Best Practices for BGP Address Planning:

Proper address planning ensures efficient and reliable BGP operations.

It involves using specific addresses for BGP sessions and advertising appropriate routes.

Analyzing the Options:

A. Advertising external networks into IGP: Not recommended as it can lead to IGP table bloating and instability.

B. Using system addresses for setting up iBGP sessions: This is considered a best practice. System addresses (loopback addresses) are stable and do not change even if the physical interface goes down, ensuring session stability.

C. Exporting the networks associated with the external links of eBGP peers into BGP: This is normal practice but not directly related to address planning best practices.

D. Optimizing BGP route table by exporting IGP routes into BGP: Typically, you would do the opposite (importing BGP routes into IGP selectively).

Conclusion:

Using system addresses (loopback addresses) for setting up iBGP sessions is a widely accepted best practice. It enhances the reliability and stability of the BGP sessions.

References

Nokia Border Gateway Protocol (BGP) Fundamentals documentation

Best practices in BGP design and implementation

Network stability and reliability considerations in BGP configurations

Understanding the AS Path Traversal:

The question states that an update travels from AS 137000 to AS 135000, passing through both 32-bit and 16-bit AS Paths.

The Autonomous Systems (AS) involved in the diagram are AS 137000, AS 140000, AS 200 (16-bit), and AS 135000.

Analyzing the AS Path:

The update starts at AS 137000.

It then moves to AS 140000.

From AS 140000, it reaches AS 200 (which is a 16-bit AS).

Finally, it reaches AS 135000.

AS Path Seen by AS 200:

When the update reaches AS 200, the AS Path recorded will include all the Autonomous Systems it has traversed so far, in reverse order.

Therefore, the AS Path seen by AS 200 will include AS 137000 and AS 140000, in that order.

Verifying the Answer:

The AS Path seen by AS 200 should show the sequence of ASes the update has traversed before reaching AS 200.

Thus, the correct AS Path is: 137000 140000.

References

Nokia Border Gateway Protocol (BGP) Fundamentals documentation

BGP Path Attributes and AS Path documentation

Standard BGP routing behavior and AS Path recording principles

BGP Message Types: BGP uses several types of messages for various functions: Open, Update, Notification, and Keep-Alive.

Keep-Alive Message: This type of message is sent periodically to verify connectivity between BGP peers. After the BGP session is established with Open messages, Keep-Alive messages ensure the session remains active and monitor the health of the connection.

Connectivity Verification: During the establishment of a BGP session, the Open message is initially used. Post-establishment, Keep-Alive messages are sent at regular intervals to maintain the connection and to verify that both peers are still reachable and operational.

References:

Nokia BGP Fundamentals Documentation, Chapter on BGP Message Types

RFC 4271 (A Border Gateway Protocol 4), Sections 4.2 and 4.4