Open Shortest Path First(OPSF)

introduction

In the 1980s the end, distance vector routing protocol deficiency - has become more and more obvious. An attempt to improve the network expansibility of the effort is to use based on link - state to calculate routing, not by jogging number or other distance vector.

Links are network in the connection between the two routers. Link-state including the transmission speed and delayed level attribute.

This chapter examines the in-depth Internet engineering task force (IETF) link - state, internal gateway routing protocol type: Open Shortest Path Shortest Path First priority (Open, OSPF). OSPF first embodied in RFC1131. This temporary norms were soon RFC1247 eliminated. Two OSPF between qualitative difference between that RFC1247 OSPF called OSPF version 2. OSPF version 2 continues to mature and evolution. After some changes appeared in RFC1583, 2178 and 2328 (the latest version). Due to the Internet and IP were highly dynamic, therefore OSPF are likely to continue development in order to keep up with the Internet and IP progress.

OSPF origin

In order to satisfy building -- IETF is more and more big based on IP networks need, formed a group, specially for the development of open mode, link - state routing protocols, in order to in large, heterogeneous IP network. New routing protocol to have achieved some success a series of personal, and manufacturers related, the shortest path priority (SPF) routing protocol based on the market, SPF widely used. Including OSPF inside, all the SPF algorithm based on a mathematical routing protocol -- Dijkstra algorithm. This algorithm can make routing based on link - state, rather than distance vector.

OSPF by IETF in the late 1980s development, OSPF is SPF class routing protocol of open version. Initial OSPF regulating embodied in RFC1131. The first edition (OSPF version 1) were soon make a significant improvement, this new version is replaced RFC1247 ontology now document. RFC1247OSPF called OSPF version 2 is to explicitly pointed out in its stability and functional substantial improvement. This OSPF version has many updated document, each updates are on open standards elaborate improvement. The next some normative appeared in RFC1583, 2178 and 2328.

The latest edition of OSPF version 2 embodied in RFC2328. The latest edition will only and RFC2138, by 1583 and 1247 by the regulations of version to interoperability. This chapter to the current open OSPF standard cycle development process not discussed, but focus on discussing the latest version RFC2328 standardization of OSPF function, characteristics and use.

Understand RFC2328OSPF, version 2

OSPF is designed specifically for autonomous systems within the IP routing protocol. So then, it cannot transmit other routing network such as the IPX or AppleTalk message. If a user's network must apply a variety of routing protocols, be about to consider using other rather than OSPF routing protocol.

Based on IP data header in the SPF destination IP address to calculate routing and does not provide a destination to the IP routing computation. Moreover, various OSPF information directly sealed in the IP: no other protocol (TCP and UDP, etc) to transmission.

OSPF was also designed for rapid detection of autonomous systems within the topology changes, and changes in the discovery after convergence to new topology. Routing decisions in autonomous system within the router Internet between link-state based. These routers each maintain a the same database, which records the network link-states. This database contains is router state, one of the available interface, can arrive of the adjacent routers and link, status information.

Routing table updates, namely Link - state radio (Link - state of, lsas), directly send it to router area of all adjacent routers. This update process technology termed igapo (which), this term will give people negative impression, cause of OSPF performance features of the wrong cognition.

In fact, OSPF networks can be very fast convergence. Network in all routers running the same routing algorithms and directly between transmit a message routing table updates. These messages are used to establish the network and link view. Each router's network view use class - UNIX tree structure and to their own as roots. The tree called shortest path tree (shortest path tree), record - reaching autonomous systems inside each destination of the shortest path. Autonomous outside the system of destination can pass to an external network border gateway to obtain, border gateway appeared in the shortest path tree structure on the leaves. Such a destination and/or network link - state data maintenance, because they are not located outside OSPF network. Therefore, they do not appear in the shortest path tree branches. 

OSPF (Open Shortest Path First)

OSPF agreement

OSPF (Open Shortest Path First) is an internal Gateway Protocol (Interior Gateway Protocol, abbreviation IGP), used in a single autonomous system (autonomous system, AS) in decision-making routing. Compared with RIP, OSPF is link-state routing protocol, and RIP is distance vector routing protocol. OSPF agreement management distance (AD) is 110.

