CCIE Security Exam: MTU and PDU Calculation

# MTU and PDU Calculation

### Question

A device is sending a PDU of 5000 B on a link with an MTU of 1500 B.

If the PDU includes 20 B of IP header, which statement is true?

### Explanations

Click on the arrows to vote for the correct answer

A. B. C. D.

C.

When a device sends data packets larger than the maximum transmission unit (MTU) of a network link, the packets must be fragmented into smaller pieces that can be transmitted across the link. The maximum payload size of each packet is equal to the MTU size minus the size of the IP header. In this case, the device is sending a PDU (Protocol Data Unit) of 5000 bytes on a link with an MTU of 1500 bytes and an IP header size of 20 bytes.

To determine the payload size of each packet, we first subtract the size of the IP header from the MTU: 1500 - 20 = 1480

Therefore, each packet can have a payload size of up to 1480 bytes. To determine how many packets are needed to transmit the entire PDU, we divide the PDU size by the payload size: 5000 / 1480 = 3.38

This means that we need 4 packets to transmit the entire PDU, with the first 3 packets having a payload size of 1480 bytes and the last packet having a payload size of 560 bytes (since the remaining bytes after the first 3 packets is 560).

Therefore, the correct answer is A: The first three packets will have a packet payload size of 1400.