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Difficulty: Very hardNetwork Topologies and Types

An enterprise organization with 8 branch locations is migrating its site-to-site connectivity from a legacy dedicated full-mesh physical WAN to a Dynamic Multipoint VPN (DMVPN) deployment featuring redundant dual central hubs over commercial internet links. Which of the following statements accurately describe the topological changes and operational characteristics resulting from this migration? (Select TWO.)

  1. The physical topology shifts to a dual-hub-and-spoke model reducing static WAN connections from 28 to 16, while the logical topology can dynamically establish direct spoke-to-spoke tunnels on demand.Answer
  2. B
    The underlying logical topology must maintain permanent, static point-to-point tunnels between every spoke pair to achieve direct spoke-to-spoke routing without traversing the central hubs.
  3. The virtual overlay operates logically as a non-broadcast multi-access (NBMA) network, requiring encapsulation protocols like Multipoint GRE to transport multicast routing protocol traffic across internet transports.Answer
  4. D
    Physical fault tolerance is compromised such that the total outage of one central hub will isolate all 8 branch offices from communicating with each other.

Answer

The physical topology shifts to a dual-hub-and-spoke model reducing static WAN connections from 28 to 16 while supporting dynamic logical spoke-to-spoke tunnels, and the virtual overlay functions as an NBMA network requiring mGRE to carry multicast and broadcast traffic.
Migrating from a static full-mesh WAN to a dual-hub DMVPN architecture changes the physical connection count from 28 static links (calculated via N(N−1)2\frac{N(N-1)}{2} for 8 nodes) down to 16 static hub links (8 sites×2 hubs8 \text{ sites} \times 2 \text{ hubs}). Furthermore, public internet transport functions logically as an NBMA network, requiring encapsulation like Multipoint GRE (mGRE) to handle broadcast and multicast traffic for dynamic routing protocols.

Step-by-Step Solution

1
Calculate physical WAN link requirements for full mesh versus dual hub-and-spoke.
Full mesh for N=8N=8 sites requires 8(8−1)2=28\frac{8(8-1)}{2} = 28 physical links. Dual-hub topology requires each of the 8 sites to connect to 2 hubs, totaling 8×2=168 \times 2 = 16 static hub-spoke links.
Topological transition reduces static provisioning overhead while using dynamic overlay capabilities.
2
Analyze logical overlay capabilities of DMVPN.
DMVPN combines Multipoint GRE (mGRE) and Next Hop Resolution Protocol (NHRP) to dynamically establish temporary spoke-to-spoke tunnels across an NBMA internet transport.
This allows direct spoke communication without static full-mesh tunnel configuration or permanent routing through central hubs.

Key Concept

Physical vs. Logical Topologies and DMVPN Overlay Mechanics
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Topics

Network Topologies and Types

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