Understanding MTU (Maximum Transmission Unit) Networking

Prerequisites: Basic Understanding of IP Addresses, Packets and Network Interfaces

Imagine downloading a large file from a website.

Your computer sends and receives information through several network devices, including switches, routers and firewalls.

But network links cannot transmit packets of unlimited size.

Each link has limits on the amount of information it can carry in one transmission.

One important limit is called the Maximum Transmission Unit (MTU).

If an IP packet exceeds the supported MTU, it may need to be fragmented, reduced in size by its sender or dropped, depending on the protocol and network conditions.

Incorrect MTU handling can cause:

• Websites to load partially or fail
• VPN connectivity problems
• Slow file transfers
• Application timeouts
• Packet fragmentation
• Intermittent network communication

In this article, you will learn:

• What MTU means
• The difference between IP packets and Ethernet frames
• Why 1,500 bytes is commonly used
• What jumbo frames are
• How MTU and TCP MSS are related
• How Path MTU Discovery works
• How to test MTU using Windows and Linux
• How to troubleshoot MTU problems

What Is MTU and How Does It Work?

MTU stands for Maximum Transmission Unit.

It defines the maximum size of a protocol data unit that can be transmitted over a network link at the relevant layer.

MTU is measured in bytes.

One byte contains eight bits.

When discussing IPv4 or IPv6 over Ethernet, an IP MTU of 1,500 bytes normally means the link can carry an IP packet up to 1,500 bytes, including its IP header.

It does not include the surrounding Ethernet header, VLAN tags or Frame Check Sequence (FCS).

Why Is 1,500 Bytes Commonly Used?

Standard Ethernet commonly supports a maximum MAC-client data payload of 1,500 bytes.

When that payload carries an IP packet, the corresponding IP MTU is normally 1,500 bytes.

However, 1,500 bytes is not the universal maximum for all networks.

Different link technologies and configurations support different MTU values.

Common MTU Examples

These are common values or protocol requirements. Actual interface limits depend on network technology and equipment.

Understanding Ethernet Frame Size

A standard Ethernet II frame contains:

• Destination MAC address
• Source MAC address
• EtherType field
• Ethernet payload
• Frame Check Sequence

The conventional Ethernet header is 14 bytes.

The FCS is 4 bytes.

Therefore:

Total Untagged Ethernet Frame Size = Ethernet Header + Payload + FCS

For a 1,500-byte IP packet:

14 + 1500 + 4 = 1518 bytes

What Happens When a VLAN Tag Is Added?

IEEE 802.1Q VLAN tagging adds 4 bytes to the Ethernet frame.

For one VLAN tag:

14 + 4 + 1500 + 4 = 1522 bytes

The IP packet remains 1,500 bytes.

VLAN tagging increases the Ethernet frame size without necessarily reducing the IP MTU.

Ethernet Frame Size Comparison

These totals exclude the Ethernet preamble, Start Frame Delimiter (SFD) and inter-frame gap.

Additional encapsulation, such as stacked VLAN tags, can further increase the frame size.

What Are Jumbo Frames and How Do They Relate to MSS?

What Is a Jumbo Frame?

A jumbo frame is an Ethernet frame designed to carry a larger-than-standard Ethernet payload.

In IP networking, this commonly means an Ethernet link configured to carry IP packets larger than 1,500 bytes.

Jumbo-frame terminology and supported maximum sizes vary by vendor.

For example, many network environments use an IP MTU of 9,000 bytes.

That does not mean the complete Ethernet frame is exactly 9,000 bytes.

Example — 9,000-Byte MTU

These calculations assume the stated payload fits within the device’s supported Ethernet framing capabilities.

A device advertising a maximum frame size of 9,000 bytes may not support a 9,000-byte IP MTU, depending on how that device defines its limit.

Is a 1,522-Byte VLAN Frame a Jumbo Frame?

Not necessarily.

A standard Ethernet frame with one VLAN tag can be 1,522 bytes while still carrying only 1,500 bytes of IP data.

This is normal VLAN encapsulation overhead.

