Open Shortest Path First (OSPF) is an adaptive routing protocol for Internet Protocol (IP) networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system (AS). It is defined as OSPF Version 2 in RFC 2328 (1998) for IPv4.[1] The updates for IPv6 are specified as OSPF Version 3 in RFC 5340 (2008).[2]
OSPF is perhaps the most widely-used interior gateway protocol (IGP) in large enterprise networks. IS-IS, another link-state routing protocol, is more common in large service provider networks. The most widely-used exterior gateway protocol is the Border Gateway Protocol (BGP), the principal routing protocol between autonomous systems on the Internet.
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Contents |
OSPF is an interior gateway protocol that routes Internet Protocol (IP) packets solely within a single routing domain (autonomous system). It gathers link state information from available routers and constructs a topology map of the network. The topology determines the routing table presented to the Internet Layer which makes routing decisions based solely on the destination IP address found in IP datagrams. OSPF was designed to support variable-length subnet masking (VLSM) or Classless Inter-Domain Routing (CIDR) addressing models.
OSPF detects changes in the topology, such as link failures, very quickly 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, a shortest path first algorithm.
The link-state information is maintained on each router as a link-state database (LSDB) which is a tree-image of the entire network topology. Identical copies of the LSDB are periodically updated through flooding on all OSPF routers.
The OSPF routing policies to construct a route table are governed by link cost factors (external metrics) associated with each routing interface. Cost factors may be the distance of a router (round-trip time), network 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.
An OSPF network may be structured, or subdivided, 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 octet-based dot-decimal notation, familiar from IPv4 address notation.
By convention, area 0 (zero) or 0.0.0.0 represents the core or backbone region of an OSPF network. The identifications of other areas may be chosen at will, often, administrators select the IP address of a main router in an area as the area's identification. Each additional area must have a direct or virtual connection to the backbone OSPF area. Such connections are maintained by an interconnecting router, known as 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 does not use a TCP/IP transport protocol (UDP, TCP), but is encapsulated directly in IP datagrams with protocol number 89. This is in contrast to other routing protocols, such as the Routing Information Protocol (RIP), or the Border Gateway Protocol (BGP). OSPF handles its own error detection and correction functions.
OSPF uses multicast addressing for route flooding on a broadcast network link. For non-broadcast networks special provisions for configuration facilitate neighbor discovery.[1] OSPF multicast IP packets never traverse IP routers, they never travel more than one hop. OSPF reserves the multicast addresses 224.0.0.5 for IPv4 or FF02::5 for IPv6 (all SPF/link state routers, also known as AllSPFRouters) and 224.0.0.6 for IPv4 or FF02::6 for IPv6 (all Designated Routers, AllDRouters), as specified in RFC 2328[3] and RFC 5340[4].
For routing multicast IP traffic, OSPF supports the Multicast Open Shortest Path First protocol (MOSPF) as defined in RFC 1584.[5] Neither Cisco nor Juniper Networks include MOSPF in their OSPF implementations. PIM (Protocol Independent Multicast) in conjunction with OSPF or other IGPs, (Interior Gateway Protocol), is widely deployed.
The OSPF protocol, when running on IPv4, can operate securely between routers, optionally using a variety of authentication methods to allow only trusted routers to participate in routing. OSPFv3, running on IPv6, no longer supports protocol-internal authentication. Instead, it relies on IPv6 protocol security (IPsec).
OSPF version 3 introduces modifications to the IPv4 implementation of the protocol.[2] 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 values.
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. This detection occurs when a router identifies itself in a hello OSPF protocol packet. This is called a two-way state and is 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 to send "hello packets" and link state updates.
As a link state routing protocol, OSPF establishes and maintains neighbor relationships in order to exchange routing updates with other routers. The neighbor relationship table is called an adjacency database in OSPF. Provided that OSPF is configured correctly, OSPF forms neighbor relationships only with the routers directly connected to it. In order to form a neighbor relationship between two routers, the interfaces used to form the relationship must be in the same area. An interface can only belong to a single area.(A neighbor state simulation video shows how neighbor state changes from Down to Full Adjacency with exchanging Hello, DD, Request, Update, and Ack).
