--- title: Running OSPF over Unnumbered Ethernet Interfaces «... description: A deep dive into running OSPF over unnumbered IPv4 Ethernet interfaces. IPv6 sidesteps the issue via link-local addresses, but IPv4 requires careful handling.... canonical: https://daily.dev/posts/running-ospf-over-unnumbered-ethernet-interfaces-ipspace-net-blog-r8ukf0cif twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Running OSPF over Unnumbered Ethernet Interfaces « ipSpace.net blog | daily.dev og:description: A deep dive into running OSPF over unnumbered IPv4 Ethernet interfaces. IPv6 sidesteps the issue via link-local addresses, but IPv4 requires careful handling.... og:url: https://daily.dev/posts/running-ospf-over-unnumbered-ethernet-interfaces-ipspace-net-blog-r8ukf0cif og:image: https://api.daily.dev/og/posts/R8ukF0cIf.png og:image:alt: Running OSPF over Unnumbered Ethernet Interfaces « 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. # Running OSPF over Unnumbered Ethernet Interfaces « ipSpace.net blog **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 6 min read · 0 upvotes · 0 comments ## Summary A deep dive into running OSPF over unnumbered IPv4 Ethernet interfaces. IPv6 sidesteps the issue via link-local addresses, but IPv4 requires careful handling. OSPF adjacency works on point-to-point unnumbered links because the network mask check is skipped for P2P links (RFC 2328 Section 10.5), and multicast delivery via AllSPFRouters (224.0.0.5) handles information exchange. Route selection relies on glue ARP entries to resolve next-hop addresses on unnumbered interfaces. The approach is confirmed to work on Cisco IOS/IOS XE/IOS XR/NX-OS, Arista EOS, Junos, Cumulus Linux, FRR, Nokia SR OS, and VyOS. Unnumbered multi-access links remain unsupported for OSPF due to the subnet assumption baked into the protocol. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Can OSPF form adjacency over unnumbered IPv4 point-to-point Ethernet interfaces? Yes, OSPF adjacency works on unnumbered IPv4 point-to-point links. RFC 2328 Section 10.5 explicitly says the network mask in Hello packets should be ignored on point-to-point networks and virtual links, bypassing the mask mismatch check. Multicast delivery via AllSPFRouters (224.0.0.5) handles packet exchange. Route selection requires a glue ARP entry mapping the neighbor's loopback address to the outgoing interface. _Engineers building unnumbered fabric designs track platform-specific OSPF behavior like this on daily.dev._ ### Why does OSPF not work over unnumbered multi-access (broadcast) Ethernet interfaces? OSPF fails on unnumbered multi-access links because the protocol assumes all non-P2P interfaces have an IP subnet. The network mask check in Hello packets cannot be bypassed on broadcast networks, and Cisco IOS XE avoids the problem entirely by not sending Hello packets out of unnumbered interfaces unless they are configured as point-to-point OSPF networks. IS-IS avoids this limitation because it does not use IP to transport routing messages. _Networking engineers choosing between OSPF and IS-IS for unnumbered fabrics find relevant comparisons on daily.dev._ ### Which router platforms support OSPF over unnumbered IPv4 interfaces? OSPF over unnumbered IPv4 Ethernet interfaces is confirmed to work on Arista EOS (requires a nerd knob), Cisco IOS, IOS XE, IOS XR, NX-OS, Juniper Junos, Cumulus Linux, FRR, Nokia SR OS, and VyOS. _Keeping up with platform support changes for niche routing features is easier when the networking community shares findings on daily.dev._ ## Similar posts on daily.dev - [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 - [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 --- Tags: [#networking](https://daily.dev/tags/networking) [View this post on daily.dev](https://daily.dev/posts/running-ospf-over-unnumbered-ethernet-interfaces-ipspace-net-blog-r8ukf0cif) ```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":"Running OSPF over Unnumbered Ethernet Interfaces « ipSpace.net blog","url":"https://daily.dev/posts/running-ospf-over-unnumbered-ethernet-interfaces-ipspace-net-blog-r8ukf0cif","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/running-ospf-over-unnumbered-ethernet-interfaces-ipspace-net-blog-r8ukf0cif"},"datePublished":"2026-08-11T08:54:09.299Z","dateModified":"2026-08-11T09:20:22.519Z","description":"A deep dive into running OSPF over unnumbered IPv4 Ethernet interfaces. 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