Cisco 400-007 - Cisco Certified Design Expert (CCDE v3.1)

BFD (Bidirectional Forwarding Detection) operates by exchanging control packets between peers, typically using UDP encapsulation. In single-hop deployments, BFD sessions are often established using:

Source IP = destination IP = local interface IP (identical source and destination addresses), especially in certain implementations like Cisco single-hop BFD.

This allows rapid fault detection without routing dependency.

The IPS must permit these packets to avoid disrupting BFD functionality.

Why other options are incorrect:

A: Fragmentation is irrelevant to BFD.

B, C, D, F: BFD does not use broadcast, multicast, or 0.0.0.0 addresses.

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Let’s calculate Total Cost of Ownership (TCO) for 24 months based on the table and given scaling requirements:

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Metro Ethernet:

CAPEX = $45,000

Installation Fee = $5,000

OPEX = $125,000/year → $250,000 for 2 years

Total = 45,000 + 5,000 + 250,000 = $300,000

DWDM:

CAPEX = $250,000

Installation Fee = $30,000

OPEX = $100,000/year → $200,000 for 2 years

Total = 250,000 + 30,000 + 200,000 = $480,000

CWDM:

CAPEX = $150,000

Installation Fee = $25,000

OPEX = $100,000/year → $200,000 for 2 years

Total = 150,000 + 25,000 + 200,000 = $375,000

MPLS:

CAPEX = $50,000

Installation Fee = $75,000

OPEX = $150,000/year → $300,000 for 2 years

Total = 50,000 + 75,000 + 300,000 = $425,000

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Metro Ethernet clearly results in the lowest total cost while meeting dual 10G and scalable growth to 20 connections due to its simple provisioning and operational flexibility.

This approach fully aligns with CCDE v3.1 business-driven cost modeling principles: balancing cost, scalability, and operational simplicity.

Why other options are incorrect:

B, C, D: All are more expensive after full 2-year calculation for the required scale.

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C (Class-Based Weighted Fair Queuing - CBWFQ): CBWFQ allows assigning bandwidth limits (in this case, 2 Mbps) to specific traffic classes, effectively controlling scavenger traffic without dropping excess packets unnecessarily.

Other options explained:

A: Policing drops excess traffic immediately rather than queuing.

B: LLQ is for delay-sensitive traffic, not scavenger control.

D: Shaping is useful but typically applied at interface level, not specific traffic classes.

Comprehensive and Detailed Explanation:

C: Using dynamic routing protocols (e.g., OSPF, EIGRP, or BGP) provides real-time path selection, automatic failover, and improved resiliency. It eliminates manual reconfiguration delays associated with static routing, directly addressing user experience issues during failure.

Other options:

A: QoS improves traffic prioritization, not failover/resiliency.

B: Bandwidth upgrades do not address the failure-handling problem.

D: Adding a third link increases cost without solving the root issue—ineffective routing response.

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UplinkFast is an STP optimization feature designed primarily for access layer switches to reduce convergence time when a direct link failure occurs. When a primary uplink fails, UplinkFast immediately activates a backup uplink port without waiting for traditional STP convergence.

It is most effective on access layer switches where redundant uplinks to distribution switches exist.

On distribution switches, it may also be useful if they have multiple uplinks toward the core.

UplinkFast is not recommended for core switches as these typically don’t have redundant uplinks toward lower-tier devices.

It is not designed to resolve interoperability issues between different STP versions or to adjust hello timers.

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In a DMVPN (Dynamic Multipoint VPN) design:

One site (here, Chicago) functions as the Next Hop Server (NHS), which facilitates dynamic spoke-to-spoke tunnel establishment.

DMVPN is based on multipoint GRE tunnels combined with NHRP (Next Hop Resolution Protocol).

The ability to detect remote tunnel endpoint failures is essential for reliable routing convergence and tunnel restoration.

While GRE (Option C) is the encapsulation mechanism used in DMVPN, it does not inherently provide failure detection. Likewise, VPLS and L2TPv3 are Layer 2 VPN technologies and not applicable in DMVPN design.

To meet the requirement of peer failure detection, the correct mechanism is:

B. IP SLA (Service-Level Agreement): This is a feature that actively monitors the health and reachability of tunnel endpoints through periodic probes (e.g., ICMP echo). When the peer becomes unreachable, routing protocol adjacencies can be withdrawn, and alternate paths selected.

Using IP SLA in a DMVPN design helps detect endpoint failure scenarios such as:

A spoke going down unexpectedly

Loss of return traffic

Partial link failures

This design pattern is aligned with CCDE v3.1 "Protocol Design Implications", emphasizing resiliency, fault detection, and efficient routing convergence in overlay VPN designs like DMVPN.

C. Fragment data packets: On slow WAN links, large data packets can delay small voice packets (serialization delay). Fragmentation ensures that voice packets are interleaved and not delayed behind large data frames (Link Fragmentation and Interleaving - LFI).

E. Prioritize voice packets: QoS mechanisms (LLQ, priority queuing) ensure that voice traffic is serviced first, reducing jitter and delay.

These are both critical QoS design principles for real-time application performance in constrained branch WAN scenarios, as outlined in CCDE v3.1.

Why other options are incorrect:

A: Increasing bandwidth may help but doesn’t address serialization or prioritization.

B: Memory upgrades do not directly improve voice quality.

D: Fiber upgrades do not address WAN-specific serialization or QoS.

The SDN architecture separates the control plane from the data plane. In this model, the SDN controller is the central element responsible for:

Receiving application intent via northbound APIs

Translating intent into traffic policies

Communicating with infrastructure devices using southbound APIs

The SDN controller enforces policies such as path selection, QoS, and access control rules across the network fabric, ensuring traffic flows align with administrator-defined logic. Neither northbound nor southbound APIs enforce policy themselves; they serve as communication interfaces.

The packet forwarding engine (Option A) simply performs data-plane functions based on instructions, but does not interpret or apply policies.

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A (Bottom-Up Approach): Starts with the evaluation of current infrastructure (hardware, protocols, configurations), followed by design adjustments based on identified gaps.

C (Top-Down Approach): Starts from business and application requirements, translating them into technical design and network architecture.

Both approaches are widely used in network design methodology depending on business requirements and organizational maturity, directly referenced in CCDE v3.1 design models.

Why other options are incorrect:

B & D: These are not valid or recognized network design approaches.

E: Three-tier is a network architecture model, not a design approach.

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Just-in-time (JIT) manufacturing relies on precision timing and real-time data exchanges between production systems and external suppliers. Network latency can directly impact the efficiency and viability of such a system. For this reason, the architect must prioritize a network design that ensures minimal delay and rapid delivery of application and sensor data.

According to CCDE v3.1 design principles, when latency is tied directly to business outcomes (e.g., manufacturing workflow dependencies), the network must be designed with a focus on:

End-to-end low-latency infrastructure

Predictable jitter and delay control

Application prioritization mechanisms (QoS)

High availability and redundancy

This allows the manufacturer to optimize production throughput while reducing delays caused by communication lag with external component providers.

Why other options are incorrect:

A. Automation helps with operational efficiency, but it does not directly ensure low-latency communication.

B. Zero Trust is a security model, not latency-related.

D. Modularity is about design structure, not latency performance.

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