300-320 Question #137: Real Exam Question with Answer & Explanation

The correct answer is B: Differentiate cost on a 10 Gigabit Ethernet trunk link.. OSPF calculates interface cost using the formula: Cost = Reference Bandwidth / Interface Bandwidth. The default reference bandwidth is 100 Mbps. With the default, FastEthernet (100 Mbps), GigabitEthernet (1000 Mbps), and 10 GigabitEthernet (10,000 Mbps) all yield a cost of 1 (int

Question

Which option describes the purpose of the auto-cost reference bandwidth 10000 command under the OSPF process?

Options

ARecognize redundant 10 Megabit links.
BDifferentiate cost on a 10 Gigabit Ethernet trunk link.
CChange the default bandwidth for a 100 Megabit link.
DAdjust the link bandwidth for routing protocols.

Explanation

OSPF calculates interface cost using the formula: Cost = Reference Bandwidth / Interface Bandwidth. The default reference bandwidth is 100 Mbps. With the default, FastEthernet (100 Mbps), GigabitEthernet (1000 Mbps), and 10 GigabitEthernet (10,000 Mbps) all yield a cost of 1 (integer floor), making OSPF unable to distinguish between them. The command 'auto-cost reference-bandwidth 10000' sets the reference to 10,000 Mbps (10 Gbps). Now: 10GigE = cost 1, GigE = cost 10, FastEthernet = cost 100. This gives OSPF the ability to differentiate a 10 Gigabit Ethernet trunk link from slower interfaces and calculate more accurate, meaningful costs. The command does not change actual interface bandwidth, affect non-OSPF protocols, or target 10 Megabit or 100 Megabit links specifically.

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