A. OSPF origin

In order to satisfy build bigger IETF based on IP network requirements, formed a group, specially for the development of open mode, link-state routing protocol for use in large, heterogeneous I P network. New routing protocol to have achieved some success a series of personal, and manufacturers related, the shortest path priority (SPF) routing protocol based in the marketplace, and widely used. Including OSPF inside, all S P F routing protocol based on a mathematical algorithm Dijkstra algorithm. - This algorithm can make routing based on link - state, rather than distance vector. OSPF by IETF in the late 1980s development, OSPF is SPF class routing protocol of open version. Initial OSPF regulating embodied in RFC1131. The first edition (OSPF version 1) were soon make a significant improvement, this new version is replaced RFC1247 ontology now document. RFC 1247 OSPF called OSPF version 2 is to explicitly pointed out in its stability and functional substantial improvement. This OSPF version has many updated document, each updates are on open standards elaborate improvement. The next some normative appeared in 1583 RFC 2178 and 2328., The latest edition of OSPF version 2 embodied in RFC 2328. The latest edition will only and 2138, by the RFC 1583 and 1247 by the regulations of version to interoperability.

Links are the router interface of another story, so OSPF also called interface state routing protocol. OSPF routers network interfaces between announcement by the state to establish link-state database and generate the shortest path tree, each OSPF routers use these shortest path tectonic routing tables.

OSPF routing protocol is a kind of typical link-state (Link) - state routing protocols, commonly used for the same routing domain. Here, routing domain refers to an Autonomous System (Autonomous System), namely AS a group, it is to show through unified routing policy or routing protocol exchange routing information network. In this yard, all OSPF routers are maintaining a same describe this AS structure of database which contains a routing domain corresponding link state information, OSPF routers it is through this database calculates OSPF routing tables.

As a kind of link-state routing protocol, OSPF will link-state advertisements data lsas (Link State of) send in a certain area of all routers, this point and different distance vector routing protocol. Running distance vector routing protocol router is some or all of the routing table transfer to neighboring routers.

2. OSPF hello agreement

1 Hello agreement purpose:

1. Used to find neighbors

2. In become neighbor before, we must have some parameters Hello bag to negotiate

3. Hello wrapped in between keepalive neighbor plays a role

4. Allow two-way communication between neighbor

5. Used in NBMA (Nonbroadcast Multi - access) network on election the DR and BDR

2. Hello Packet contains the following information:

1. The source of the router RID

2. Source router's Area ID

3. Source router interface of the mask

4. The source of the router interface authentication type and authentication information

5. Source router interface Hello packets sent time intervals

6. Source router interface of invalid time interval

Priority 7.

8. DR/BDR interface IP address

9. Five marker bits (flag bit)

10. The source of all the neighborhood RID router

Three. OSPF network type

OSPF definition of five species of network type:

1 point - to - the point

2. Broadcast

3. Non - broadcast

4. The point - to - multipoint

5. Insgroupsto link

1.1. Point-to-point network, such as T1 line, is to connect separate router's network, a pair of point-to-point network the effective neighbors always can form the adjacency relations, in this kind of Internet, OSPF packets target address is used, the multicast address 224.0.0.5 called AllSPFRouters

2.1. J radio type networks, such as Ethernet Token Ring, and FDDI, such networks will elect a DR and BDR, DR/BDR sends the OSPF packets target address for 224.0.0.5, carrying these OSPF packets frame target MAC address for 0005; 0100.5E00. And besides DR/BDR outside send OSPF packets target address for 224.0.0.6, this address that AllDRouters.

3.1. NBMA networks, such as x.25, Frame Relay, and ATM, do not have a radio capabilities, so neighbor artificially to specify, in such networks will elect the DR and BDR, OSPF packets adopts unicast way

4.1. Point to the multi-point network is a special configuration NBMA networks, can be seen as a point-to-point link collection. In such networks don't election the DR and BDR.

5.1. Virtual links: OSPF packets sent on unicast way

All network also can be summarized into two types: network

1. A Transit

2 a Stub 

Open Shortest Path First(OSPF)

OSPF (Open Shortest Path First) protocol is the Internet Network Engineering (IETF) developed an internal gateway protocol (IGP), the gateway and router are within an autonomous system. OSPF is a link state protocol, or shortest path first (SPF) protocol. Although the protocol relies on IP environment than some of the technology, but the agreement specific to IP, but also the function of subnet addressing. The agreement in accordance with the purpose of IP datagram for routing IP address, once you've decided how to choose the path of a IP datagram, the datagram will be sent to the chosen path, no additional header, that is, there is no additional package. This method is different with many networks because they use some type of internal network of the UDP encapsulation header to control the subnet routing protocol.