RFC 2326 PDF

Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. All Rights Reserved. OSPF is a link-state routing protocol. It is designed to be run internal to a single Autonomous System.

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Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited.

All Rights Reserved. OSPF is a link-state routing protocol. It is designed to be run internal to a single Autonomous System.

From this database, a routing table is calculated by constructing a shortest- path tree. OSPF recalculates routes quickly in the face of topological changes, utilizing a minimum of routing protocol traffic.

OSPF provides support for equal-cost multipath. An area routing capability is provided, enabling an additional level of routing protection and a reduction in routing protocol traffic. In addition, all OSPF routing protocol exchanges are authenticated.

All differences are backward-compatible in nature. Please send comments to ospf gated. Table of Contents 1 Introduction This means that it distributes routing information between routers belonging to a single Autonomous System.

In addition, much work has been done to produce a protocol that responds quickly to topology changes, yet involves small amounts of routing protocol traffic. IP packets are routed "as is" -- they are not encapsulated in any further protocol headers as they transit the Autonomous System. OSPF is a dynamic routing protocol. It quickly detects topological changes in the AS such as router interface failures and calculates new loop-free routes after a period of convergence.

This period of convergence is short and involves a minimum of routing traffic. This database is referred to as the link-state database.

Each participating router has an identical database. The router distributes its local state throughout the Autonomous System by flooding.

From the link-state database, each router constructs a tree of shortest paths with itself as root. This shortest-path tree gives the route to each destination in the Autonomous System. Externally derived routing information appears on the tree as leaves.

When several equal-cost routes to a destination exist, traffic is distributed equally among them. The cost of a route is described by a single dimensionless metric.

OSPF allows sets of networks to be grouped together. Such a grouping is called an area. The topology of an area is hidden from the rest of the Autonomous System. This information hiding enables a significant reduction in routing traffic. An area is a generalization of an IP subnetted network. Each route distributed by OSPF has a destination and mask. Two different subnets of the same IP network number may have different sizes i. This is commonly referred to as variable length subnetting. A packet is routed to the best i.

Host routes are considered to be subnets whose masks are "all ones" 0xffffffff. All OSPF protocol exchanges are authenticated. A variety of authentication schemes can be used; in fact, separate authentication schemes can be configured for each IP subnet. Externally derived routing data e. Each external route can also be tagged by the advertising router, enabling the passing of additional information between routers on the boundary of the Autonomous System.

Definitions of commonly used terms This section provides definitions for terms that have a specific meaning to the OSPF protocol and that are used throughout the text. Router A level three Internet Protocol packet switch. Formerly called a gateway in much of the IP literature. Autonomous System A group of routers exchanging routing information via a common routing protocol.

Abbreviated as AS. Interior Gateway Protocol The routing protocol spoken by the routers belonging to an Autonomous system. Abbreviated as IGP. This number uniquely identifies the router within an Autonomous System.

We consider these to be separate networks. This specification displays network masks as hexadecimal numbers. Such a mask is often displayed elsewhere in the literature as Point-to-point networks A network that joins a single pair of routers. A 56Kb serial line is an example of a point-to-point network. Broadcast networks Networks supporting many more than two attached routers, together with the capability to address a single physical message to all of the attached routers broadcast.

The Hello Protocol itself takes advantage of the broadcast capability. The OSPF protocol makes further use of multicast capabilities, if they exist.

Each pair of routers on a broadcast network is assumed to be able to communicate directly. An ethernet is an example of a broadcast network. Non-broadcast networks Networks supporting many more than two routers, but having no broadcast capability. However, due to the lack of broadcast capability, some configuration information may be necessary to aid in the discovery of neighbors. On non-broadcast networks, OSPF protocol packets that are normally multicast need to be sent to each neighboring router, in turn.

OSPF runs in one of two modes over non-broadcast networks. The second mode, called Point-to-MultiPoint, treats the non- broadcast network as a collection of point-to-point links. An interface has state information associated with it, which is obtained from the underlying lower level protocols and the routing protocol itself.

An interface to a network has associated with it a single IP address and mask unless the network is an unnumbered point-to-point network. An interface is sometimes also referred to as a link. Neighboring routers Two routers that have interfaces to a common network. Adjacency A relationship formed between selected neighboring routers for the purpose of exchanging routing information. Not every pair of neighboring routers become adjacent.

Link state advertisement Unit of data describing the local state of a router or network. Each link state advertisement is flooded throughout the routing domain. Throughout this memo, link state advertisement is abbreviated as LSA. On broadcast networks the Hello Protocol can also dynamically discover neighboring routers. The Designated Router is elected by the Hello Protocol. The Designated Router concept enables a reduction in the number of adjacencies required on a broadcast or NBMA network.

This in turn reduces the amount of routing protocol traffic and the size of the link-state database. Lower-level protocols The underlying network access protocols that provide services to the Internet Protocol and in turn the OSPF protocol. Examples of these are the X. Brief history of link-state routing technology OSPF is a link state routing protocol. Such protocols are also referred to in the literature as SPF-based or distributed- database protocols.

This section gives a brief description of the developments in link-state technology that have influenced the OSPF protocol. This protocol is described in [ Ref3 ]. It has formed the starting point for all other link-state protocols. Modifications to this protocol were proposed in [ Ref4 ]. These modifications dealt with increasing the fault tolerance of the routing protocol through, among other things, adding a checksum to the LSAs thereby detecting database corruption.

The paper also included means for reducing the routing traffic overhead in a link-state protocol. This was accomplished by introducing mechanisms which enabled the interval between LSA originations to be increased by an order of magnitude.

This protocol is described in [ Ref2 ]. The protocol includes methods for data and routing traffic reduction when operating over broadcast networks.

This is accomplished by election of a Designated Router for each broadcast network, which then originates an LSA for the network. The Designated Router concept has been greatly enhanced to further reduce the amount of routing traffic required. Multicast capabilities are utilized for additional routing bandwidth reduction. Packet formats, protocol constants and configuration items are specified in the appendices.

Labels such as HelloInterval encountered in the text refer to protocol constants. They may or may not be configurable. Architectural constants are summarized in Appendix B. Configurable constants are summarized in Appendix C. The detailed specification of the protocol is presented in terms of data structures.

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Schulzrinne Request for Comments: Columbia U. Category: Standards Track A. Rao Netscape R. Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. All Rights Reserved.

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Frederick Blue Coat Systems Inc. Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. All Rights Reserved.

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