400-007 Exam Dumps - Cisco CCDE v3.0 Questions and Answers

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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D (Fate sharing):Fate sharing refers to a design where multiple dependent states fail together (i.e., when physical link fails, BGP session and its routes fail too).

Other options explained:

A: Fault isolation refers to containing failures, not dependent failure.

B: Resiliency is the ability to recover, not simultaneous failure.

C: Redundancy would prevent total loss if implemented.

VPLS replicates multicast streams as multiple unicast streams, causing scalability problems with high-bandwidth multicast sources. The most scalable solution is:

Replace VPLS with a Layer 3 MVPN (Multicast VPN): MVPN provides efficient multicast replication in the provider core, dramatically reducing bandwidth consumption compared to Layer 2 replication.

Why other options are incorrect:

A: VPLS handles multicast poorly at scale because of head-end replication.

B: GRE tunnels are operationally complex and do not scale efficiently for large deployments.

C: IGMP snooping may help locally but does not solve replication inefficiency in the core.

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NETCONF (Network Configuration Protocol) is a standardized protocol used for automating configuration and management of network devices. According to IETF standards (RFC 6241), the required encoding for NETCONF is:

XML (Extensible Markup Language)

Cisco IOS XE supports NETCONF over SSH and uses XML to structure both configuration data and RPC messages. XML allows strict schema validation (via YANG models), extensibility, and rich hierarchical structure required for modern network automation.

Other formats:

HTML is not a data encoding method.

YAML is typically used with configuration management tools, not NETCONF.

JSON is supported in RESTCONF, not in traditional NETCONF.

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Bridge Assurance is a mechanism used with Rapid PVST+ and MST that helps prevent loops caused by unidirectional failures on point-to-point links. When enabled, both ends of a point-to-point STP link must continuously exchange BPDUs. If one side stops receiving BPDUs, the port transitions into a blocking state, preventing loops caused by silent failures.

This directly addresses unidirectional link failures that otherwise may not be detected quickly by traditional STP timers.

It is typically deployed between switches in the core and distribution layers, where point-to-point links exist.

Other options explained:

A: Incorrect — STP TCNs are not triggered simply by station connection events.

B: Incorrect — Path optimization is handled by STP/RSTP convergence, not bridge assurance directly.

C: Incorrect — Bridge assurance is not designed for unmanaged switches at the access layer.

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