Causes of Incomplete ARP Entry

An ISP technician gets a call from a business that just changed equipment but can no longer connect to the Internet.

The technician checks the ARP table on the ISP switch, and there is no corresponding MAC address present.

Instead, the entry is Incomplete.

Which of the following could cause this condition?

A.

VLAN mismatch

B.

Duplex/Speed mismatch

C.

Duplicate IP address

D.

TX/RX reverse.

B.

When the ARP table on the ISP switch shows an incomplete entry for a device on the network, it means that the switch has not yet learned the MAC address associated with the IP address of that device. This can happen for several reasons, including the following:

A. VLAN mismatch: If the device is on a different VLAN than the switch, the switch will not be able to learn its MAC address. This could happen if the VLAN configuration on the switch or the device was not set up correctly.

B. Duplex/Speed mismatch: If the switch and the device are not set to the same duplex and speed settings, they may not be able to communicate properly. This can result in the switch not being able to learn the MAC address of the device.

C. Duplicate IP address: If there is another device on the network with the same IP address as the new device, the switch may not be able to learn the MAC address of either device, resulting in an incomplete ARP entry.

D. TX/RX reverse: If the transmit and receive cables are reversed between the switch and the device, they will not be able to communicate properly. This can result in the switch not being able to learn the MAC address of the device.

Out of the options provided, the most likely cause of the incomplete ARP entry is B, Duplex/Speed mismatch. However, further troubleshooting would be needed to confirm this.