A network administrator upgrades an enterprise core network link between two OSPF-enabled … — CompTIA Network+

A network administrator upgrades an enterprise core network link between two OSPF-enabled routers from $1\text{ Gbps}$ to $10\text{ Gbps}$ . However, after the upgrade, the routers continue to load-balance traffic equally between the new $10\text{ Gbps}$ primary link and a parallel $1\text{ Gbps}$ backup link. Upon inspecting the OSPF database, both interfaces are assigned an OSPF cost metric of $1$ . Which of the following best explains why OSPF calculates an identical path metric for both the $1\text{ Gbps}$ and $10\text{ Gbps}$ links?

The OSPF default reference bandwidth is $100\text{ Mbps}$ , which calculates a cost fractional value below $1$ for both links and rounds up to a minimum interface cost of $1$ .Answer
B
OSPF uses Administrative Distance rather than interface bandwidth to determine path metrics, assigning a fixed metric of $1$ to all internal link-state routes.
C
The split horizon mechanism prevents OSPF routers from advertising metric costs lower than $1$ across interfaces operating within the same backbone area.
D
An autonegotiation failure forced a duplex mismatch on the $10\text{ Gbps}$ interface, causing OSPF to fall back to a default legacy cost of $1$ .

Answer

The OSPF default reference bandwidth is 100 Mbps, which calculates a cost fractional value below 1 for both links and rounds up to a minimum interface cost of 1.

OSPF calculates path cost using the formula Cost = Reference Bandwidth / Interface Bandwidth. Because the default reference bandwidth in standard OSPF deployments is 100 Mbps, any interface speed equal to or greater than 100 Mbps (such as 1 Gbps or 10 Gbps) calculates to a value less than or equal to 1. Since OSPF costs must be positive integers, both 1 Gbps and 10 Gbps links receive a cost of 1, treating them as equal-cost paths until the auto-cost reference-bandwidth command is configured.

Step-by-Step Solution

Identify the OSPF metric calculation formula

\text{OSPF Cost} = \frac{\text{Reference Bandwidth}}{\text{Interface Bandwidth}}

OSPF determines interface metrics based on relative link speed.

Calculate the default cost for a

1\text{ Gbps}

link

\frac{100\text{ Mbps}}{1000\text{ Mbps}} = 0.1 \rightarrow \text{rounded up to } 1

The default reference bandwidth in OSPF is

100\text{ Mbps}

, and minimum OSPF metric cost is

1

Calculate the default cost for a

10\text{ Gbps}

link

\frac{100\text{ Mbps}}{10000\text{ Mbps}} = 0.01 \rightarrow \text{rounded up to } 1

Since OSPF metrics must be positive integers, any calculation yielding less than 1 defaults to 1.

Determine the operational outcome

Both paths evaluate to cost

1

, triggering Equal-Cost Multi-Path (ECMP) routing.

To differentiate speeds greater than

100\text{ Mbps}

, the network engineer must manually adjust the `auto-cost reference-bandwidth` setting.

Key Concept

OSPF Reference Bandwidth and Cost Calculation

Question

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Step-by-Step Solution

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