Open Shortest Path First
It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS).
OSPF gathers link state information from available routers and constructs a topology map of the network.
It gathers link state information from available routers and constructs a topology map of the network.
OSPF detects changes in the topology, such as link failures, and converges on a new loop-free routing structure within seconds.
This provides a dynamic process of traffic load balancing between routes of equal cost.
OSPF divides the network into routing areas to simplify administration and optimize traffic and resource utilization.
Areas are identified by 32-bit numbers, expressed either simply in decimal, or often in the same octet-based dot-decimal notation used for IPv4 addresses.
[5] Protocol Independent Multicast (PIM) in conjunction with OSPF or other IGPs, is widely deployed.
All IP prefix information has been removed from the link-state advertisements and from the hello discovery packet making OSPFv3 essentially protocol-independent.
Despite the expanded IP addressing to 128 bits in IPv6, area and router Identifications are still based on 32-bit numbers.
Neighboring routers in the same broadcast domain or at each end of a point-to-point link communicate with each other via the OSPF protocol.
OSPF uses both unicast and multicast transmission modes to send "hello" packets and link-state updates.
During its course, each router conversation transitions through a maximum of eight conditions defined by a state machine:[1][11] In broadcast multiple-access networks, neighbor adjacency is formed dynamically using multicast hello packets to 224.0.0.5.
In a non-broadcast multiple-access (NBMA) network, a neighbor adjacency is formed by sending unicast packets to another router.
OSPF can handle thousands of routers with more a concern of reaching capacity of the forwarding information base (FIB) table when the network contains lots of routes and lower-end devices.
Many resources[14] refer to OSPF guides from over 20 years ago where it was impressive to have 64 MB of RAM.
A case study simulates an NSSA getting around the stub-area problem of not being able to import external addresses.
In Cisco's implementation, the external routes can be summarized before injecting them into the totally stubby area.
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.
DR's can also be elected on NBMA (Non-Broadcast Multi-Access) networks such as Frame Relay or ATM.
DR and non-DR routers evolve from 2-way to full adjacency relationships by exchanging DD, Request, and Update.
Special techniques, often vendor-dependent, may be needed to support the DR function on non-broadcast multiaccess (NBMA) media.
The messages establish relationships between neighboring devices (called adjacencies) and communicate key parameters about how OSPF is to be used in the autonomous system or area.
Router-LSAs, describe the following types of interfaces: To inform other areas about inter-area routers.
A customer can use OSPF over a MPLS VPN, where the service provider uses BGP or RIP as their interior gateway protocol.
OSPF-TE is an extension to OSPF extending the expressivity to allow for traffic engineering and use on non-IP networks.
RFC 3717 documents work in optical routing for IP based on extensions to OSPF and IS-IS.
OSPF is a widely deployed routing protocol that can converge a network in a few seconds and guarantee loop-free paths.
[citation needed] 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.
