MTU Size Calculator

About the MTU Size Calculator

Maximum Transmission Unit (MTU) is the largest packet size a network link can transmit without fragmentation. Standard Ethernet MTU is 1500 bytes. However, encapsulation protocols (VXLAN, GRE, IPsec) add headers that reduce the available payload, potentially causing fragmentation and performance degradation.

This calculator determines the effective payload size after accounting for various encapsulation overheads. It helps network engineers set inner MTU values correctly to avoid fragmentation when using overlay networks, VPNs, or tunneling protocols.

MTU mismatches are a subtle but impactful source of network issues. When a packet exceeds the MTU of a link, it must be fragmented (adding overhead and delay) or dropped (causing TCP retransmissions). Proper MTU configuration prevents both scenarios.

When This Page Helps

MTU mismatches cause fragmentation, packet drops, and mysterious connectivity issues. This calculator ensures correct MTU settings for encapsulated traffic, preventing performance degradation.

How to Use the Inputs

1. Enter the physical link MTU (typically 1500 for standard Ethernet, 9000 for jumbo frames).
2. Select the encapsulation type (none, VXLAN, GRE, IPsec, etc.).
3. Review the encapsulation overhead and effective payload.
4. Set inner interface MTU to the effective payload value.

Example Calculation

Tips & Best Practices

• Use jumbo frames (MTU 9000) on the physical network to accommodate overlay overhead.
• Set container/VM MTU to physical MTU minus encapsulation overhead.
• Test MTU with ping -f -s <size> (ping with don't-fragment flag).
• AWS EC2 instances in the same VPC support jumbo frames (MTU 9001).
• Kubernetes CNI plugins typically auto-configure pod MTU based on node MTU.
• Monitor TCP MSS (Maximum Segment Size) to verify effective MTU.

MTU and Network Performance

Larger MTU means fewer packets for the same data, reducing per-packet overhead (headers, interrupts, context switches). This is why jumbo frames (MTU 9000) improve throughput by 10–30% in data center networks. However, larger packets also mean more data retransmitted on packet loss.

Overlay Network MTU Management

Modern cloud and container networking relies heavily on overlay protocols (VXLAN, Geneve, WireGuard). Each adds overhead that reduces effective MTU. The best practice is jumbo frames on the physical/underlay network, preserving full 1500-byte MTU for tenant/overlay traffic.

Path MTU Discovery

Path MTU Discovery (PMTUD) uses ICMP Packet Too Big messages to determine the smallest MTU along a network path. When firewalls block ICMP, PMTUD fails silently, causing TCP connections to stall for large packets. Always allow ICMP Type 3 Code 4 through firewalls.

Frequently Asked Questions

• What is the standard Ethernet MTU?

  Standard Ethernet MTU is 1500 bytes. This is the payload of an Ethernet frame, not including the 14-byte Ethernet header and 4-byte FCS. Most networks default to this value. Jumbo frames increase MTU to 9000 bytes but require end-to-end support.

• What causes MTU-related issues?

  When a packet exceeds a link's MTU and the Don't Fragment (DF) bit is set, the packet is dropped. TCP negotiates MSS to avoid this, but ICMP-based Path MTU Discovery can be blocked by firewalls, causing black-hole routing for large packets.

• Should I use jumbo frames?

  In data centers and between cloud instances in the same VPC, jumbo frames improve throughput by reducing per-packet overhead. However, all devices on the path must support jumbo frames. Don't use them for internet-facing traffic.

• How does VXLAN affect MTU?

  VXLAN adds 50 bytes of overhead: 14 (outer Ethernet) + 20 (outer IP) + 8 (UDP) + 8 (VXLAN header). With a 1500-byte physical MTU, the inner MTU is reduced to 1450 bytes. Use jumbo frames (9000+) on the underlay to avoid this reduction.

• What about IPsec MTU impact?

  IPsec overhead varies by cipher suite: AES-128-GCM adds ~50 bytes, AES-256-CBC adds ~73 bytes. Include ESP header, IV, padding, and authentication. For a 1500-byte MTU with standard AES-GCM, the effective payload is ~1450 bytes.

• How do I test effective MTU?

  Use ping with the don't-fragment flag: ping -f -s 1472 target (1472 + 28 bytes IP/ICMP header = 1500). Decrease the size until pings succeed to find the actual path MTU. This is manual PMTUD (Path MTU Discovery).