Up until now, the discussion has been about BGP-4, its operation, and the configuration options available. However, for most, BGP-4 is the means by which information about the Internet is brought into the internal organizational routing domain. Disseminating this information throughout the autonomous system is the subject of this next section.
If the organization is not an ISP, there is a fair chance that the network is running an IGP within the autonomous system. The IP routing table generated by this protocol or protocols is distinct from the BGP-4 routing table, although as you have seen, they communicate freely. The use of synchronization is a case in point. However, populating these tables with routes from the other table needs to be manually configured.
Obviously, routes can be injected from the IGP into BGP-4, or from the BGP-4 into the IGP.
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