Designing Cisco Enterprise Networks (ENSLD)

To enable successful route exchanges between eBGP neighbors who share the same AS number, two important BGP features are 'as-override' and 'allow-as-in'. 'As-override' is used by service providers to change the AS number in the AS path, preventing route rejection due to loop prevention mechanisms. 'Allow-as-in' permits the AS number to be present in the AS path multiple times, which is necessary when routes are being reused within the same AS. Together, these features facilitate route exchanges in scenarios where eBGP peers share an identical AS number.

The dual stack topology supports running both IPv4 and IPv6 simultaneously on the same network devices. This enables the customer to gradually migrate services from IPv4 to IPv6 while maintaining the functionality of the existing IPv4 topology until the transition is complete. This approach ensures seamless connectivity and service continuity during the migration process, meeting the requirement to enable IPv6 connectivity while preserving the IPv4 topology services before fully decommissioning IPv4.

To ensure traffic is load balanced across the two links to the ISP when running BGP, the feature that is used is Multipath Load Sharing. This feature allows BGP to use multiple paths to balance the traffic load efficiently between them, ensuring both links are utilized.

When two companies with overlapping IP address ranges merge, NAT (Network Address Translation) can be used to enable communication without reconfiguring the entire IP address space. Using a single IP address to create overload NAT (also known as PAT - Port Address Translation) allows multiple users from the newly acquired company to access the server in Company A while conserving IP addresses. This method maps multiple private IP addresses to a single public IP address, using different ports to differentiate the sessions, thus conserving IP address space and providing the required access.