It does not automatically mean the network is using jumbo IP packets.

Some networking documentation uses the term baby giant for slightly oversized Ethernet frames. The exact classification is vendor-dependent.

Why Are Jumbo Frames Used?

Jumbo frames are commonly used in:

• Data centers
• Storage networks
• Virtualization environments
• Server-to-server communication
• Selected backup and replication networks

Larger payloads can reduce packet-processing overhead for certain workloads.

However, jumbo frames do not automatically improve application performance.

Important Jumbo Frame Requirements

• Both endpoints must support the configured IP MTU.
• Switches must support the resulting complete Ethernet frame size.
• Intermediate Layer 3 links must support the applicable IP packet size or handle it appropriately.
• VLAN tags and additional encapsulation must be considered.
• Vendor-defined frame-size limits must be verified.

What Is MSS?

MSS stands for Maximum Segment Size.

It represents the maximum TCP data segment size advertised by an endpoint, excluding the TCP and IP headers.

For a basic IPv4 TCP connection without IP or TCP options:

MSS = MTU − IPv4 Header − TCP Header

For MTU 1500:

1500 − 20 − 20 = 1460 bytes

For a basic IPv6/TCP packet without extension headers or TCP options, the corresponding calculation uses the 40-byte IPv6 base header.

In practice, actual transmitted segment sizes depend on effective path limits, negotiated MSS and protocol overhead.

What Is Path MTU Discovery?

An IP packet can cross multiple network links before reaching its destination.

Each link may support a different IP MTU.

The smallest effective IP MTU along the path determines the maximum unfragmented IP packet size that can travel through it.

This is called Path MTU (PMTU).

Example — Different MTUs Along a Path

In this example, the smallest IP MTU is 1,400 bytes.

A 1,500-byte IP packet cannot traverse the 1,400-byte bottleneck unchanged.

How IPv4 Path MTU Discovery Works

IPv4 Path MTU Discovery is defined in RFC 1191.

A simplified process is:

1. The sender transmits an IPv4 packet with the Don’t Fragment (DF) flag set.
2. A router receives the packet.
3. The router determines that the IP packet exceeds the outgoing link MTU.
4. The router drops the packet because fragmentation is prohibited.
5. The router sends an ICMP Fragmentation Needed message.
6. The sender can adjust its estimated Path MTU.

The relevant ICMPv4 error is:

Destination Unreachable — Fragmentation Needed (Type 3, Code 4)

How IPv6 Handles MTU

Intermediate IPv6 routers do not fragment IPv6 packets.

When an IPv6 packet exceeds the outgoing link MTU, the router discards it and sends an ICMPv6 Packet Too Big message.

The source must adjust packet sizing or perform source fragmentation when applicable.

The minimum IPv6 link MTU is 1,280 bytes.

How to Test MTU Using Windows and Linux

One way to investigate IPv4 Path MTU is to send ICMP Echo Requests of different sizes with fragmentation prohibited.

This method can help identify packet-size limitations.

However, ping results should not be treated as a complete or guaranteed measurement of the optimal MTU.

Understanding the 28-Byte Calculation

For a basic IPv4 ICMP Echo Request:

Therefore:

Total IPv4 Packet Size = ICMP Payload + 28 Bytes

If the IP MTU is 1,500 bytes:

1500 − 28 = 1472 bytes

A 1,472-byte ICMP payload therefore produces a 1,500-byte IPv4 packet, assuming the standard headers without IP options.

When carried in an untagged Ethernet frame:

1500 + 14 + 4 = 1518 bytes

Method 1 — Windows MTU Testing

Open Windows Command Prompt.

To test a 1,500-byte IPv4 packet:

ping -4 -f -l 1472 192.0.2.20

Parameters:

Replace the example IP address with an authorized, reachable destination.

Illustrative Successful Output:

Reply from 192.0.2.20: bytes=1472 time=2ms TTL=64
Reply from 192.0.2.20: bytes=1472 time=2ms TTL=64

If the packet cannot be sent without fragmentation, Windows may report:

Packet needs to be fragmented but DF set.

