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