OSPF routing protocol through the establishment of interactions to exchange routing information, but not all neighboring OSPF routers will establish interactive relationships. OSPF adjacency will be established following a brief overview of the process.
OSPF protocol packets through the Hello protocol to establish and maintain neighbor relationships, but also use it to ensure two-way communication between adjacent routers. OSPF routers periodically send Hello packets, when the router see themselves listed in the data package of other router Hello, this will be established between the two routers, two-way communication. In the multi-access environment, Hello packets are also used to find the designated router DR, DR to control through the establishment of interactions with which the router.
OSPF routers establish two-way communication which is the second step after the database synchronization, database synchronization is all the link state routing protocols most common. In OSPF routing protocols, database synchronization relationship only in the establishment of interactions between the routers.
OSPF database synchronization database described by OSPF packet (DatabaseDescriptionPackets) carried out. OSPF routers periodically generate a database description packet, the packet is ordered, that comes with serial number, and the data packet to the neighboring router broadcasts. Adjacent router database can describe the serial number of packets of data with its own database for comparison, if it receives more data than data from the serial number database large, the neighboring router, the serial number will be larger for the data request, the data obtained with a request to update its link state database.
We can next to the router from the OSPF Hello packet sent to establish a full database synchronization to establish the process of OSPF interactions into several different states, namely:
Down: This is a reciprocal relationship between OSPF initialization state, said that within a certain time is not received from a neighboring router, the information is sent. In the non-broadcast network environment, OSPF router may also send in a Down state router Hello packets.
Attempt: This state only in NBMA environment, such as Frame Relay, X.25 or ATM environment, effectively, that is not received within a certain time to a neighboring routers, but OSPF routers still must pass to a lower frequency to the adjacent router sends Hello packets to keep in touch.
Init: in the state, OSPF router has received neighboring router's Hello packet is sent, but its own IP address does not appear in the Hello packet, the means of both two-way communication has not been established.
2-Way: This state can be said to begin to establish a real step interactively. In this state, the router that the router itself is already in neighboring Hello packets, the two-way communication has been established. Designated router and backup designated router selection is done in this state. In this state, OSPF routers can be one of routers according to whether the designated router or point to point according to the link or virtual link is to decide whether to establish interactive relationships.
Exstart: This state is the establishment of the first step interactive state. In this state, the router for data exchange to determine the initial database description packet sequence number to ensure that routers always get the latest link state information. Meanwhile, in this state the router must also decide between the standby router, the relationship between the status of the router at the master is back up to the status of the router link state information request.
Exchange: In this state, the router sends to the neighboring OSPF routers exchange database description packets to the link state information, each packet has a packet sequence number. In this state, the router may also send a link to the neighboring router state request packets to request the corresponding data. From this state, we say that state of OSPF in the Flood.
Loading: In the loading state, OSPF routers will find the neighboring routers of its new link state data and the data itself has expired request to the neighboring router, and wait for the answer to the adjacent router.
Full: This is a relationship between two OSPF routers to establish the final state interaction, in time, establish interactive relationships between the router database synchronization has completed the work, they have the same link state database.
Inter-Domain Routing
The previous section describes the OSPF routing protocol in a single area calculation. In a single area OSPF, OSPF routing protocol will not generate more routing information. For the rest of the region's OSPF router communication, the region's border router will have some other information on area radio, the additional information described in the same AS in the other regions of the routing information. Specific routing information exchange process is as follows:
The definition in the OSPF, all areas must be linked with regional 0, so each area must have a 0 area border routers associated with the region, this will be the area border routers to link the internal structure of the data by region SummaryLink broadcasting to the region 0, which is broadcast to all other regions of the border router. At this point, and the border area 0 routers associated with area 0 and all other regions of the link state information through the information, these border routers can calculate the route to the appropriate destination, and the routing information broadcast to to link the region with a view to enable the routers within the region and outside of the region to find the best route to communicate.
AS external routes
Autonomous domain border router, AS, AS external routing information will be broadcast to the entire AS in addition to all areas of residual domain. In order for these AS external routing information into effect, AS all the routers within (except residues within the router) must know the location of AS border router, the routing information from non-residue domain within the regional boundary router broadcasts its chain Road type broadcast packet type 4.
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