
 * 2) Attempt: The attempt state is similar to the down state, except that a router is in the process of efforts to establish a conversation with another router, but is only used on non-broadcast multiple-access networks (NBMAs).
 * 3) Init: The init state indicates that a hello packet has been received from a neighbor, but the router has not established a two-way conversation.
 * 4) Two-way: The two-way state indicates the establishment of a bidirectional conversation between two routers. This state immediately precedes the establishment of adjacency. This is the lowest state of a router that may be considered as a DR.

Database exchange

 * 1) Exchange start (exstart): The exstart state is the first step of adjacency of two routers.
 * 2) Exchange: In the exchange state, a router is sending its link-state database information to the adjacent neighbor. At this state, a router can exchange all OSPF routing protocol packets.
 * 3) Loading: In the loading state, a router requests the most recent link-state advertisements (LSAs) from its neighbor discovered in the previous state.
 * 4) Full: The full state concludes the conversation when the routers are fully adjacent, and the state appears in all router- and network-LSAs. The link-state databases of the neighbors are fully synchronized.

Broadcast networks
In broadcast multiple-access networks, neighbor adjacency is formed dynamically using multicast hello packets to.

IP 192.0.2.1 > 224.0.0.5: OSPFv2, hello IP 192.0.2.2 > 224.0.0.5: OSPFv2, hello IP 192.0.2.1 > 192.0.2.2: OSPFv2, database description IP 192.0.2.2 > 192.0.2.1: OSPFv2, database description

Passive network
A network where OSPF adverts the network, but the OSPF will not start neighbour adjacency.

Non-broadcast networks
In a non-broadcast multiple-access (NBMA) network, a neighbor adjacency is formed by sending unicast packets to another router. A non-broadcast network can have more than two routers, but broadcast is not supported.

IP 192.0.2.1 > 192.0.2.2: OSPFv2, hello IP 192.0.2.2 > 192.0.2.1: OSPFv2, hello IP 192.0.2.1 > 192.0.2.2: OSPFv2, database description IP 192.0.2.2 > 192.0.2.1: OSPFv2, database description

Examples of non-broadcast networks:
 * X.25 public data network
 * Wireguard
 * Serial interface

 * Requires all routers to be able to communicate directly, on the same network.
 * Designated Router is elected for the network.
 * LSA is generated for the network.

OSPF areas
A network is divided into OSPF areas that are logical groupings of hosts and networks. An area includes its connecting router having an interface for each connected network link. Each router maintains a separate link-state database for the area whose information may be summarized towards the rest of the network by the connecting router. Thus, the topology of an area is unknown outside the area. This reduces the routing traffic between parts of an autonomous system.

OSPF can handle thousands of routers with more a concern of reaching capacity of the forwarding information base (FIB) table when the network contains lots of routes and lower-end devices. Modern low-end routers have a full gigabyte of RAM, which allows them to handle many routers in an area 0. Many resources refer to OSPF guides from over 20 years ago where it was impressive to have 64 MB of RAM.

Areas are uniquely identified with 32-bit numbers. The area identifiers are commonly written in the dot-decimal notation, familiar from IPv4 addressing. However, they are not IP addresses and may duplicate, without conflict, any IPv4 address. The area identifiers for IPv6 implementations (OSPFv3) also use 32-bit identifiers written in the same notation. When dotted formatting is omitted, most implementations expand area 1 to the area identifier 0.0.0.1, but some have been known to expand it as 1.0.0.0.

Several vendors (Cisco, Allied Telesis, Juniper, Alcatel-Lucent, Huawei, Quagga), implement totally stubby and NSSA totally stubby area for stub and not-so-stubby areas. Although not covered by RFC standards, they are considered by many to be standard features in OSPF implementations.

OSPF defines several area types:
 * Backbone
 * Non-backbone/regular
 * Stub
 * Totally stubby
 * Not-so-stubby
 * Totally not-so-stubby
 * Transit

Backbone area
The backbone area (also known as area 0 or area 0.0.0.0) forms the core of an OSPF network. All other areas are connected to it, either directly or through other routers. OSPF requires this to prevent routing loops. 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. In OSPF the term autonomous system boundary router (ASBR) is historic, in the sense that many OSPF domains can coexist in the same Internet-visible autonomous system, RFC 1996.

