--- title: Running IS-IS over Unnumbered Ethernet Interfaces «... description: IS-IS can run over unnumbered Ethernet interfaces, including multi-access segments, unlike OSPF which requires IP addresses on interfaces. Testing on Arista... canonical: https://daily.dev/posts/running-is-is-over-unnumbered-ethernet-interfaces-ipspace-net-blog-ofr6rlroc twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Running IS-IS over Unnumbered Ethernet Interfaces « ipSpace.net blog | daily.dev og:description: IS-IS can run over unnumbered Ethernet interfaces, including multi-access segments, unlike OSPF which requires IP addresses on interfaces. Testing on Arista... og:url: https://daily.dev/posts/running-is-is-over-unnumbered-ethernet-interfaces-ipspace-net-blog-ofr6rlroc og:image: https://api.daily.dev/og/posts/ofR6rLROc.png og:image:alt: Running IS-IS 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 IS-IS over Unnumbered Ethernet Interfaces « ipSpace.net blog **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 5 min read · 0 upvotes · 0 comments ## Summary IS-IS can run over unnumbered Ethernet interfaces, including multi-access segments, unlike OSPF which requires IP addresses on interfaces. Testing on Arista EOS and Cisco IOS XE shows full adjacency formation, correct pseudonode LSP generation, and working IP routing with ARP glue entries. Cisco NXOS requires 'medium p2p' to use ip unnumbered, which forces point-to-point IS-IS behavior even on multi-access links, meaning no pseudonode LSP is generated — this can cause interoperability issues in multi-vendor environments. Junos vSRX works for unnumbered IPv4 P2P links and IPv6 works universally due to link-local addresses. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Can IS-IS run over unnumbered Ethernet interfaces on multi-access segments? Yes, IS-IS can run over unnumbered Ethernet interfaces on multi-access segments. Unlike OSPF, IS-IS treats IPv4 and IPv6 prefixes as TLVs and makes no assumptions about interface IP addresses. On Arista EOS and Cisco IOS XE, adjacencies form correctly, a pseudonode LSP is generated for the LAN segment, and IP routing works with ARP glue entries resolving loopback addresses. _Network engineers building unnumbered fabrics track platform-specific IS-IS behavior like this on daily.dev._ ### Why does Cisco NXOS require 'medium p2p' for IS-IS unnumbered interfaces and what are the side effects? Cisco NXOS requires 'medium p2p' to enable 'ip unnumbered' on interfaces. This forces IS-IS to treat all links as point-to-point, so no pseudonode LSP is generated even on multi-access segments. Adjacencies still form on shared Ethernet, but each router advertises direct P2P links to neighbors instead of a shared pseudonode — breaking multi-vendor interoperability on the same segment. _Engineers running mixed-vendor IS-IS deployments find NXOS edge cases like this 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 - [Multivendor SR-MPLS « ipSpace.net blog](https://daily.dev/posts/multivendor-sr-mpls-ipspace-net-blog-qekokwduk) · ipSpace.net Blog · 0 upvotes · 0 comments --- [View this post on daily.dev](https://daily.dev/posts/running-is-is-over-unnumbered-ethernet-interfaces-ipspace-net-blog-ofr6rlroc) ```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 IS-IS over Unnumbered Ethernet Interfaces « ipSpace.net blog","url":"https://daily.dev/posts/running-is-is-over-unnumbered-ethernet-interfaces-ipspace-net-blog-ofr6rlroc","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/running-is-is-over-unnumbered-ethernet-interfaces-ipspace-net-blog-ofr6rlroc"},"datePublished":"2026-08-11T08:54:05.547Z","dateModified":"2026-08-11T09:05:02.020Z","description":"IS-IS can run over unnumbered Ethernet interfaces, including multi-access segments, unlike OSPF which requires IP addresses on interfaces. 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Unlike OSPF, IS-IS treats IPv4 and IPv6 prefixes as TLVs and makes no assumptions about interface IP addresses. On Arista EOS and Cisco IOS XE, adjacencies form correctly, a pseudonode LSP is generated for the LAN segment, and IP routing works with ARP glue entries resolving loopback addresses. Network engineers building unnumbered fabrics track platform-specific IS-IS behavior like this on daily.dev."}},{"@type":"Question","name":"Why does Cisco NXOS require 'medium p2p' for IS-IS unnumbered interfaces and what are the side effects?","acceptedAnswer":{"@type":"Answer","text":"Cisco NXOS requires 'medium p2p' to enable 'ip unnumbered' on interfaces. This forces IS-IS to treat all links as point-to-point, so no pseudonode LSP is generated even on multi-access segments. Adjacencies still form on shared Ethernet, but each router advertises direct P2P links to neighbors instead of a shared pseudonode — breaking multi-vendor interoperability on the same segment. Engineers running mixed-vendor IS-IS deployments find NXOS edge cases like this on daily.dev."}}]} ```