Open Shortest Path First
Encyclopedia
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
. The updates for IPv6
are specified as OSPF Version 3 in RFC 5340 (2008).
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.
(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 packets. 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. OSPF multicast IP packets never traverse IP routers, they never travel more than one hop. OSPF reserves the multicast address
es 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 and RFC 5340.
For routing multicast IP traffic, OSPF supports the Multicast Open Shortest Path First
protocol (MOSPF) as defined in RFC 1584. 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. 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.
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 shows how neighbor state changes from Down to Full Adjacency progressively with exchanging Hello, DD, Request, Update, and Ack packets).
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. (An ABR simulation shows how an ABR lets areas know each others’ network addresses by flooding Summary LSA.)
Several special area types are defined.
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.
(AS) and routing from within the area is based entirely on a default route. A Stub Area simulation shows how an ABR deletes type 4, 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 areas exist in the not-so-stubby area (NSSA). In addition, several other proprietary variations have been implemented by systems vendors, such as the totally stubby area (TSA) and the NSSA not so stubby area, 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 of the area is a default route which is the only Type-3 LSA advertised into the area. When there is only one route out of the area, fewer routing decisions have to be made by the route processor, which lowers system resource utilization.
NSSA totally 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.
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.
Metrics, however, are only directly comparable when of the same type. Four types of metrics are recognized. An intra-area route is always preferred to an External route regardless of metric. In decreasing preference, these types are:
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 path
s within the link-state database.
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.
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 internal router has all its interfaces in a single area.
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 Simulation shows a step-by-step DR election example: How neighbor list, neighbor state, DR, and BDR are changed when receiving Hello) 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 a multi-access network segment 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 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 sides of the link must become fully adjacent and the bandwidth between them cannot be further optimized. DR LSDB Synch Simulation shows how DR and non-DR routers evolve from 2-way to full adjacency relationships by exchanging DD, Request, and Update.
As per Appendix A.3 of RFC 5340 (OSPFv3 for IPv6) there are 5 OSPF Packet formats as follows:
The five different formats for each "Type" of OSPF v3 packet are listed below:
The OSPFv3 (24 Bit) Options Field
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:
. 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, after first considering the pros and cons of IS-IS in service provider environments.
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 that may compel major ISPs to select 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, but that is quite uncommon and the network must be specifically designed to minimize overhead to achieve stable operation.
Adaptive routing
Adaptive routing describes the capability of a system, through which routes are characterized by their destination, to alter the path that the route takes through the system in response to a change in conditions...
routing protocol
Routing protocol
A routing protocol is a protocol that specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network, the choice of the route being done by routing algorithms. Each router has a priori knowledge only of...
for Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) networks. It uses a link state routing
Link-state routing protocol
A link-state routing protocol is one of the two main classes of routing protocols used in packet switching networks for computer communications . Examples of link-state routing protocols include OSPF and IS-IS....
algorithm and falls into the group of interior routing protocols, operating within a single autonomous system
Autonomous system (Internet)
Within the Internet, an Autonomous System is a collection of connected Internet Protocol routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet....
(AS). It is defined as OSPF Version 2 in RFC 2328 (1998) for IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
. The updates for IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
are specified as OSPF Version 3 in RFC 5340 (2008).
OSPF is perhaps the most widely-used interior gateway protocol
Interior gateway protocol
An interior gateway protocol is a routing protocol that is used to exchange routing information within an autonomous system ....
(IGP) in large enterprise networks. IS-IS
IS-IS
Intermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
, another link-state routing protocol, is more common in large service provider networks. The most widely-used exterior gateway protocol
Exterior Gateway Protocol
The Exterior Gateway Protocol is a now obsolete routing protocol for the Internet originally specified in 1982 by Eric C. Rosen of Bolt, Beranek and Newman, and David L. Mills. It was first described in RFC 827 and formally specified in RFC 904...
is the Border Gateway Protocol
Border Gateway Protocol
The Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
(BGP), the principal routing protocol between autonomous systems on the Internet.
Overview
OSPF is an interior gateway protocol that routes Internet ProtocolInternet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) packets solely within a single routing domain (autonomous system
Autonomous system (Internet)
Within the Internet, an Autonomous System is a collection of connected Internet Protocol routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet....
). 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
Internet layer
The internet layer or IP layer is a group of internetworking methods in the Internet protocol suite, commonly also called TCP/IP, which is the foundation of the Internet...
which makes routing decisions based solely on the destination IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
found in IP packets. OSPF was designed to support variable-length subnet masking (VLSM) or Classless Inter-Domain Routing
Classless Inter-Domain Routing
Classless Inter-Domain Routing is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet...