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.

Regular area
A regular area is just a non-backbone (nonzero) area without specific feature, generating and receiving summary and external LSAs. The backbone area is a special type of such area.

Stub area

 * In hello packets the E-flag is not high, indicating "External routing: not capable"

A stub area is an area that does not receive route advertisements external to the AS and routing from within the area is based entirely on a default route. An ABR deletes type 4 and 5 LSAs from internal routers, sends them a default route of 0.0.0.0 and turns itself into a default gateway. This reduces LSDB and routing table size for internal routers.

Modifications to the basic concept of stub area have been implemented by systems vendors, such as the totally stubby area (TSA) and the not-so-stubby area (NSSA), both an extension in Cisco Systems routing equipment.

Totally stubby area
A totally stubby area is similar to a stub area. However, this area does not allow summary routes in addition to not having external routes, that is, inter-area (IA) routes are not summarized into totally stubby areas. The only way for traffic to get routed outside 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.

 * Occasionally, it is said that a TSA can have only one ABR.

Not-so-stubby area

 * In hello packets the N-flag is set high, indicating "NSSA: supported"

A not-so-stubby area (NSSA) is a type of stub area that can import autonomous system external routes and send them to other areas, but still cannot receive AS-external routes from other areas.

NSSA is an extension of the stub area feature that allows the injection of external routes in a limited fashion into the stub area. A case study simulates an NSSA getting around the stub-area problem of not being able to import external addresses. It visualizes the following activities: the ASBR imports external addresses with a type 7 LSA, the ABR converts a type 7 LSA to type 5 and floods it to other areas, the ABR acts as an ASBR for other areas. The ASBRs do not take type 5 LSAs and then convert to type 7 LSAs for the area.

Totally not-so-stubby area
An addition to the standard functionality of an NSSA, the totally stubby NSSA is an NSSA that takes on the attributes of a TSA, meaning that type 3 and 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 boundary 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.

 * Redistribution into an NSSA area creates a special type of LSA known as type 7, which can exist only in an NSSA area. An NSSA ASBR generates this LSA, and an NSSA ABR router translates it into a type 5 LSA, which gets propagated into the OSPF domain.

A newly acquired subsidiary is one example of where it might be suitable for an area to be simultaneously not-so-stubby and totally stubby if the practical place to put an ASBR is on the edge of a totally stubby area. In such a case, the ASBR does send externals into the totally stubby area, and they are available to OSPF speakers within that area. In Cisco's implementation, the external routes can be summarized before injecting them into the totally stubby area. In general, the ASBR should not advertise default into the TSA-NSSA, although this can work with extremely careful design and operation, for the limited special cases in which such an advertisement makes sense.

By declaring the totally stubby area as NSSA, no external routes from the backbone, except the default route, enter the area being discussed. The externals do reach area 0.0.0.0 via the TSA-NSSA, but no routes other than the default route enter the TSA-NSSA. Routers in the TSA-NSSA send all traffic to the ABR, except to routes advertised by the ASBR.

Router types
OSPF defines the following overlapping categories of routers:
 * Internal router (IR): An internal router has all its interfaces belonging to the same area.
 * Area border router (ABR): An area border router is a router that connects one or more areas to the main backbone network. It is considered a member of all areas it is connected to. An ABR keeps multiple instances of the link-state database in memory, one for each area to which that router is connected.
 * Backbone router (BR): A backbone router has an interface to the backbone area. Backbone routers may also be area routers, but do not have to be.
 * Autonomous system boundary router (ASBR): An autonomous system boundary router is a router that is connected by using more than one routing protocol and that exchanges routing information with routers autonomous systems. ASBRs typically also run an exterior routing protocol (e.g., BGP), or use static routes, or both. An ASBR is used to distribute routes received from other, external ASs throughout its own autonomous system. An ASBR creates External LSAs for external addresses and floods them to all areas via ABR. Routers in other areas use ABRs as next hops to access external addresses. Then ABRs forward packets to the ASBR that announces the external addresses.

The router type is an attribute of an OSPF process. A given physical router may have one or more OSPF processes. 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 area border router and an autonomous system boundary router.

