--- title: MTU Settings in Virtual Network Devices « ipSpace.net blog description: MTU mismatches between virtual network devices can silently break protocols like OSPF and BGP. The root cause traced here: containerlab sets vEth link MTU to... canonical: https://daily.dev/posts/mtu-settings-in-virtual-network-devices-ipspace-net-blog-uyt1mcpz6 twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: MTU Settings in Virtual Network Devices « ipSpace.net blog | daily.dev og:description: MTU mismatches between virtual network devices can silently break protocols like OSPF and BGP. The root cause traced here: containerlab sets vEth link MTU to... og:url: https://daily.dev/posts/mtu-settings-in-virtual-network-devices-ipspace-net-blog-uyt1mcpz6 og:image: https://api.daily.dev/og/posts/uyT1Mcpz6.png og:image:alt: MTU Settings in Virtual Network Devices « ipSpace.net blog og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # MTU Settings in Virtual Network Devices « ipSpace.net blog **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 3 min read · 0 upvotes · 0 comments ## Summary MTU mismatches between virtual network devices can silently break protocols like OSPF and BGP. The root cause traced here: containerlab sets vEth link MTU to 9500 by default, so SR Linux (a real container) inherits 9500, while SR OS (a VM wrapped in a container via vrnetlab) defaults to the Ethernet standard of 1500. Cumulus VX and FRR containers also ran at 9500. The fix was implementing an explicit MTU parameter in netlab and defaulting container-based network devices to 1500. The post also explains why paravirtual drivers like virtio can pass MTU hints to guest OSes, while emulated NICs like E1000 cannot, and notes that Cumulus Linux changed its default MTU to 9216 in release 4.2. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does containerlab set MTU to 9500 on vEth links by default? Containerlab explicitly sets the MTU to 9500 on point-to-point vEth links by design. This means container-native network OSes like SR Linux inherit that 9500 MTU, while VM-based devices packaged as containers (such as SR OS via vrnetlab/QEMU) default to the Ethernet standard of 1500, causing silent MTU mismatches that can break OSPF and BGP adjacencies. _Engineers running mixed containerlab topologies track these defaults on daily.dev before they become a lab-breaking surprise._ ### Why does SR Linux have a different default MTU than SR OS in containerlab? SR Linux is a native container and inherits its MTU from the container orchestration system — containerlab sets vEth links to 9500, so SR Linux gets 9500. SR OS is not available as a real container; what runs in containerlab is a VM packaged inside a container file with QEMU (via vrnetlab), so it uses the standard Ethernet default of 1500 bytes. _Developers choosing between SR Linux and SR OS for lab topologies find the trade-offs covered on daily.dev._ ### When did Cumulus Linux change its default MTU and to what value? Cumulus Linux changed its default MTU to 9216 bytes starting in release 4.2. _Network engineers upgrading Cumulus Linux versions watch for default-changing releases like this on daily.dev._ ## Similar posts on daily.dev - [Networking Aspects of Running VMs in Containers « ipSpace.net blog](https://daily.dev/posts/networking-aspects-of-running-vms-in-containers-ipspace-net-blog-gttjndloc) · ipSpace.net Blog · 0 upvotes · 0 comments - [Performance Has Layers](https://daily.dev/posts/performance-has-layers-ega2gcrnh) · Oxide · 97 upvotes · 2 comments - [The OSPF MTU Mismatch Saga « ipSpace.net blog](https://daily.dev/posts/the-ospf-mtu-mismatch-saga-ipspace-net-blog-jmtvuzjm6) · ipSpace.net Blog · 0 upvotes · 0 comments - [Dear Junos, Tunnels Are Not Virtual Links « ipSpace.net blog](https://daily.dev/posts/dear-junos-tunnels-are-not-virtual-links-ipspace-net-blog-1zdexbued) · ipSpace.net Blog · 0 upvotes · 0 comments - [The Never-Ending IPv6 Loopback Prefix Saga « ipSpace.net blog](https://daily.dev/posts/the-never-ending-ipv6-loopback-prefix-saga-ipspace-net-blog-4nlhgh10l) · ipSpace.net Blog · 0 upvotes · 0 comments --- [View this post on daily.dev](https://daily.dev/posts/mtu-settings-in-virtual-network-devices-ipspace-net-blog-uyt1mcpz6) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"MTU Settings in Virtual Network Devices « ipSpace.net blog","url":"https://daily.dev/posts/mtu-settings-in-virtual-network-devices-ipspace-net-blog-uyt1mcpz6","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/mtu-settings-in-virtual-network-devices-ipspace-net-blog-uyt1mcpz6"},"datePublished":"2026-08-11T08:53:54.418Z","dateModified":"2026-08-11T09:03:47.170Z","description":"MTU mismatches between virtual network devices can silently break protocols like OSPF and BGP. 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