# Diagnosing MTU Mismatches Between QEMU TAP Interfaces and VM Network Devices

DevFeed: [Diagnosing MTU Mismatches Between QEMU TAP Interfaces and VM Network Devices](<https://devfeed.tech/articles/the-linux-bridge-mtu-hell-11152.md>)

Original publisher: [Read original article](<https://blog.ipspace.net/2025/03/linux-bridge-mtu-hell/>)

Published: 2025-03-11T07:11:00Z

Content type: article

Language: en

Sources: [ipSpace.net blog](<https://devfeed.tech/sources/ipspace-net-blog.md>)

Topics: [Linux](<https://devfeed.tech/topics/linux.md>), [qemu](<https://devfeed.tech/topics/qemu.md>), [networking](<https://devfeed.tech/topics/networking.md>), [Vagrant](<https://devfeed.tech/topics/vagrant.md>), [Operating system](<https://devfeed.tech/topics/operating-system.md>)

Tags: [article](<https://devfeed.tech/tags/article.md>), [connectivity](<https://devfeed.tech/tags/connectivity.md>), [linux](<https://devfeed.tech/tags/linux.md>), [networking](<https://devfeed.tech/tags/networking.md>), [qemu](<https://devfeed.tech/tags/qemu.md>), [switching](<https://devfeed.tech/tags/switching.md>), [vagrant](<https://devfeed.tech/tags/vagrant.md>), [virtio](<https://devfeed.tech/tags/virtio.md>), [virtualization](<https://devfeed.tech/tags/virtualization.md>)

## AI overview

The article investigates asymmetric connectivity when oversized packets traverse two virtual machines connected through a Linux bridge. Packet captures show that QEMU does not adjust the Linux TAP interface MTU when a VM changes its virtual NIC MTU, creating an MTU mismatch that can prevent packets from reaching the outgoing interface.

## Source excerpt

It all started with an innocuous article describing the MTU basics. As the real purpose of the MTU is to prevent packet drops due to fixed-size receiver buffers, and I waste spend most of my time in virtual labs, I wanted to check how various virtual network devices react to incoming oversized packets. As the first step, I created a simple netlab topology in which a single link had a slightly larger than usual MTU... and then all hell broke loose. Read more ...