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MTU Path Discovery Tool

mtupath mtupingnetworkingtroubleshooting

Updated Mar 6, 2026

A practical MTU troubleshooting helper that calculates the correct ICMP payload size for any given MTU, explains the math (IPv4: MTU − 28, IPv6: MTU − 48), and generates ready-to-paste ping commands with the Don't Fragment bit set for Linux, Windows, macOS, Cisco IOS, and Junos.

Includes MTU presets for common scenarios (Ethernet, PPPoE, VPN, jumbo frames), a binary-search helper for narrowing down path MTU, a reference table, and a concise explainer on how MTU testing works.

Configuration

IP Version:
Presets:

Calculated Payload

IPv4 ICMP payload formula
1500 − 28 = 1472
Header overhead: 20 (IP header) + 8 (ICMP header) = 28 bytes
MTU
1500
Overhead
28
Max Payload
1472

Platform Commands

Linux
ping -M do -s 1472 8.8.8.8
-M do sets the DF (Don't Fragment) bit via socket option.
Windows
ping -f -l 1472 8.8.8.8
-f sets the DF flag. Run as Administrator for best results.
macOS
ping -D -s 1472 8.8.8.8
-D sets the DF bit. Behavior identical to BSD ping.
Cisco IOS
ping
  Protocol [ip]: ip
  Target IP address: 8.8.8.8
  Repeat count [5]: 5
  Datagram size [100]: 1472
  Timeout in seconds [2]: 2
  Extended commands [n]: y
  ...defaults...
  Set DF bit in IP header? [no]: yes
  ...defaults...
Extended ping with DF bit. Use from privileged EXEC mode.
Junos
ping 8.8.8.8 do-not-fragment size 1472
do-not-fragment flag sets DF. Works in operational mode.

Try Smaller Payloads (binary search)

Click a value to copy. Start from 1472 and work down until pings succeed to find the path MTU.

Common MTU Reference

MTUIPv4 PayloadIPv6 PayloadScenario
150014721452Standard Ethernet
149214641444PPPoE
147214441424Tunnel / encapsulation
146014321412VPN overhead
140013721352Conservative VPN
900089728952Jumbo frames

How MTU Testing Works

What is MTU? The Maximum Transmission Unit is the largest packet size (in bytes) that a network interface can send in a single frame. Standard Ethernet MTU is 1500 bytes.

What is Path MTU? The smallest MTU of any link along the entire path between source and destination. A single link with a lower MTU bottlenecks the whole path.

Why subtract 28 (IPv4) or 48 (IPv6)? The ping payload sits inside an ICMP packet, which sits inside an IP packet. The IP and ICMP headers consume 28 bytes, so the maximum payload you can send is MTU minus that overhead.

Why set the Don't Fragment (DF) bit? Without DF, oversized packets are silently fragmented by routers. With DF set, routers that cannot forward the full packet will respond with an ICMP "Fragmentation Needed" message, revealing the bottleneck MTU.

Interpreting Results

  • Ping succeeds: The path can carry packets of that size without fragmentation.
  • Ping fails / "Packet needs to be fragmented": Something in the path has a lower MTU than your test size. Try a smaller payload.
  • Example: If payload 1472 fails but 1464 succeeds, the path MTU is likely 1492 (1464 + 28).
  • Binary search: Start with the max payload and drop by 8–10 bytes at a time until pings succeed. Then calculate MTU = successful payload + overhead.
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