Open Shortest Path First

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.

Virtual link over Virtual links, tunneling and shamelinks, 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.[8] If the same IP or router ID is used the link creates two equal-cost routes to the destination.[9]
• Sham link[10][11][12] A link that connects sites that belong to the same OSPF area and share an OSPF backdoor link via MPLS VPN backbone.

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:[1][13]

Neighbor state changes (Hello Protocol)

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.

Neighbor state changes (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.

In broadcast multiple-access networks, neighbor adjacency is formed dynamically using multicast hello packets to 224.0.0.5.

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

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

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.

Example of backbone, area 0 with 2 routers, R1 and R2

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.[17] 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.[18][19]

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.

Figure show 3 routers with 2 areas, area 0 and area 122, sharing one router.

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.

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.

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.[20]

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.[21]

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.

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.

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.

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.[1]^: 75

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.

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. 192.168.0.1 would be higher than 10.1.1.2).
• Usually the router with the second-highest priority number becomes the BDR.
• The priority values range between 0 – 255,[22] 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.

1. DR sends LSU to 224.0.0.5
2. BDR sends LSUAck to 224.0.0.5
3. DR Other sends LSUAck to 224.0.0.6

1. BDR sends LSU to 224.0.0.5
2. BDR sends LSUAck to 224.0.0.5
3. DR Other sends LSUAck to 224.0.0.6

1. DR Other sends LSU to 224.0.0.6
2. BDR sends LSA to 224.0.0.5
3. BDR sends LSUAck to 224.0.0.5
4. Non-source routers, DR Other sends LSUAck to 224.0.0.6

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 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 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 link state advertisements

LS type	LS name	Generated by	Description
1	Router-LSAs	Each internal router within an area	The link-state ID of the type 1 LSA is the originating router ID. Router-LSAs, describe the following types of interfaces: Point-to-point connection to another router Connection to a transit network Connection to a stub network (Reserved in v3) Virtual link
2	Network-LSAs	The DR	Originated for broadcasts and NBMA networks by the designated router. This LSA contains the list of routers connected to the network. The link-state ID of the type 2 LSA is the IP interface address of the DR.
3	Summary-LSAs	The ABR	Type 3 summary-LSAs describe routes to networks. To inform other areas about inter-area routers. These routes can also be summarised.
4	ASBR-summary	The ABR	Type 4 describe routes to AS boundary routers beyond its area. The area border router (ABR) generates this LSA to inform other routers in the OSPF domain, that the matching router is an autonomous system boundary router (ASBR), so that the external LSAs (Type 5 / Type 7) it sent may be properly resolved outside its own area.
5	AS-external-LSAs	The ASBR	Type 5 These describe routes advertised by the ASBR. LSAs contain information imported into OSPF from other routing processes. Together with Type 4 they describe they way to an external route.
7	NSSA external link-state advertisements	The ASBR, within a not-so-stubby area	Type 7-LSAs are identical to type-5 LSAs. Type-7 LSAs are only flooded within the NSSA. At the area border router, selected type-7 LSAs are translated into type 5-LSAs and flooded into the backbone.
8	Link-LSA (v3)	Each internal router within a link	Provide it local router's link-local address to all other routers on the local network.
9	Intra-Area-Prefix-LSAs (v3)	Each internal router within an area	Replaces some of the functionality of Router-LSAs; stub network segment, or an attached transit network segment.

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

• 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

• Explicit support for multiple instances per link[28]

• 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.

• 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-TE is an extension to OSPF extending the expressivity to allow for traffic engineering and use on non-IP networks.[34] 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.

RFC 3717 documents work in optical routing for IP based on extensions to OSPF and IS-IS.[35]

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.