An OSPF domain 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. While most OSPF implementations will right-justify an area number written in a format other than dotted decimal format (e.g., area 1), it is wise to always use dotted-decimal formats. 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.
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.(See a short video of ABR simulation. How ABR let areas know each others’ subnet addresses by flooding Summary LSA.)
Several "special" area types are defined:
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, 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. Note that 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, RFC1996 (ASGuidelines 1996, p. 25).[6]
The backbone area is responsible for distributing routing information between nonbackbone areas. The backbone must be contiguous, but it does not need to be physically contiguous; backbone connectivity can be established and maintained through the configuration of virtual links.
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.
A stub area is an area which does not receive route advertisements external to the autonomous system (AS) and routing from within the area is based entirely on a default route. (A Stub Area simulation video shows how ABR deletes type 4, 5 LSA of internal routers, sends them a default route 0.0.0.0, and makes itself a default gateway for internal routers to access external addresses.)This reduces the size of the routing databases for the area's internal routers.
Modifications to the basic concept of stub areas exist in the not-so-stubby area (NSSA). In addition, several other proprietary variation have been implemented by systems vendors, such as the totally stubby area (TSA) and the NSSA totally stubby area, both an extension in Cisco Systems routing equipment.
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 NSSA simulation video shows how a NSSA gets around the Stub Area problem: No External LSA. In NSSA, ASBR can import external addresses with type 7 LSA. Then ABR converts type 7 to type 5 LSA and floods them to other areas. In fact, ABR necomes ASBR for other areas.)
Several vendors, (Cisco, Juniper, Huawei, Quagga), now implement below two extensions to stub and NSSA area and although not covered by RFC they are considered by many to be standard features in OSPF implementations.
Stub areas
An area can simultaneously be not-so-stubby and totally stubby. This is done when the practical place to put an ASBR, as, for example, with a newly acquired subsidiary, 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 ASBR.
A transit area is an area with two or more OSPF border routers and is used to pass network traffic from one adjacent area to another. The transit area does not originate this traffic and is not the destination of such traffic.
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 the speed (bandwidth) of the interface addressing the given route, although that tends to need network-specific scaling factors now that links faster than 100 Mbit/s are common. Cisco uses a metric like 10^8/bandwidth (the base value, 10^8 by default, can be adjusted). So, a 100Mbit/s link will have a cost of 1, a 10Mbit/s a cost of 10 and so on. But for links faster than 100Mbit/s, the cost would be <1.
Metrics, however, are only directly comparable when of the same type. There are four types of metrics, with the most preferred type listed in order below. An intra-area route is always preferred to an inter-area route regardless of metric, and so on for the other types.
OSPF-TE is an extension to OSPF extending the expressivity to allow for traffic engineering and use on non-IP networks (RFC 3630).[9] 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.
Uses of OSPF-TE include the following.
RFC 3717 documents work in optical routing for IP, based on "constraint-based" extensions to OSPF and IS-IS.[10]
OSPF defines the following router types:
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 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.
These router types should not be confused with the terms designated router (DR), or backup designated router (BDR), which are attributes of a router interface, not the router itself.
An area border router (ABR) 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 copies of the link-state database in memory, one for each area to which that router is connected.
An autonomous system boundary router (ASBR) is a router that is connected to more than one Routing protocol and that exchanges routing information with routers in other protocols. 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.(See a short video of ASBR simulation. How ASBR creates External LSA for external addresses and floods them to all areas. )
An internal router is a router that has OSPF neighbor relationships with interfaces in the same area.
Backbone routers are all routers that are connected to the OSPF backbone, irrespective whether they are also area border routers or internal routers of the backbone area. An area border router is always a backbone router, since all areas must be either directly connected to the backbone or connected to the backbone via a virtual link (spanning across another area to get to the backbone).