Each router has an identifier, customarily written in the dotted-decimal format (e.g., 1.2.3.4) of an IP address. This identifier must be established in every OSPF instance. If not explicitly configured, the highest logical IP address will be duplicated as the router identifier. However, since the router identifier is not an IP address, it does not have to be a part of any routable subnet in the network, and often isn't to avoid confusion.

Non-point-to-point network
On networks (same subnet) with networks type of:
 * Broadcast
 * Non-Broadcast Multi-Access (NBMA)

A system of designated router (DR) and backup designated router (BDR), is used to reducing network traffic by providing a source for routing updates. This is done using multicast addresses:

 * Multicast address, all routers in the topology will listen on that multicast address.
 * Multicast address, DR and BDR will listen on that multicast address.

The DR and BDR maintains a complete topology table of the network and sends the updates to the other routers via multicast. All routers in a multi-access network segment will form a leader/follower relationship with the DR and BDR. They will form adjacencies with the DR and BDR only. Every time a router sends an update, it sends it to the DR and BDR on the multicast address. The DR will then send the update out to all other routers in the area, to the multicast address. 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 OSPF broadcast networks. DR's can also be elected on NBMA (Non-Broadcast Multi-Access) networks such as Frame Relay or ATM. DRs or BDRs are not elected on point-to-point links (such as a point-to-point WAN connection) because the two routers on either side of the link must become fully adjacent and the bandwidth between them cannot be further optimized. DR and non-DR routers evolve from 2-way to full adjacency relationships by exchanging DD, Request, and Update.

Designated router
A designated router (DR) is the router interface elected among all routers on a particular multiaccess network segment, generally assumed to be broadcast multiaccess. Special techniques, often vendor-dependent, may be needed to support the DR function on non-broadcast multiaccess (NBMA) media. It is usually wise to configure the individual virtual circuits of an NBMA subnet as individual point-to-point lines; the techniques used are implementation-dependent.

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. The BDR is the OSPF router with the second-highest priority at the time of the last election.

A given router can have some interfaces that are designated (DR) and others that are backup designated (BDR), and others that are non-designated. If no router is a DR or a BDR on a given subnet, the BDR is first elected, and then a second election is held for the DR.

DR Other
A router that has not been selected to be designated router (DR) or backup designated router (BDR). The router forms an adjacency to both the designated router (DR) and the backup designated router (BDR).

For other non (B)DR, the adjacency stops at 2-ways State.

Designated router election
The DR is elected based on the following default criteria:
 * If the priority setting on an OSPF router is set to 0, that means it can NEVER become a DR or BDR.
 * If no DR exists on the network, routes will wait until Wait Timer runs out.
 * When a DR fails and the BDR takes over, there is another election to see who becomes the replacement BDR.
 * The router sending the Hello packets with the highest priority wins the election.
 * If two or more routers tie with the highest priority setting, the router sending the Hello with the highest RID (Router ID) wins. NOTE: a RID is the highest logical (loopback) IP address configured on a router, if no logical/loopback IP address is set then the router uses the highest IP address configured on its active interfaces (e.g. would be higher than ).
 * Usually the router with the second-highest priority number becomes the BDR.
 * The priority values range between 0 – 255, with a higher value increasing its chances of becoming DR or BDR.
 * If a higher priority OSPF router comes online after the election has taken place, it will not become DR or BDR until (at least) the DR and BDR fail.
 * If the current DR 'goes down' the current BDR becomes the new DR and a new election takes place to find another BDR. If the new DR then 'goes down' and the original DR is now available, still previously chosen BDR will become DR.

When DR has Routing update

 * 1) DR sends LSU to
 * 2) BDR sends LSUAck to
 * 3) DR Other sends LSUAck to

When BDR has Routing update

 * 1)  BDR sends LSU to
 * 2)  BDR sends LSUAck to
 * 3)  DR Other sends LSUAck to

When DR Other has Routing update

 * 1)  DR Other sends LSU to
 * 2)  BDR sends LSA to
 * 3)  BDR sends LSUAck to
 * 4)  Non-source routers, DR Other sends LSUAck to

Protocol messages
Unlike other routing protocols, OSPF does not carry data via a transport protocol, such as the User Datagram Protocol (UDP) or the Transmission Control Protocol (TCP). Instead, OSPF forms IP datagrams directly, packaging them using protocol number 89 for the IP Protocol field. OSPF defines five different message types, for various types of communication. Multiple packets can be sent per frame.

