Cisco Certified Network Associate (CCNA) Practice Exam

The command used to lower the default Maximum Transmission Unit (MTU) size for a Point-to-Point Protocol over Ethernet (PPPoE) configuration is "ip mtu 1492." This command is particularly relevant in PPPoE environments because the standard Ethernet MTU is 1500 bytes. However, PPPoE introduces a slight overhead due to the additional header it adds to each packet, which effectively reduces the available MTU size for the data being sent.

By setting the MTU size to 1492, you account for this overhead (8 bytes) and ensure that the packets transmitted do not get fragmented, which can lead to performance issues and connection problems. This is crucial for maintaining efficient and reliable network communication over PPPoE connections.

The other options either do not apply in the context of PPPoE or are incorrectly formatted for the purpose of configuring the MTU. The "ip mtu" command itself is necessary to specify the MTU size on a specific interface, while "mtu" alone is often used for layer two interfaces but may not always be recognized in all Cisco devices or contexts.