A designated router (DR) is the router interface elected among all routers on a particular multiaccess network segment, generally assumed to be broadcast multiaccess.A video DR election basic flow visualizes neighbor discovery (Hello), flooding (224.0.0.6), DR election (priority, RID). Special techniques, often vendor-dependent, may be needed to support the DR function on nonbroadcast multiaccess (NBMA) media. It is usually wise to configure the individual virtual circuits of a NBMA subnet as individual point-to-point lines; the techniques used are implementation-dependent.
Do not confuse the DR with an OSPF router type. A given physical router can have some interfaces that are designated (DR), others that are backup designated (BDR), and others that are non-designated. If no router is DR or BDR on a given subnet, the DR is first elected, and then a second election is held if there is more than one BDR.(A DR Election Detail video shows a progressive election process with topology, Hello, neighbor list)[11] 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. All routers in an area will form a slave/master relationship with the DR. 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 224.0.0.6. The DR will then send the update out to all other routers in the area, to the multicast address 224.0.0.5. 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 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 sides of the link must become fully adjacent and the bandwidth between them cannot be further optimized.A video here shows how DR and non-DR routers evolve from 2-way to full adjacency by exchanging DD, Request, and Update.
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 second highest priority at the time of the last election.
The "Main OSPF Packet Header" is the same for all 5 types of packets (with exception of the Type field) where as the following sub-headers will vary from type to type and are shown below the Main OSPF Packet Header.
| Octet | Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | Version | Type | Packet Length | |||||||||||||||||||||||||||||
| 4 | 32 | Router ID | |||||||||||||||||||||||||||||||
| 8 | 64 | Area ID | |||||||||||||||||||||||||||||||
| 12 | 96 | Checksum | Instance ID | 0 | |||||||||||||||||||||||||||||
As per Appendix A.3 of RFC 5340 (OSPFv3 for IPv6) there are 5 OSPF Packet formats as follows:
| Type | Description |
| 1 | Hello |
| 2 | Database Description |
| 3 | Link State Request |
| 4 | Link State Update |
| 5 | Link State Acknowledgement |
The five different formats for each "Type" of OSPF v3 packet are listed below:
| Octet | Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 3 {Ver} | 1 {Type} | Packet Length | |||||||||||||||||||||||||||||
| 4 | 32 | Router ID | |||||||||||||||||||||||||||||||
| 8 | 64 | Area ID | |||||||||||||||||||||||||||||||
| 12 | 96 | Checksum | Instance ID | 0 | |||||||||||||||||||||||||||||
| 16 | 128 | Interface ID | |||||||||||||||||||||||||||||||
| 20 | 160 | Rtr Priority | Options (Explained below) | ||||||||||||||||||||||||||||||
| 24 | 192 | HelloInterval | RouterDeadInterval | ||||||||||||||||||||||||||||||
| 28 | 224 | Designated Router ID | |||||||||||||||||||||||||||||||
| 32 | 256 | Backup Designated Router ID | |||||||||||||||||||||||||||||||
| 36 | 288 | Neighbor ID | |||||||||||||||||||||||||||||||
| ~ | ~ | ... | |||||||||||||||||||||||||||||||
| Octet | Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 3 {Ver} | 2 {Type} | Packet Length | |||||||||||||||||||||||||||||
| 4 | 32 | Router ID | |||||||||||||||||||||||||||||||
| 8 | 64 | Area ID | |||||||||||||||||||||||||||||||
| 12 | 96 | Checksum | Instance ID | 0 | |||||||||||||||||||||||||||||
| 16 | 128 | 0 | Options (Explained below) | ||||||||||||||||||||||||||||||