OSPF uses 5 packet types:
 * Hello
 * Database description
 * Link state request
 * Link state update
 * Link state acknowledgement

Hello Packet
OSPF's Hello messages are used as a form of greeting, to allow a router to discover other adjacent routers on its local links and networks. The messages establish relationships between neighboring devices (called adjacencies) and communicate key parameters about how OSPF is to be used in the autonomous system or area. During normal operation, routers send hello messages to their neighbors at regular intervals (the hello interval); if a router stops receiving hello messages from a neighbor, after a set period (the dead interval) the router will assume the neighbor has gone down.

Database description (DBD)
Database description messages contain descriptions of the topology of the autonomous system or area. They convey the contents of the link-state database (LSDB) for the area from one router to another. Communicating a large LSDB may require several messages to be sent by having the sending device designated as a leader device and sending messages in sequence, with the follower (recipient of the LSDB information) responding with acknowledgments.

Link state packets

 * Link state request (LSR): Link state request messages are used by one router to request updated information about a portion of the LSDB from another router. The message specifies the link(s) for which the requesting device wants more current information.

 * Link state update (LSU): Link-state update messages contain updated information about the state of certain links on the LSDB. They are sent in response to a link state request message, and also broadcast or multicast by routers on a regular basis. Their contents are used to update the information in the LSDBs of routers that receive them.
 * Link state acknowledgment (LSAck): Link-state acknowledgment messages provide reliability to the link-state exchange process, by explicitly acknowledging receipt of a Link State Update message.

OSPF v2 area types and accepted LSAs
Not all area types use all LSA. Below is a matrix of accepted LSAs.

Routing metrics
OSPF uses path cost as its basic routing metric, which was defined by the standard not to equate to any standard value such as speed, so the network designer could pick a metric important to the design. In practice, it is determined by comparing the speed of the interface to a reference-bandwidth for the OSPF process. The cost is determined by dividing the reference bandwidth by the interface speed (although the cost for any interface can be manually overridden). If a reference bandwidth is set to '10000', then a 10 Gbit/s link will have a cost of 1. Any speeds less than 1 are rounded up to 1. Here is an example table that shows the routing metric or 'cost calculation' on an interface.
 * Type-1 LSA has a size of 16-bit field (65,535 in decimal)
 * Type-3 LSA has a size of 24-bit field (16,777,216 in decimal)

OSPF is a layer 3 protocol. If a layer 2 switch is between the two devices running OSPF, one side may negotiate a speed different from the other side. This can create an asymmetric routing on the link (Router 1 to Router 2 could cost '1' and the return path could cost '10'), which may lead to unintended consequences.

Metrics, however, are only directly comparable when of the same type. Four types of metrics are recognized. In decreasing preference (for example, an intra-area route is always preferred to an external route regardless of metric), these types are:
 * 1) Intra-area
 * 2) Inter-area
 * 3) External Type 1, which includes both the external path cost and the sum of internal path costs to the ASBR that advertises the route,
 * 4) External Type 2, the value of which is solely that of the external path cost,

OSPF v3
OSPF version 3 introduces modifications to the IPv4 implementation of the protocol. Despite the expansion of addresses to 128 bits in IPv6, area and router identifications are still 32-bit numbers.

High-level changes

 * Except for virtual links, all neighbor exchanges use IPv6 link-local addressing exclusively. The IPv6 protocol runs per link, rather than based on the subnet.
 * All IP prefix information has been removed from the link-state advertisements and from the hello discovery packet, making OSPFv3 essentially protocol-independent.
 * Three separate flooding scopes for LSAs:
 * Link-local scope: LSA is flooded only on the local link and no further.
 * Area scope: LSA is flooded throughout a single OSPF area.
 * AS scope: LSA is flooded throughout the routing domain.
 * Use of IPv6 link-local addresses, for neighbor discovery, auto-configuration.
 * Authentication has been moved to the IP Authentication Header

Changes introduced in OSPF v3, then backported by vendors to v2

 * Explicit support for multiple instances per link

Packet format changes

 * OSPF version number changed to 3
 * From the LSA header, the options field has been removed.
 * In hello packets and database description, the options field is changed from 16 to 24 bits.
 * In hello packet, the address information has been removed. The interface ID has been added.
 * In router-LSAs, two options bits, the R-bit and the V6-bit, have been added.
 * R-bit: allows for multi-homed hosts to participate in the routing protocol.
 * V6-bit: specializes the R-bit.
 * Add instance ID, which allows multiple OSPF protocol instances on the same logical interface.

