A customer has a headquarters site and six existing branch offices, with plans to add six more branches soon. They want an economical WAN technology that allows many branches to connect to headquarters, and there are no free physical ports left on the headquarters router. Which WAN technology is the most appropriate recommendation?

Introduction / Context:
When connecting multiple branch offices to a central headquarters site, network designers must balance cost, scalability, and router port utilization. Legacy WAN technologies such as Frame Relay, PPP, and ISDN appear frequently in exam scenarios because they clearly illustrate these trade-offs and design choices.

Given Data / Assumptions:

• The customer has a central headquarters site and six existing branches.
• They plan to add six more branches soon, for a total of twelve.
• The headquarters router has no free physical WAN ports available for additional serial links.
• The customer wants an economical WAN solution that scales to many branches.

Concept / Approach:
Traditional point-to-point leased lines using PPP or HDLC require a separate physical interface for each link at the headquarters site. With twelve branches, this would mean twelve separate WAN interfaces on the headquarters router, which is not possible when no ports are available. Frame Relay, on the other hand, allows multiple virtual circuits (DLCIs) to be multiplexed over a single physical interface, making it much more scalable in a hub-and-spoke design. ISDN can provide dial-up connectivity but becomes expensive and inefficient for permanent or frequent connections.

Step-by-Step Solution:
1. Evaluate PPP and HDLC: both are used over point-to-point links, typically requiring one physical interface per branch at headquarters.2. Recognize that the headquarters router has no free ports, so adding more dedicated physical circuits is not practical.3. Consider ISDN: while it can provide dial-up access, it is cost effective only for intermittent connections and does not scale well for a growing set of permanent branch links.4. Recall that Frame Relay enables many virtual circuits over a single physical access circuit to the Frame Relay cloud.5. Because a single headquarters serial interface can support many branch circuits via Frame Relay, this technology fits the requirement of economically connecting present and future branches with limited physical ports.

Verification / Alternative check:
Topology diagrams of classic hub-and-spoke Frame Relay designs show a single headquarters router interface connected to the Frame Relay service provider, with multiple DLCIs mapping to each branch site. This model clearly satisfies the requirement of supporting many branches without additional router ports. In contrast, diagrams of PPP or HDLC deployments show a separate physical connection to each branch.

Why Other Options Are Wrong:
Option A, PPP, and option B, HDLC, both require a separate leased line and corresponding physical interface from headquarters to each branch, which conflicts with the constraint of no free ports. Option D, ISDN, is often used for backup or low usage scenarios rather than as the primary, always on connection for many branches. It would likely be more expensive and less scalable than Frame Relay for this customer.

Common Pitfalls:
A common mistake is to choose PPP or HDLC simply because they are familiar protocols, without considering the physical port limitation and the need to scale to many sites. Another pitfall is overlooking the cost implications of running many dedicated leased lines versus using a virtual circuit based technology. Remember that Frame Relay was historically popular precisely because it could aggregate many logical connections onto a single access circuit at the hub.

Final Answer:
Frame Relay using a hub-and-spoke topology from the headquarters router