(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
Shortest path tree
A shortest path tree, in graph theory, is a subgraph of a given graph constructed so that the distance between a selected root node and all other nodes is minimal. It is a tree because if there are two paths between the root node and some vertex v A shortest path tree, in graph theory, is a...
for each route using a method based on Dijkstra's algorithm
Dijkstra's algorithm
Dijkstra's algorithm, conceived by Dutch computer scientist Edsger Dijkstra in 1956 and published in 1959, is a graph search algorithm that solves the single-source shortest path problem for a graph with nonnegative edge path costs, producing a shortest path tree...
, 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
Network topology
Network topology is the layout pattern of interconnections of the various elements of a computer or biological network....
. 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
Dot-decimal notation
Dot-decimal notation is a presentation format for numerical data. It consists of a string of decimal numbers, each pair separated by a full stop ....
, familiar from IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
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
Routing Information Protocol
The Routing Information Protocol is a distance-vector routing protocol, 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 the source to a destination. The maximum number of hops allowed for RIP is 15....
(RIP), or the Border Gateway Protocol
Border Gateway Protocol
The Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
(BGP). OSPF handles its own error detection and correction functions.
OSPF uses multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
addressing for route flooding on a broadcast network link. 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. OSPF reserves the multicast address
Multicast address
A multicast address is a logical identifier for a group of hosts in a computer network, that are available to process datagrams or frames intended to be multicast for a designated network service...
es 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 and RFC 5340.
For routing multicast IP traffic, OSPF supports the Multicast Open Shortest Path First
Multicast Open Shortest Path First
The Multicast Open Shortest Path First protocol is an extension to the Open Shortest Path First protocol to support multicast routing, allowing routers to share information about group memberships....
protocol (MOSPF) as defined in RFC 1584. Neither Cisco nor Juniper Networks include MOSPF in their OSPF implementations. PIM (Protocol Independent Multicast
Protocol Independent Multicast
Protocol-Independent Multicast is a family of multicast routing protocols for Internet Protocol networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN or the Internet...
) in conjunction with OSPF or other IGPs, (Interior Gateway Protocol
Interior gateway protocol
An interior gateway protocol is a routing protocol that is used to exchange routing information within an autonomous system ....
), 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
IPsec
Internet Protocol Security is a protocol suite for securing Internet Protocol communications by authenticating and encrypting each IP packet of a communication session...
).
OSPF version 3 introduces modifications to the IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
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
Subnetwork
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....
. 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
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
, area and router identifications are still based on 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
values.
Neighbor relationships
Routers in the same broadcast domainBroadcast domain
A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments....
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
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
and multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
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 shows how neighbor state changes from Down to Full Adjacency progressively with exchanging Hello, DD, Request, Update, and Ack packets).
Area types
An OSPF domainBroadcast domain
A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments....
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
Dot-decimal notation
Dot-decimal notation is a presentation format for numerical data. It consists of a string of decimal numbers, each pair separated by a full stop ....
of an IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
address. However, they are not IP addresses and may duplicate, without conflict, any IPv4 address. The area identifiers for IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
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. (An ABR simulation shows how an ABR lets areas know each others’ network addresses by flooding Summary LSA.)
Several special area types are defined.
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, 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 .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.
Stub area
A stub area is an area which does not receive route advertisements external to the autonomous systemAutonomous system (Internet)
Within the Internet, an Autonomous System is a collection of connected Internet Protocol routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet....
(AS) and routing from within the area is based entirely on a default route. A Stub Area simulation shows how an ABR deletes type 4, 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 areas exist in the not-so-stubby area (NSSA). In addition, several other proprietary variations have been implemented by systems vendors, such as the totally stubby area (TSA) and the NSSA not so stubby area, both an extension in Cisco Systems
Cisco Systems
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
routing equipment.
Not-so-stubby area
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.Proprietary extensions
Several vendors (Cisco, Juniper, Alcatel-Lucent, 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.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 of the area is a default route which is the only Type-3 LSA advertised into the area. When there is only one route out of the area, fewer routing decisions have to be made by the route processor, which lowers system resource utilization.
- Occasionally, it is said that a TSA can have only one ABR.
NSSA totally 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 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.
