When two OSPF routers have established bidirectional (two-way) communication, they synchronize their routing (link-state) databases. For point-to-point links, the two routers communicate this information directly between themselves. On network links (that is, multiaccess network—either broadcast or nonbroadcast), this synchronization takes place between the new OSPF router and the DR. The exchange protocol is first used to synchronize the routing (link-state) databases. After synchronization, any changes in the router's links use the flooding protocol to update all the OSPF routers.
Note that this protocol is asymmetric. The first step in the exchange process is to determine which is the master and which is the slave. After agreeing on these roles, the two routers begin to exchange the description of their respective link-state databases. This information is passed between the two routers via the exchange protocol packet layout, as shown in Figure 3-24.
Figure 3-24 Exchange Protocol Packet Layout
0 1234567890 1234567890 1234567890 1
DD Sequence Number
Link State Type
Link State ID
Link State Sequence Number
Link State Checksum
As they receive and process these database description packets, the routers make a separate list that contains the records they need to exchange later. When the comparisons are complete, the routers then exchange the necessary updates that were put into the list so that their databases can be kept up to date.
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