400-007 · Question #294
In a redundant hub and spoke "wheel" design, all spokes are connected to the hub, and spokes are connected to other spokes as well. During failure on one spoke link, the traffic from that site can…
The correct answer is D. Increase unequal-cost parallel paths. E. Use two links to each remote site instead of one. In a hub-and-spoke wheel design where traffic oscillation causes cascading overloads, resilience is improved by increasing available bandwidth per site and enabling proportional load distribution across multiple paths.
Question
In a redundant hub and spoke "wheel" design, all spokes are connected to the hub, and spokes are connected to other spokes as well. During failure on one spoke link, the traffic from that site can be sent to a neighboring site for it to be forwarded to the hub site. But during peak hours, a link is overloaded and traffic is re-routed to a neighbor, which subsequently becomes overloaded. This overload results in network traffic oscillation as the load varies at each spoke site. This design provides more redundancy but not more resiliency because the routing protocol must process many alternate paths to determine the lowest cost path. Which two design changes help to improve resilience in this case? (Choose two.)
Options
- AIncrease the number of redundant paths considered during the routing convergence calculation.
- BEliminate links between every spoke.
- CIncrease routing protocol convergence timers.
- DIncrease unequal-cost parallel paths.
- EUse two links to each remote site instead of one.
How the community answered
(26 responses)- A4% (1)
- B8% (2)
- C19% (5)
- D69% (18)
Why each option
In a hub-and-spoke wheel design where traffic oscillation causes cascading overloads, resilience is improved by increasing available bandwidth per site and enabling proportional load distribution across multiple paths.
Adding more redundant paths to the convergence calculation increases routing protocol processing overhead and slows convergence, which worsens the oscillation problem rather than resolving it.
Eliminating inter-spoke links removes the alternate forwarding paths that provide redundancy, reducing both redundancy and resilience in the design.
Increasing convergence timers slows the routing protocol's reaction to topology changes, meaning overload conditions persist longer before traffic is redirected, which degrades resilience rather than improving it.
Increasing unequal-cost parallel paths - via features like EIGRP variance - allows traffic to be distributed proportionally across multiple paths simultaneously rather than failing over sequentially, which directly prevents any single path from becoming saturated and triggering the cascading oscillation described.
Adding a second link to each remote site doubles the available bandwidth at that spoke, so peak-hour congestion on one link does not force traffic through a neighboring spoke, eliminating the cascading overload scenario without introducing additional routing protocol complexity.
Concept tested: WAN hub-and-spoke resilience with unequal-cost load balancing
Source: https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13675-17.html
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