| 20 | 160 | Interface MTU | 0 | 0 | 0 | 0 | 0 | 0 | I | M | M S |
||||||||||||||||||||||
| 24 | 192 | DD sequence number | |||||||||||||||||||||||||||||||
| 28 | 224 | An LSA Header | |||||||||||||||||||||||||||||||
| 32 | 256 | ||||||||||||||||||||||||||||||||
| 36 | 288 | ||||||||||||||||||||||||||||||||
| 40 | 320 | ||||||||||||||||||||||||||||||||
| 44 | 352 | ||||||||||||||||||||||||||||||||
| ~ | ~ | ... | |||||||||||||||||||||||||||||||
| Octet | Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 3 {Ver} | 3 {Type} | Packet Length | |||||||||||||||||||||||||||||
| 4 | 32 | Router ID | |||||||||||||||||||||||||||||||
| 8 | 64 | Area ID | |||||||||||||||||||||||||||||||
| 12 | 96 | Checksum | Instance ID | 0 | |||||||||||||||||||||||||||||
| 16 | 128 | 0 | LS Type | ||||||||||||||||||||||||||||||
| 20 | 160 | Link State ID | |||||||||||||||||||||||||||||||
| 24 | 192 | Advertising Router | |||||||||||||||||||||||||||||||
| ~ | ~ | ... | |||||||||||||||||||||||||||||||
| Octet | Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 3 {Ver} | 4 {Type} | Packet Length | |||||||||||||||||||||||||||||
| 4 | 32 | Router ID | |||||||||||||||||||||||||||||||
| 8 | 64 | Area ID | |||||||||||||||||||||||||||||||
| 12 | 96 | Checksum | Instance ID | 0 | |||||||||||||||||||||||||||||
| 16 | 128 | # LSAs | |||||||||||||||||||||||||||||||
| 20 | 160 | LSAs | |||||||||||||||||||||||||||||||
| 24 | 192 | ||||||||||||||||||||||||||||||||
| 28 | 224 | ||||||||||||||||||||||||||||||||
| 32 | 256 | ||||||||||||||||||||||||||||||||
| 36 | 288 | ||||||||||||||||||||||||||||||||
| ~ | ~ | ... | |||||||||||||||||||||||||||||||
| Octet | Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 3 {Ver} | 5 {Type} | Packet Length | |||||||||||||||||||||||||||||
| 4 | 32 | Router ID | |||||||||||||||||||||||||||||||
| 8 | 64 | Area ID | |||||||||||||||||||||||||||||||
| 12 | 96 | Checksum | Instance ID | 0 | |||||||||||||||||||||||||||||
| 16 | 128 | An LSA Header (Shown below) | |||||||||||||||||||||||||||||||
| 20 | 160 | ||||||||||||||||||||||||||||||||
| 24 | 192 | ||||||||||||||||||||||||||||||||
| 28 | 224 | ||||||||||||||||||||||||||||||||
| 32 | 256 | ||||||||||||||||||||||||||||||||
| ~ | ~ | ... | |||||||||||||||||||||||||||||||
The OSPFv3 (24 Bit) Options Field
| Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| * | * | DC | R | N | x | E | V6 |
In broadcast multiple-access networks, neighbor adjacency is formed dynamically using multicast hello packets to 224.0.0.5. A DR and BDR are elected normally, and function normally.
For non-broadcast multiple-access networks (NBMA), RFC 2328 defined the following two official modes for OSPF:
Cisco has defined the following three additional modes for OSPF in NBMA topologies:
Other testing solutions for conformance and load or stress testing is available from vendors such as:
OSPF was the first widely deployed routing protocol that could converge a network in the low 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 more than IS-IS. 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 ISP's today may well choose to use the features of the now-efficient implementations of OSPF,[15] after first considering the pros and cons of IS-IS in service provider environments.[16].
As mentioned, OSPF can provide better load-sharing on external links than other IGPs. 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.
In contrast, 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.
The only real limiting factor which major ISPs might go for IS-IS over OSPF is if they have a network with more than 850 routers. There is mention of an OSPF network with over 1000 routers[17], but that is quite uncommon and the network must be specifically designed to minimize overhead to achieve stable operation.