LSA format changes

 * The LSA type field is changed to 16 bits.
 * Add support for handling unknown LSA types
 * Three bits are used for encoding flooding scope.
 * With IPv6, addresses in LSAs are expressed as prefix and prefix length.
 * In router-LSAs and network-LSAs, the address information is removed.
 * Router-LSAs and network-LSAs are made network-protocol independent.
 * A new LSA type is added, link-LSA, which provides the router's link-local address to all other routers attached to the logical interface, provides a list of IPv6 prefixes to associate with the link, and can send information that reflect the router's capabilities.
 * LSA Type-3 summary-LSAs have been renamed "inter-area-prefix-LSAs".
 * LSA Type-4 summary LSAs have been renamed "inter-area-router-LSAs".
 * Intra-area-prefix-LSA is added, an LSA that carries all IPv6 prefix information.

OSPF over MPLS VPN

A customer can use OSPF over a MPLS VPN, where the service provider uses BGP or RIP as their interior gateway protocol. When using OSPF over MPLS VPN, the VPN backbone becomes part of the OSPF backbone area 0. In all areas, isolated copies of the IGP are run.

Advantages:
 * The MPLS VPN is transparent to the customer's OSPF standard routing.
 * Customer's equipment only needs to support OSPF.
 * Reduce the need for tunnels (Generic Routing Encapsulation, IPsec, wireguard) to use OSPF.

To achieve this, a modified OSPF-BGP redistribution is used. All OSPF routes retain the source LSA type and metric. To prevent loops, an optional DN bit is set by the service provider in LSAs from the provider equipment to indicate that a route has already been sent to the customer's equipment.

Traffic engineering
OSPF-TE is an extension to OSPF extending the expressivity to allow for traffic engineering and use on non-IP networks. Using OSPF-TE, more information about the topology can be exchanged using opaque LSA carrying type–length–value elements. These extensions allow OSPF-TE to run completely out of band of the data plane network. This means that it can also be used on non-IP networks, such as optical networks.

OSPF-TE is used in GMPLS networks as a means to describe the topology over which GMPLS paths can be established. GMPLS uses its own path setup and forwarding protocols, once it has the full network map.

In the Resource Reservation Protocol (RSVP), OSPF-TE is used for recording and flooding RSVP signaled bandwidth reservations for label-switched paths within the link-state database.

Optical routing
documents work in optical routing for IP based on extensions to OSPF and IS-IS.

Multicast Open Shortest Path First
The Multicast Open Shortest Path First (MOSPF) protocol is an extension to OSPF to support multicast routing. MOSPF allows routers to share information about group memberships.

Notable implementations

 * Allied Telesis implements OSPFv2 & OSPFv3 in Allied Ware Plus (AW+)
 * Arista Networks implements OSPFv2 and OSPFv3
 * BIRD implements both OSPFv2 and OSPFv3
 * Cisco IOS and NX-OS
 * Cisco Meraki
 * D-Link implements OSPFv2 on Unified Services Router.
 * Dell's FTOS implements OSPFv2 and OSPFv3
 * ExtremeXOS
 * FRRouting, the successor of Quagga
 * GNU Zebra, a GPL routing suite for Unix-like systems
 * Juniper Junos
 * NetWare implements OSPF in its Multi Protocol Routing module.
 * OpenBSD includes OpenOSPFD, an OSPFv2 implementation.
 * Quagga, a fork of GNU Zebra for Unix-like systems
 * Windows NT 4.0 Server, Windows 2000 Server and Windows Server 2003 implemented OSPFv2 in the Routing and Remote Access Service, although the functionality was removed in Windows Server 2008.
 * XORP, a routing suite implementing RFC2328 (OSPFv2) and RFC2740 (OSPFv3) for both IPv4 and IPv6

Applications
OSPF is a widely deployed routing protocol that can converge a network in a few seconds and guarantee loop-free paths. It has many features that allow the imposition of policies about the propagation of routes that it may be appropriate to keep local, for load sharing, and for selective route importing. IS-IS, in contrast, can be tuned for lower overhead in a stable network, the sort more common in ISP than enterprise networks. There are some historical accidents that made IS-IS the preferred IGP for ISPs, but ISPs today may well choose to use the features of the now-efficient implementations of OSPF, after first considering the pros and cons of IS-IS in service provider environments.