Testing a Smaller Packet

To test an IPv4 packet of 1,492 bytes:

ping -4 -f -l 1464 192.0.2.20

Calculation:

1464 + 28 = 1492 bytes

IPv4 MTU Calculation Examples

These are IP packet sizes.

The complete Ethernet frame will be larger because of Ethernet framing overhead.

Method 2 — Linux MTU Testing

On Linux systems using the common iputils ping implementation:

ping -4 -M do -s 1472 -c 3 192.0.2.20

Parameters:

If the kernel determines that the packet exceeds the known MTU, an error may appear.

Illustrative Output:

ping: local error: message too long, mtu=1492

To inspect configured interface MTUs:

ip link show

Common MTU Problems and Troubleshooting

What Is an MTU Black Hole?

An MTU black hole can occur when packets exceeding a path’s effective MTU are dropped and the sender does not receive usable information about the problem.

For example:

1. A TCP connection completes its three-way handshake.
2. The sender begins transmitting larger TCP/IP packets.
3. An intermediate link cannot forward an oversized IP packet.
4. Necessary ICMP feedback is blocked or lost.
5. Data transfer stalls or retransmissions occur.

This is why indiscriminately blocking ICMP and ICMPv6 can interfere with normal network operation.

Common MTU Troubleshooting Symptoms

Should You Change the MTU After a Ping Test?

Not automatically.

Before changing any MTU:

1. Verify the existing IP MTU.
2. Identify the bottleneck link.
3. Check the supported complete Ethernet frame size where relevant.
4. Determine whether VLAN or tunnel encapsulation is present.
5. Verify Path MTU Discovery.
6. Check TCP MSS when troubleshooting TCP applications.
7. Change configuration only when the network design and evidence support it.

For example, a switch supporting a maximum complete Ethernet frame size of 9,000 bytes should not automatically be assumed to support a 9,000-byte IP MTU.

If the device counts the Ethernet header and FCS within its 9,000-byte limit, the supported IP packet size will be smaller.

Key Takeaways

MTU and Frame Size

• MTU stands for Maximum Transmission Unit.
• Standard Ethernet commonly supports a 1,500-byte IP MTU.
• A 1,500-byte IP packet produces a 1,518-byte untagged Ethernet frame, including FCS.
• A single 802.1Q VLAN tag increases the total frame size to 1,522 bytes.
• Ethernet header and FCS overhead are not included in the conventional 1,500-byte IP MTU.

Jumbo Frames and MSS

• Jumbo Ethernet frames carry larger-than-standard payloads.
• An IP MTU of 9,000 bytes results in a 9,018-byte untagged Ethernet frame, including FCS.
• A single VLAN tag increases that example to 9,022 bytes.
• Jumbo-frame limits vary across vendors and platforms.
• MSS describes TCP data size rather than complete IP or Ethernet frame size.

Path MTU and Troubleshooting

• Path MTU is the smallest effective IP MTU across the route.
• IPv4 supports DF-based Path MTU Discovery.
• IPv6 routers do not fragment packets.
• Windows and Linux ping commands can help investigate packet-size limitations.
• IPv4 ICMP payload testing commonly uses 28 bytes of combined IP and ICMP header overhead.
• ICMP filtering can interfere with Path MTU Discovery.

Maximum Transmission Unit is a fundamental networking concept that defines the maximum size of a protocol data unit on a particular link.

For conventional Ethernet carrying IP, a 1,500-byte MTU normally refers to the IP packet size, not the total Ethernet frame size.

The complete Ethernet frame is larger because it includes Layer 2 headers and the Frame Check Sequence.

Jumbo-frame networks support larger payloads, but the permitted IP MTU and complete frame size must be checked against the actual device capabilities.

Understanding MTU, Ethernet frame size, MSS and Path MTU Discovery helps network administrators investigate fragmentation, packet drops, VPN connectivity issues and application performance problems.

Correct terminology is especially important when working with switches, routers, firewalls, VLANs and high-throughput networks.

Useful Links and References

SanchitGurukul Resources

Official External References