To ensure internal routers have reachability to networks learned from external peers, which configuration excerpt is correct?

The configuration excerpt that ensures internal routers have reachability to networks learned from external peers involves the use of the export command in an internal BGP (iBGP) setup. This command allows routes learned from external BGP (eBGP) peers to be advertised to iBGP peers.

In BGP, when an internal router receives routes from an external peer, those routes do not automatically propagate to other internal routers unless specifically configured to do so. By using an export command, you enable the advertisement of these eBGP-learned routes within the iBGP network. This is critical in scenarios where internal routers need to make routing decisions based on external network information, thereby maintaining consistent and accurate routing tables across the internal network.

This configuration promotes the sharing of routing information among iBGP peers, which is essential for ensuring that all routers within the autonomous system have a complete view of reachable networks, even those learned from external sources. This is particularly important in larger, more complex network environments where routing efficiency and accuracy are paramount.