Detection of loss of a fragment of MLP
All OSPF routers share in forming of the routing information of autonomous system by drive of the special messages containing information on a current status of a fragment of a network. These messages are called LSA (Link State Advertisement) — declaration of link state. B that case if declaration introduces a router status, it shall contain information on the status of its interfaces and the close to it routers. Each such declaration extends on all autonomous system. The set of such LSA creates the database of routing in each of routers.
Messages of LSA are surely created in case of any state change of a controlled component of a network. For support of bigger reliability of the message of LSA can be created also in the absence of any changes in a network through rather larger intervals, for example, once for half an hour.
Particular B, such messages are formed if on a network the new router appeared or the data transfer link state changed. The received messages form the database of network condition (Link State Data Base) in each router. When obtaining the message about changes in structure of a network the router enters appropriate
changes in the copy of the database. Thus, in each instant all databases of the routers which are in one autonomous system are identical and adequately display structure of this system. To define a route along which the datagram shall be passed the router based on the copy databases builds a tree of the shortest paths in which peak places itself. Creation of the shortest paths the router executes every time when there is a change network condition.
B of dependence on the nature of state changes of a network, routers of the OSPF protocol can form several different LSA types. Messages of each type are created by routers of one or several types (IR, ABR and so on). Distinguish five types of messages of LSA of the routing protocol OSPF:
□ LSAtype 1 — (type 1), declaration of ports of the router;
□ LSA type 2 — (type 2), declaration of the Broadcast network;
□ LSA types 3, 4 — (type 3, 4), declaration of the added ports;
□ LSA type 5 — (type 5), declaration of external ports.
Messages of LSA of 1 type
Messages of the first type (router link advertisement) are created by the router for each area in which it has active interfaces. Messages of LSA of this type contain the integrated information on a status of ports of the router in the field and the identifier of this router. When forming messages of LSA for identification of routers IP addresses of their interfaces are used. By default the least address of its active interface becomes the identifier of the router. B some cases can be reasonable use of the dummy address which is appropriated to its virtual interface of the router. Messages of the first type extend only within one area.
Messages of LSA of 2 type
Messages of LSA of the second type (network link advertisement) are created only on the Broadcast (Ethernet) or NBMA networks (Non Broadcast Multi Access). To exclude a possibility of repeated information representation about one network, for the routers connected to it the special procedure of a choice is performed. Following the results of accomplishment of this procedure one of routers (DR — Designated Router) is selected from quality assigned for this network and creates for it messages of the second type. The message of LSA of 2 type contains identifiers of all routers connected to the specified network.
Routing on the TCP/IP networks
Messages of 3, 4 LSA
Messages of LSA of 3, 4 type — (summary link advertisement) are created by boundary routers of area and are sent to adjacent areas. Each message of this type represents a total route which can be used for information exchange with the specified area within one autonomous system. B of a particular, LSA of 3 type describe routes to networks, LSA of 4 type — to the boundary routers of autonomous system which are in the field.
Messages of LSA of 5 type
Messages of LSA of 5 type (external link advertisement) are created by boundary routers of autonomous system and contain information on the routes external in relation to this autonomous system. Information of messages of this type extends on all areas of autonomous system except for the certain expressly selected areas. Such areas usually have no external relations, except communication with trunk area and therefore are called deadlock (Stub-Area). It is apparent that it is illogical to send specific information on external networks to such area as for it “all roads carry to Rome”, in this case — to the trunk area.
Creation and service of a topological basis of routes of the OSPF protocol
Essential singularity of an algorithm of the routing protocol OSPF is the special procedure of information exchange between routers on networks multiple access. The routers connected to the same network are called adjacent routers (Neighboring Routers). Routers of the OSPF protocol establish and service the neighbour’s relations, using the special additional Hello protocol. The composition of the routers connected to a network decides on the help of this protocol, their serviceability dynamically is defined and it is sampleed one of them as the assigned router. The assigned router creates the messages containing the list of the routers connected to a network and passes contents of the current database on the request received from one of them. For request drive to the assigned router the multicast address 22.214.171.124 is used. If for any reasons the assigned router ceased to function, its functions automatically pass to the spare assigned router (BDR — Backup Designated Router) selected along with the main. For information exchange about routes adjacent routers shall establish partnership (Adjacency). B some cases for establishment of partnership from the router can be required accomplishment of the procedure
authentications. By drive of the messages of LSA to a network multiple access usual (not DR) the router as a destination address uses a multicast address 126.96.36.199, thus this message is addressed to the main and the spare assigned routers. Ha the first look this stage of information exchange can seem excessive. Really, when using other address (for example, Broadcast) the router could direct directly the information to all the neighbors at the same time. However use of the mode of multi-address exchange in pure form does not allow to provide required reliability of data transfer as in this case some of receivers cannot determine the fact of loss of information. For relaying of the received message in a network multiple access the main (or spare) the assigned router uses as a destination address a multicast address 188.8.131.52, therefore, this message is received by all routers of the OSPF protocol of this network. B those cases when routers of the OSPF protocol are connected by forward channels “tsnka-point” the procedure described above is not used.
For drive of the routing information routers of the OSPF protocol use different types of declarations of a status of network components (LSA). After check of correctness and relevance of the information which is contained in the next received message of LSA of 1 type — 4, the router if necessary adjusts the database and sends the message to the neighbors in the autonomous region. Process of distribution of LSA within autonomous system is called deluging (Flooding). Messages of LSA of 5 type extend in all autonomous system except for its stub areas.
For storage of the routing information routers of the OSPF protocol use special topological databases which are called Link-State Database. The database is created of the messages of LSA received by the router and displays a current status and structure of information bonds in the considered field of routing. All routers located within one area shall have identical contents of databases.
The topological database is presented in the form of the table which columns and lines form identifiers of routers and networks of this area. Contents of a cell of this table which is on traverse of the Fvlrt column and the line Nm correspond to the cost of direct delivery of the datagram from the Fvlrt router in the Nt network. If specifying of delivery cost is absent, it means that the Fvlrt router and the Nm network are directly not bound. Ha the base of this table each router builds a tree of the shortest routes connecting him to remaining components of area and actually a routing table.
Routing on the TCP/IP networks
In spite of the fact that the described mode of creation of route tables seems excessively unwieldy on comparing with the mode used by algorithms “Distance-Vector” it provides a row of indisputable benefits of algorithms “Link-State”. K to such benefits belong uses of several parallel routes which are earlier marked an opportunity and the almost instantaneous completion of transient phenomena in case of state change of a system.
Messages of the OSPF protocol
For drive of technology messages of the OSPF protocol specialized datagrams with the established protocol No. 89 type are used. These datagrams consist of title which format is identical to all types of messages, and information part with variable structure. The format of title of the message of the OSPF protocol is given in fig. 4.10.
0 8 16 31
SOURCE Router IP ADDRESS
AUTHENTICATION (0 – 3)
AUTHENTICATION (4 – 7)
Fig. 4.10. Format of title of the message of the OSPF protocol
B the VERSION field is placed OSPF protocol version number. Field value of TYPE defines type of the message of the OSPF protocol. Possible types of messages and their assignment are given in tab. 4.1.
Table 4.1. Types and assignment of messages of the OSPF protocol
TYPE Message type Assignment of the message
1 Hello Hello protocol
2 Database Description Description of the database
3 Link-State request Request of link state