Transit area
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.Path preference
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. Four types of metrics are recognized. An intra-area route is always preferred to an External route regardless of metric. In decreasing preference, these types are:
- Intra-area
- Inter-area
- External Type 1, which includes both the external path cost and the sum of internal path costs to the ASBR that advertises the route,
- External Type 2, the value of which is solely that of the external path cost
Traffic engineering
OSPF-TE is an extension to OSPF extending the expressivity to allow for traffic engineering and use on non-IP networks (RFC 3630). More information about the topology can be exchanged using opaque LSALink-state advertisement
The link-state advertisement is a basic communication means of the OSPF routing protocol for the Internet Protocol . It communicates the router's local routing topology to all other local routers in the same OSPF area. OSPF is designed for scalability, so some LSAs are not flooded out on all...
carrying type-length-value
Type-length-value
Within data communication protocols, optional information may be encoded as a type-length-value or TLV element inside of the protocol. TLV is also known as tag-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
Resource Reservation Protocol
The Resource Reservation Protocol is a Transport Layer protocol designed to reserve resources across a network for an integrated services Internet. RSVP operates over an IPv4 or IPv6 Internet Layer and provides receiver-initiated setup of resource reservations for multicast or unicast data flows...
(RSVP), OSPF-TE is used for recording and flooding RSVP signaled bandwidth reservations for Label switched path
Label Switched Path
In MPLS networking, a Label Switched Path is a path through an MPLS network, set up by a signaling protocol such as LDP, RSVP-TE, BGP or CR-LDP. The path is set up based on criteria in the forwarding equivalence class ....
s within the link-state database.
Other extensions
RFC 3717 documents work in optical routing for IP, based on "constraint-based" extensions to OSPF and IS-IS.OSPF router types
OSPF defines the following router types:- Area border router (ABR)
- Autonomous system boundary router (ASBR)
- Internal router (IR)
- Backbone router (BR)
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.
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.
Area border router
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.Autonomous system boundary router
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. (An interactive ASBR simulation shows how an ASBR creates External LSA for external addresses and floods them to all areas via ABR.) Routers in other areas use ABR as next hop to access external addresses. Then ABR forwards packets to the ASBR that announces the external addresses.Internal router
An internal router is a router that has OSPF neighbor relationships with interfaces in the same area.An internal router has all its interfaces in a single area.
Backbone router
Backbone routers are all routers that are connected to the OSPF backbone, irrespective of 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).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. A DR Election Simulation visualizes the basic neighbor discovery process (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 Simulation shows a step-by-step DR election example: How neighbor list, neighbor state, DR, and BDR are changed when receiving Hello) 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 (Backup Designated Router).
- 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, 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.
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 a multi-access network segment 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 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 sides of the link must become fully adjacent and the bandwidth between them cannot be further optimized. DR LSDB Synch Simulation shows how DR and non-DR routers evolve from 2-way to full adjacency relationships by exchanging DD, Request, and Update.
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 second highest priority at the time of the last election.OSPF v3 Packet Formats
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 Octet (computing) An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:... |
Bit Bit A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states... |
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 Octet (computing) An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:... |
Bit Bit A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states... |
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 Octet (computing) An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:... |
Bit Bit A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states... |
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 Octet (computing) An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:... |
Bit Bit A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states... |
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 Octet (computing) An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:... |
Bit Bit A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states... |
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 Octet (computing) An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:... |
Bit Bit A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states... |
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
-
- This "Options Field" is used in OSPF Hello packets, Database Description packets, and certain LSAs (router-LSAs, network-LSAs, inter-area-router-LSAs, and link-LSAs).
-
- (Note: Previous OSPF versions {v1 & v2} DO NOT not support all of the options/fields listed here.)
| 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 |
-
- Explanation of the bits in the Options field:
-
-
- There are currently only 7-bits assigned.
-
-
-
- V6-bit: "V6" stands for IP[v6] routing calculations are to be used.
-
-
-
- E-bit: "E" stands for [E]xternal as in AS-External-LSA flooding as specified in OSPFv2.
-
-
-
- x-bit: This is currently depreciated. It was previously used by MOSPF.
-
-
-
- N-bit: "N" stands for [N]SSA (Not So Stubby Area) and used for routers which are attached to NSSA networks.
-
-
-
- R-bit: "R" stands for [R]outer and specifies whether the router is Active or not.
-
-
-
- DC-bit: "DC" stands for [D]emand [C]ircuts and is specified in RFC 1793.
-
-
-
* -bits: These two bits are reserved for migration of OSPFv2 protocol extensions.
-
-
-
- The remaining 16-bits have yet to be assigned.
-
OSPF in broadcast and non-broadcast networks
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:
- nonbroadcast
- point-to-multipoint
Cisco has defined the following three additional modes for OSPF in NBMA topologies:
- point-to-multipoint nonbroadcast
- broadcast
- point-to-point
Implementations
- BIRDBird Internet routing daemonBIRD is an open source implementation of a TCP/IP routing daemon for Unix like systems. Developed as a school project at the Faculty of Mathematics and Physics, Charles University, Prague, with major contributions from developers Martin Mares, Pavel Machek and Ondrej Filip...
implements both OSPFv2 and OSPFv3 - GNU ZebraGNU ZebraZebra is a routing software package that provides TCP/IP based routing services with routing protocols support such as RIP, OSPF and BGP. Zebra also supports special BGP Route Reflector and Route Server behavior. In addition to traditional IPv4 routing protocols, Zebra also supports IPv6 routing...