When the default route to an ISP is injected into OSPF from multiple ASBRs as a Type I external route and the same external cost specified, other routers will go to the ASBR with the least path cost from its location. This can be tuned further by adjusting the external cost. If the default route from different ISPs is injected with different external costs, as a Type II external route, the lower-cost default becomes the primary exit and the higher-cost becomes the backup only.

title: Open Shortest Path First


text:
Routing Information Protocol

The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from source to destination. The largest number of hops allowed for RIP is 15, which limits the size of networks that RIP can support.

RIP implements the split horizon, route poisoning, and holddown mechanisms to prevent incorrect routing information from being propagated.

In RIPv1 routers broadcast updates with their routing table every 30 seconds. In the early deployments, routing tables were small enough that the traffic was not significant. As networks grew in size, however, it became evident there could be a massive traffic burst every 30 seconds, even if the routers had been initialized at random times.

In most networking environments, RIP is not the preferred choice of routing protocol, as its time to converge and scalability are poor compared to EIGRP, OSPF, or IS-IS. However, it is easy to configure, because RIP does not require any parameters, unlike other protocols.

RIP uses the User Datagram Protocol (UDP) as its transport protocol, and is assigned the reserved port number 520.

Development of distance-vector routing
Based on the Bellman–Ford algorithm and the Ford–Fulkerson algorithm, distance-vector routing protocols started to be implemented from 1969 onwards in data networks such as the ARPANET and CYCLADES. The predecessor of RIP was the Gateway Information Protocol (GWINFO) which was developed by Xerox in the mid-1970s to route its experimental network. As part of the Xerox Network Systems (XNS) protocol suite GWINFO transformed into the XNS Routing Information Protocol. This XNS RIP in turn became the basis for early routing protocols, such as Novell's IPX RIP, AppleTalk's Routing Table Maintenance Protocol (RTMP), and the IP RIP. The 1982 Berkeley Software Distribution of the UNIX operating system implemented RIP in the routed daemon. The 4.2BSD release proved popular and became the basis for subsequent UNIX versions, which implemented RIP in the routed or gated daemon. Ultimately, RIP had been extensively deployed before the standard, written by Charles Hedrick, was passed as RIPv1 in 1988.

The RIP hop count
The routing metric used by RIP counts the number of routers that need to be passed to reach a destination IP network. The hop count 0 denotes a network that is directly connected to the router. 16 hops denote a network that is unreachable, according to the RIP hop limit.

Versions
There are three standardized versions of the Routing Information Protocol: RIPv1 and RIPv2 for IPv4, and RIPng for IPv6.

RIP version 1
The original specification of RIP was published in 1988. When starting up, and every 30 seconds thereafter, a router with RIPv1 implementation broadcasts to a request message through every RIPv1 enabled interface. Neighbouring routers receiving the request message respond with a RIPv1 segment, containing their routing table. The requesting router updates its own routing table, with the reachable IP network address, hop count and next hop, that is the router interface IP address from which the RIPv1 response was sent. As the requesting router receives updates from different neighbouring routers it will only update the reachable networks in its routing table, if it receives information about a reachable network it has not yet in its routing table or information that a network it has in its routing table is reachable with a lower hop count. Therefore, a RIPv1 router will in most cases only have one entry for a reachable network, the one with the lowest hop count. If a router receives information from two different neighbouring router that the same network is reachable with the same hop count but via two different routes, the network will be entered into the routing table two times with different next hop routers. The RIPv1 enabled router will then perform what is known as equal-cost load balancing for IP packets.

