Pass the Cisco CCDE v3.0 400-007 Questions and answers with ExamsMirror

B (OSPF Type 1 external):Type 1 external routes include both the external cost (from redistribution) and the internal OSPF cost to the ASBR. This ensures accurate end-to-end metric calculation across domains.

Other options explained:

A: Type 2 external ignores internal OSPF cost, potentially creating suboptimal routing.

C/D: EIGRP doesn’t use OSPF metric types; redistribution into EIGRP uses EIGRP metrics.

Increasing jitter buffer size smoothens jitter but at the cost of introducing additional delay.

Excessive jitter buffering can lead to perceptible audio or video lag, impacting user experience.

C: Therefore, the undesired effect is increased transport delay, potentially leading to quality issues if the buffer size is too large.

Why other options are incorrect:

A & D: Jitter buffering does not improve jitter itself; it masks it at the expense of delay.

B: Jitter compensation doesn't increase jitter, but delay.

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PBR overrides the normal routing table behavior, potentially leading to microloops during reconvergence because forwarding decisions are based on policies rather than updated routing tables.

During fast topology changes, PBR decisions may not adapt as quickly as dynamic routing protocols, causing brief routing inconsistencies.

Why other options are incorrect:

A: PBR can add complexity but does not inherently limit scalability.

C/D: PBR does not directly affect convergence speed but can introduce inconsistency.

In PIM Sparse Mode, when the Last-Hop Router (LHR) receives multicast traffic on the shared tree (*,G), it performs an RPF check based on the RP address, not the multicast source address. In the diagram:

The LHR has IGP cost 10 towards the RP.

The shared tree (*,G) uses RP as the RPF target.

Since multicast traffic on (*,G) arrives on the interface toward RP (correct RPF interface), the RPF check succeeds.

Therefore, the multicast traffic is successfully switched toward the receivers.

This behavior is fully aligned with CCDE v3.1 multicast design principles, which emphasize clear understanding of RPF behavior both on shared trees (*,G) and source trees (S,G).

Why other options are incorrect:

A: The RPF check is not performed against the source in (*,G); it uses RP.

B: RPF checks are always performed, even on (*,G).

D: RPF is unrelated to the receiver address.

A (Change OSPF reference bandwidth): OSPF calculates path cost based on interface bandwidth using a formula where cost = reference bandwidth / interface bandwidth. The default OSPF reference bandwidth is 100 Mbps, which causes all faster links (1 Gbps and above) to appear with the same minimum cost.

Updating the reference bandwidth (e.g., to 10000 or higher) allows OSPF to assign accurate costs to faster links, enabling proper path selection that reflects real link capacities.

Other options explained:

B: Filtering is not the correct method for path optimization.

C: Changing interface bandwidth values is a workaround but not scalable or recommended compared to updating the reference bandwidth globally.

D: Summarization addresses prefix aggregation, not cost calculation.

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Cisco FabricPath is a Layer 2 multipath technology that blends traditional Layer 2 Ethernet with routing technologies to enable loop-free forwarding and scalability. Two critical mechanisms used to mitigate loops:

C. RPF (Reverse Path Forwarding) Check: FabricPath applies a reverse path forwarding check, similar to multicast, to verify the source of a frame and discard duplicates arriving on unexpected paths.

D. TTL Header: FabricPath adds a Time-To-Live (TTL) field to Layer 2 frames, which is not present in traditional Ethernet. This prevents indefinite circulation of frames by limiting their lifetime in the network—much like in IP.

These elements allow FabricPath to prevent broadcast storms and loops in multipath Layer 2 topologies, addressing legacy STP limitations with efficient loop-avoidance methods.

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In OSPF multi-area designs with multiple ABRs, when there is temporary inconsistency between the ABRs’ type-3 summary LSAs, microloops can occur during convergence. These loops happen due to slight differences in LSA arrival and SPF calculations across routers while the network is converging.

Microloops are short-lived but can cause brief packet loss.

This becomes more likely as the number of ABRs grows and IGP flooding domains increase.

CCDE v3.1 emphasizes careful ABR design and possible use of loop avoidance mechanisms (e.g., LFA, microloop prevention techniques) in large-scale OSPF designs.

Why other options are incorrect:

A: LSA flooding in OSPF scales well; type-3 LSA replication is not the primary scalability issue.

B: ABRs process all LSAs regardless; load is not split.

D: Multiple ABRs all independently summarize and advertise into the backbone.

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C (Significant effort and time): In a Waterfall model, changes introduced after the design phase require rework of multiple design documents, validations, and downstream impacts on implementation and testing. This leads to significant delays and resource consumption compared to more flexible models like Agile.

Other options explained:

A: Creativity is not the primary impact.

B: Rework is true but doesn’t fully capture the scope of additional effort.

D: Waterfall does not provide inherent flexibility to absorb late changes.

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B (Weighted Random Early Detection - WRED):WRED proactively drops packets before buffers are full to avoid global synchronization in TCP traffic and prevent tail drops, thus improving overall TCP performance under congestion.

Other options explained:

A: WFQ provides fairness but does not proactively avoid congestion.

C: LLQ is for delay-sensitive traffic, not congestion avoidance for TCP.

D: FIFO offers no QoS or congestion control.