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Difficulty: HardRouting Concepts and Dynamic Routing Protocols

A network administrator upgrades several core backbone links within an Open Shortest Path First (OSPF) routing domain from 1 Gbps to 10 Gbps. Despite the physical bandwidth upgrade, the core routers assign an identical metric cost of 1 to both the 1 Gbps and 10 Gbps interfaces, resulting in unwanted equal-cost multi-path (ECMP) load balancing across asymmetric links. Which configuration change must be applied to ensure OSPF correctly assigns a lower metric cost to the 10 Gbps interfaces?

  1. Increase the OSPF auto-cost reference bandwidth setting across all participating routers in the routing domain.Answer
  2. B
    Lower the administrative distance of OSPF on the 10 Gbps router interfaces below the default value of 110.
  3. C
    Modify the local OSPF process ID on the 10 Gbps interfaces to force Layer 2 PDU encapsulation tagging.
  4. D
    Reconfigure host default gateways on connected subnets to bypass interior gateway protocol routing decisions.

Answer

Increase the OSPF auto-cost reference bandwidth setting across all participating routers in the routing domain.
The correct answer highlights that default OSPF implementations use a reference bandwidth of 100 Mbps (10810^8 bps) to calculate link cost. As a result, both 1 Gbps and 10 Gbps interfaces evaluate to a cost less than 1, which rounds up to the minimum metric value of 1. Increasing the auto-cost reference bandwidth across all routers allows OSPF to assign distinct, lower cost metrics to faster links, resolving unwanted equal-cost load balancing.

Step-by-Step Solution

1
Analyze the default OSPF interface cost calculation formula.
Recognize that OSPF calculates cost as Cost=Reference BandwidthInterface Bandwidth\text{Cost} = \frac{\text{Reference Bandwidth}}{\text{Interface Bandwidth}}, where default reference bandwidth equals 100 Mbps (10810^8 bps).
For interfaces with bandwidth equal to or greater than 100 Mbps (such as 1 Gbps and 10 Gbps), the formula yields values less than 1, which are rounded up to the minimum metric integer of 1.
2
Determine how to restore metric differentiation for high-speed interfaces.
Adjust the auto-cost reference bandwidth parameter to a higher threshold (such as 100,000 Mbps or 100 Gbps).
With a 100,000 Mbps reference bandwidth, a 1 Gbps link has a metric cost of 100 (100,0001,000\frac{100,000}{1,000}), whereas a 10 Gbps link has a metric cost of 10 (100,00010,000\frac{100,000}{10,000}), successfully preferring the faster path.

Key Concept

OSPF Auto-Cost Reference Bandwidth Adjustment
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Topics

Routing Concepts and Dynamic Routing Protocols

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