RIPv1 enabled routers not only request the routing tables of other routers every 30 seconds, they also listen to incoming requests from neighbouring routers and send their own routing table in turn. RIPv1 routing tables are therefore updated every 25 to 35 seconds. The RIPv1 protocol adds a small random time variable to the update time, to avoid routing tables synchronizing across a LAN. It was thought, as a result of random initialization, the routing updates would spread out in time, but this was not true in practice. Sally Floyd and Van Jacobson showed in 1994 that, without slight randomization of the update timer, the timers synchronized over time.

RIPv1 can be configured into silent mode, so that a router requests and processes neighbouring routing tables, and keeps its routing table and hop count for reachable networks up to date, but does not needlessly send its own routing table into the network. Silent mode is commonly implemented to hosts.

RIPv1 uses classful routing. The periodic routing updates do not carry subnet information, lacking support for variable length subnet masks (VLSM). This limitation makes it impossible to have different-sized subnets inside of the same network class. In other words, all subnets in a network class must have the same size. There is also no support for router authentication, making RIP vulnerable to various attacks.

RIP version 2
Due to the deficiencies of the original RIP specification, RIP version 2 (RIPv2) was developed in 1993, published in 1994, and declared Internet Standard 56 in 1998. It included the ability to carry subnet information, thus supporting Classless Inter-Domain Routing (CIDR). To maintain backward compatibility, the hop count limit of 15 remained. RIPv2 has facilities to fully interoperate with the earlier specification if all Must Be Zero protocol fields in the RIPv1 messages are properly specified. In addition, a compatibility switch feature allows fine-grained interoperability adjustments.

In an effort to avoid unnecessary load on hosts that do not participate in routing, RIPv2 multicasts the entire routing table to all adjacent routers at the address Multicast address, as opposed to RIPv1 which uses broadcast. Unicast addressing is still allowed for special applications.

(MD5) authentication for RIP was introduced in 1997.

Route tags were also added in RIP version 2. This functionality allows a distinction between routes learned from the RIP protocol and routes learned from other protocols.

RIPng
RIPng (RIP next generation) is an extension of RIPv2 for support of IPv6, the next generation Internet Protocol. The main differences between RIPv2 and RIPng are:
 * Support of IPv6 networking.
 * While RIPv2 supports RIPv1 updates authentication, RIPng does not. IPv6 routers were, at the time, supposed to use IPsec for authentication.
 * RIPv2 encodes the next-hop into each route entry, RIPng requires specific encoding of the next hop for a set of route entries.

RIPng sends updates on UDP port 521 using the multicast group.

RIP messages between routers
RIP messages use the User Datagram Protocol on port 520 and all RIP messages exchanged between routers are encapsulated in a UDP datagram.

RIPv1 Messages
RIP defined two types of messages:
 * Request Message
 * Asking a neighbouring RIPv1 enabled router to send its routing table.

 * Response Message
 * Carries the routing table of a router.

Timers
The routing information protocol uses the following timers as part of its operation:

 * Update Timer
 * Controls the interval between two gratuitous Response Messages. By default the value is 30 seconds. The response message is broadcast to all its RIP enabled interface.

 * Invalid Timer
 * The invalid timer specifies how long a routing entry can be in the routing table without being updated. This is also called as expiration Timer. By default, the value is 180 seconds. After the timer expires the hop count of the routing entry will be set to 16, marking the destination as unreachable.

 * Flush Timer
 * The flush timer controls the time between the route is invalidated or marked as unreachable and removal of entry from the routing table. By default the value is 240 seconds. This is 60 seconds longer than Invalid timer. So for 60 seconds the router will be advertising about this unreachable route to all its neighbours. This timer must be set to a higher value than the invalid timer.

 * Holddown Timer
 * The hold-down timer is started per route entry, when the hop count is changing from lower value to higher value. This allows the route to get stabilized. During this time no update can be done to that routing entry. This is not part of the RFC 1058. This is Cisco's implementation.  The default value of this timer is 180 seconds.

Limitations

 * The hop count cannot exceed 15, or routes will be dropped.
 * Variable Length Subnet Masks are not supported by RIP version 1 (which is obsolete).
 * RIP has slow convergence and count to infinity problems.