, a GPLGNU General Public LicenseThe GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
routing suite for Unix-likeUnix-likeA Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems supporting OSPF - Netware implements OSPF in its Multi Protocol Routing module.
- OpenBSDOpenBSDOpenBSD is a Unix-like computer operating system descended from Berkeley Software Distribution , a Unix derivative developed at the University of California, Berkeley. It was forked from NetBSD by project leader Theo de Raadt in late 1995...
includes an OpenOSPFD implementation within the OpenBGPDOpenBGPDOpenBGPD allows general purpose computers to be used as routers. It is a Unix system daemon that provides a free, open-source implementation of the Border Gateway Protocol version 4. This allows a machine to exchange routes with other systems that speak BGP....
protocol. - QuaggaQuagga (Software)Quagga is a network routing software suite providing implementations of Open Shortest Path First , Routing Information Protocol , Border Gateway Protocol and IS-IS for Unix-like platforms, particularly Linux, Solaris, FreeBSD and NetBSD....
, a fork of GNU Zebra for Unix-likeUnix-likeA Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems - XORPXORPXORP is an open source Internet Protocol routing software suite originally designed at the International Computer Science Institute in Berkeley, California...
, a routing suite implementing RFC2328 (OSPFv2) and RFC2740 (OSPFv3) for both IPv4 and IPv6 - Windows NT 4.0Windows NT 4.0Windows NT 4.0 is a preemptive, graphical and business-oriented operating system designed to work with either uniprocessor or symmetric multi-processor computers. It was the next release of Microsoft's Windows NT line of operating systems and was released to manufacturing on 31 July 1996...
Server, Windows 2000Windows 2000Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...
Server and Windows Server 2003Windows Server 2003Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005...
implement OSPFv2 in the Routing and Remote Access Service, although the functionality was removed in Windows Server 2008.
Applications
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-ISIS-IS
Intermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
. 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, after first considering the pros and cons of IS-IS in service provider environments.
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 that may compel major ISPs to select 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, but that is quite uncommon and the network must be specifically designed to minimize overhead to achieve stable operation.
See also
- Adaptive RoutingAdaptive routingAdaptive routing describes the capability of a system, through which routes are characterized by their destination, to alter the path that the route takes through the system in response to a change in conditions...
- Enhanced Interior Gateway Routing ProtocolEnhanced Interior Gateway Routing ProtocolEnhanced Interior Gateway Routing Protocol - is a Cisco proprietary routing protocol loosely based on their original IGRP. EIGRP is an advanced distance-vector routing protocol, with optimizations to minimize both the routing instability incurred after topology changes, as well as the use of...
- Mesh networkingMesh networkingMesh networking is a type of networking where each node must not only capture and disseminate its own data, but also serve as a relay for other nodes, that is, it must collaborate to propagate the data in the network....
- Route analyticsRoute analyticsRoute analytics is an emerging network monitoring technology specifically developed to analyze the routing protocols and structures in meshed IP Networks...
- RoutingRoutingRouting is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
Further reading
- Andrew Colton, OSPF for Cisco Routers
- Jeff Doyle, Jennifer Carroll, Routing TCP/IP, Volume 1, 2nd Edition
- John T. Moy, OSPF: Anatomy of an Internet Routing Protocol.
- William R. Parkhurst, Cisco OSPF Command and Configuration Handbook. ISBN 978-1-58705-071-8
External links
- IETF OSPF Working Group
- OSPF Basics
- Design and Implementation of OpenOSPFD (Paper)
- Design and Implementation of OpenOSPFD (Presentation)
- Cisco OSPF
- Cisco OSPF Areas and Virtual Links
- Summary of OSPF v2
- OSPF and IS-IS: A Comparative Anatomy by Dave Katz, Juniper
- IS-IS and OSPF difference discussion (Vishwas Manral, Manav bhatia and Yasuhiro Ohara)
- Data Communication Lectures of Manfred Lindner - Part OSPF Fundamentals
- Data Communication Lectures of Manfred Lindner - Part OSPF Areas
- TCP/IP Guide - OSPF
- Good comparison of IS-IS vs OSPF
- Introduction to OSPF Sham Link
- An OSPF Basic Simulation. The goal is to help beginners grasp the basic ideas quickly. It visualizes the interactions of 4 types of activities on a simplified topology: OSPF tasks (neighbor discovery neighbors, address learning); OSPF packets (Hello, DD, Request, Update); Table update (LSDB, routing table); Ping verification.