Open Shortest Path First is ...

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<h2>Open Shortest Path First</h2><div class="wp-caption alignright" style="width: 310px"><a href="http://withfriendship.com/images/d/18363/Open-Shortest-Path-First-picture.gif" target="_blank"><img title="Open Shortest Path First" alt="Open Shortest Path First" src="http://withfriendship.com/images/d/18363/Open-Shortest-Path-First-picture.gif" width="300" height="270" /></a> </div><div class="wp-caption alignright" style="width: 310px"><a href="http://withfriendship.com/images/d/18363/open-shortest-path-first.jpg" target="_blank"><img title="Open Shortest Path First" alt="Open Shortest Path First" src="http://withfriendship.com/images/d/18363/open-shortest-path-first.jpg" width="150" height="135" /></a> </div><div class="wp-caption alignright" style="width: 310px"><a href="http://learntelecom.com/wp-content/uploads/2010/06/images1.jpg" target="_blank"><img title="Open Shortest Path First" alt="Open Shortest Path First" src="http://learntelecom.com/wp-content/uploads/2010/06/images1.jpg" width="150" height="135" /></a> </div>
Open Shortest Path First is a link-state 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 in large enterprise networks. IS-IS, another link-state dynamic 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.
OSPF is an interior gateway protocol that routes Internet Protocol 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, 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 1st algorithm.
The OSPF routing policies to construct a route table are governed by link cost factors 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 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, 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 domain. 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 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 and RFC 5340.
For routing multicast IP traffic, OSPF supports the Multicast Open Shortest Path First protocol 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.
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.
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 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. Generally an interface is only configured in a single area, however you can configure an interface to belong to multiple areas. In the 2nd area, such an interface must be configured as a secondary interface..
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 aren't IP addresses and may duplicate, without conflict, any IPv4 address. The area identifiers for IPv6 implementations of OSPF 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.
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.
A stub area is an area which does not receive route advertisements external to the autonomous system and routing from within the area is based entirely on a default route. 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. 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.
A not-so-stubby area is a type of stub area that can import autonomous system external routes and send them to other areas, but still cannot receive AS-external routes from other areas. NSSA is an extension of the stub area feature that allows the injection of external routes in a limited fashion into the stub area. A case study simulates an NSSA getting around the Stub Area problem of not being able to import external addresses. It visualizes the following activities: the ASBR imports external addresses with a type 7 LSA, the ABR converts a type 7 LSA to type 5 and floods it to other areas, the ABR acts as an "ASBR" for other areas. The ABRs don't take type 5 LSAs and then convert to type 7 LSAs for the area.Here is the whole idea to do it.
Several vendors, now implement the 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.
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.
<h3>Related Sites for Open Shortest Path First</h3>
<ul><li>Open Shortest Path First - DocWiki - Cisco <a href="http://docwiki.cisco.com/wiki/Open_Shortest_Path_First" target="_blank">read Open Shortest Path First</a></li>
<li>What is OSPF (Open Shortest Path First)? - Definition from WhatIs.com <a href="http://searchenterprisewan.techtarget.com/definition/OSPF" target="_blank">read Open Shortest Path First</a></li>
<li>Open Shortest Path First â€" Wikipédia, aâ€¦ <a href="http://pt.wikipedia.org/wiki/Open_Shortest_Path_First" target="_blank">read Open Shortest Path First</a></li>
<li>IETF OSPF Charter - Internet Engineering Task Force (IETF) <a href="http://www.ietf.org/html.charters/ospf-charter.html" target="_blank">read Open Shortest Path First</a></li></ul>

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tags: Open, Shortest, Path, First, Open Shortest Path First, area, ospf, routing, protocol

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