Implementations

 * Cisco IOS, software used in Cisco routers (supports version 1, version 2 and RIPng)
 * Cisco NX-OS software used in Cisco Nexus data center switches (supports RIPv2 only )
 * Junos software used in Juniper routers, switches, and firewalls (supports RIPv1 and RIPv2)
 * Routing and Remote Access, a Windows Server feature, contains RIP support
 * Quagga, a free open source software routing suite based on GNU Zebra
 * BIRD, a free open source software routing suite
 * Zeroshell, a free open source software routing suite
 * A RIP implementation first introduced in 4.2BSD, routed, survives in several of its descendants, including FreeBSD and NetBSD.
 * OpenBSD introduced a new implementation, ripd, in version 4.1 and retired routed in version 4.4.
 * Netgear routers commonly offer a choice of two implementations of RIPv2; these are labelled RIP_2M and RIP_2B. RIP_2M is the standard RIPv2 implementation using multicasting - which requires all routers on the network to support RIPv2 and multicasting, whereas RIP_2B sends RIPv2 packets using subnet broadcasting - making it more compatible with routers that do not support multicasting, including RIPv1 routers.
 * Huawei HG633 ADSL/VDSL routers support passive and active routing with RIP v1 & v2 on the LAN and WAN side.

Similar protocols
Cisco's proprietary Interior Gateway Routing Protocol (IGRP) was a somewhat more capable protocol than RIP. It belongs to the same basic family of distance-vector routing protocols.

Cisco has ceased support and distribution of IGRP in their router software. It was replaced by the Enhanced Interior Gateway Routing Protocol (EIGRP) which is a completely new design. While EIGRP still uses a distance-vector model, it relates to IGRP only in using the same composite routing metric. Both IGRP and EIGRP calculated a single composite metric for each route, from a formula of five variables: bandwidth, delay, reliability, load, and MTU; though on Cisco routers, by default, only bandwidth and delay are used in this calculation.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Routing Information Protocol


text:
Erich von Drygalski

Erich Dagobert von Drygalski (February 9, 1865 – January 10, 1949) was a German geographer, geophysicist and polar scientist, born in Königsberg, East Prussia.

Between 1882 and 1887, Drygalski studied mathematics and natural science at the University of Königsberg, Bonn, Berlin and Leipzig. He graduated with a doctorate thesis about ice shields in Nordic areas. Between 1888 and 1891, he was an assistant at the Geodetic Institute and the Central Office of International Geodetics in Berlin.

Drygalski led two expeditions between 1891 and 1893, which were supplied by the Society for Geoscience of Berlin. One expedition wintered during the winter between 1892 and 1893 in Western Greenland. He habilitated 1889 for geography and geophysics with the collected scientific evidence. In 1898, Drygalski became associate professor and 1899 extraordinary professor for geography and geophysics in Berlin.

Gauss expedition
Drygalski led the first German South Polar expedition with the ship Gauss to explore the unknown area of Antarctica lying south of the Kerguelen Islands. The expedition started from Kiel in the summer of 1901. A small party of the expedition was also stationed on the Kerguelen Islands, while the main party proceeded further south. Drygalski also paid a brief call to Heard Island and provided the first comprehensive scientific information on the island's geology, flora and fauna. Despite being trapped by ice for nearly fourteen months until February 1903, the expedition discovered new territory in Antarctica, the Kaiser Wilhelm II Land with the Gaussberg. The expedition arrived back in Kiel in November 1903. Subsequently, Drygalski wrote the narrative of the expedition and edited the voluminous scientific data. Between 1905 and 1931, he published twenty volumes and two atlases documenting the expedition and was awarded the 1933 Royal Geographical Society's Patron's Medal.

Later years
From October 1906 until his retirement, Drygalski was a professor in Munich, where he also presided the Geographic Institute, founded by him, until his death. In 1910, he also took part in Count Ferdinand von Zeppelin's expedition to Spitsbergen and participated in other expeditions to North America and northeastern Asia. He died in 1949 in Munich.

Trivia
Drygalski Island, Drygalski Fjord in South Georgia, and an avenue in the southern part of Munich were named after him, as is the crater Drygalski on the Moon. Two glaciers, including Drygalski Glacier (Antarctica) and Drygalski Glacier (Tanzania) on the slopes of Mount Kilimanjaro are also named for him. An archive in the Ludwig Maximilians University remembers his pioneering efforts. He also has a South African spider named after him, Araneus drygalskii (Strand, 1909), based on material collected on the Gauss expedition.

title: Erich von Drygalski