Maximum Transmission Unit

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Questions and Answers

What happens when R1 receives a packet from W1 that exceeds its M2?

R1 sends a request to W1 to adjust the packet size

R1 immediately sends back an ICMP message to W1

R1 tries to fragment the packet into smaller packets (correct)

R1 sends the packet back to W1 without changing it

What is the purpose of the DF bit in IP version 4?

To disallow packet fragmentation (correct)

To reduce packet size

To allow packet fragmentation

To increase packet size

What happens when R1 receives a packet from W1 that exceeds its M2 with DF bit set to 1?

R1 sends a packet back to W1 to reduce the packet size

R1 sends back an ICMP fragmentation needed and DF set message (correct)

R1 ignores the packet

R1 tries to fragment the packet

What is M2 Path Discovery in IPV6?

A feature that dynamically adjusts packet size Signup and view all the answers

What happens when the IPV6 sender receives an ICMP message indicating packet too big?

The sender reduces the packet size and resends Signup and view all the answers

What is the main difference between IPV4 and IPV6 in terms of packet handling?

IPV4 has a DF bit, IPV6 does not Signup and view all the answers

What happens when R1 receives a packet from W1 that is smaller than its M2?

R1 forwards the packet to its destination Signup and view all the answers

What is the purpose of the ICMP message in this scenario?

To notify W1 of packet drop Signup and view all the answers

What is the consequence of setting the DF bit to 1 in IPV4?

The packet will never be fragmented Signup and view all the answers

What is the main advantage of M2 Path Discovery in IPV6?

It dynamically adjusts packet size Signup and view all the answers

What is the primary function of an M2 in a network device?

To determine the largest frame or packet that can be transmitted or received on an interface Signup and view all the answers

What is the difference between an M2 at layer two and an M2 at layer three?

The M2 at layer two includes the Ethernet header size Signup and view all the answers

What would happen if the M2 on Router R1 is smaller than the M2 on Switch SW1?

Traffic would be dropped due to mismatched M2 sizes Signup and view all the answers

Why is it a best practice to reduce the M2 on a PGP link?

To accommodate the eight byte PGP header Signup and view all the answers

What is the purpose of the Ethernet header in an M2 calculation?

To account for the extra size of a frame Signup and view all the answers

What happens if the M2 on Router R1 is not matched with the M2 on Switch SW1?

Traffic would be dropped due to mismatched M2 sizes Signup and view all the answers

What is the relationship between the M2 on Router R1 and Switch SW1?

The M2 on Router R1 and Switch SW1 should match Signup and view all the answers

What is the main benefit of having matching M2 sizes on both sides of a link?

Proper communication between devices Signup and view all the answers

What would happen if the packet size exceeds the M2 on Router R1?

The packet would be dropped due to exceeding the M2 size Signup and view all the answers

What is the purpose of the M2 in a network device?

To determine the largest frame or packet that can be transmitted or received on an interface Signup and view all the answers

Study Notes

Maximum Transmission Unit (MTU)

MTU is the largest frame or packet that can be transmitted or received on an interface, applicable to any network device.
MTU can be identified at Layer 2 (frames) or Layer 3 (packets).

MTU Example

Router R1 has an MTU of 1500 bytes for packets (Layer 3) and 1518 bytes for frames (Layer 2), considering an 18-byte Ethernet header.
Matching MTUs on both sides of a link (R1 and SW1) ensure proper communication.

Consequences of Mismatched MTUs

If R1 has a smaller MTU than SW1, packets exceeding R1's MTU will be dropped, and an ICMP "fragmentation needed and DF set" message will be sent back to SW1.

Reducing MTU Size

In Point-to-Point Protocol over Ethernet (PPPoE), the MTU may be reduced by 8 bytes to accommodate the PPPoE header, resulting in a packet size of 1492 bytes.

IPv4 and IPv6 Differences

In IPv4, the Don't Fragment (DF) bit is set to 1, disallowing packet fragmentation.
In IPv6, there is no DF bit, and the receiver sends a "packet too big" ICMPv6 message to the sender.
IPv6 supports MTU Path Discovery, allowing the sender to dynamically adjust packet size based on ICMPv6 feedback.

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Description

Learn about M2, the largest frame or packet that can be transmitted or received on an interface, and how it applies to network devices at layer 2 and 3.