Cisco Updated 400-007 Exam Questions and Answers by vivienne

The goal is to prevent AS 111 from being used as a transit AS.

The most effective method is to use an AS path access list to filter outbound BGP advertisements, allowing only prefixes originated from AS 111 (AS path contains only AS 111).

This ensures AS 111 only advertises its own prefixes and does not forward third-party prefixes between providers.

Why other options are incorrect:

A: AS-set is used for aggregation, not for transit prevention.

B: Local preference controls outbound path selection, not advertisement or transit.

D: Prefix list on inbound filters received prefixes, but does not prevent transit.

—

The issue described is classic route flapping, where a single unstable prefix (10.1.5.0/24) causes repeated route withdrawals and advertisements, triggering churn in the BGP control plane across the entire network. This behavior can be mitigated using the following scalable and non-disruptive design approach:

D. Route Aggregation: Aggregating routes at the edge router (in this case, the LA router) means that multiple more specific prefixes (such as 10.1.4.0/24 through 10.1.7.0/24) are advertised as a single summarized prefix (e.g., 10.1.4.0/22). As a result, the instability of 10.1.5.0/24 will not affect upstream routers (Chicago and New York), as they will only see the aggregated route.

Key benefits aligned with CCDE v3.1 design guidance:

Isolates flapping to the edge while maintaining reachability

Reduces control plane churn in BGP

Preserves scalability and simplifies the routing table in upstream routers

Why other options are incorrect:

A (Route dampening): While effective, route dampening is less favored today due to convergence delay and side effects; it’s also a reactive measure versus a design-based solution.

B and C (Filtering): Filtering the route prevents reachability to that subnet, which violates availability requirements and causes data black-holing.

Thus, aggregation is the most scalable, nondisruptive, and design-aligned choice in this context.

A: FabricPath assigns a specific multicast (S,G) flow to one tree. A single (S,G) does not get load-balanced across multiple trees.

E: Overall multicast traffic across multiple (S,G) groups is distributed across multiple FabricPath trees, providing general load balancing and efficient link utilization across the fabric.

Why other options are incorrect:

B: Traffic load per tree may not be uniform due to different group distributions.

C: The assignment is done by the ingress switch based on hashing, not all leaf nodes assign the same tree.

D: Individual (S,G) flows do not get load-balanced across trees.

—